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Old 06-07-2020, 02:44 PM
PontiacJim1959 PontiacJim1959 is offline
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Originally Posted by 389 View Post
It doesn't matter what wheel you start with or end with, they are all dedicated hydraulic circuits..

I used to gravity bleed until I tried my brothers pump, wow what a difference! I think I got an extra inch of brake pedal..
Yep, the pump works good. They sell them in the Adam & Eve stores and they too will give you an extra inch.

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Old 06-07-2020, 04:00 PM
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If there's still air in the system you didn't perform the bleeding procedure correctly.

As someone that had to have other peoples lives riding on the performance of the brake system, and have all the liability that goes along with it, I can get the pedal just as high with gravity bleeding, as any other method.

The fluid displaces air, if done correctly the system is completely full of fluid with zero air. There are various ways to displace air from a system, if the air is entirely evacuated, the end result is going to be the same, no matter how the air is evacuated. Gravity bleeding is based on a siphon method, done correctly it does just as good as any other method.

If you pour fluid into a jar until it's full, and screw the lid on it there is no air in that vessel. Pulling vacuum, or trying to force more fluid in under pressure isn't going to fill it any further. Telling me that putting a pump on a system is superior to gravity bleeding, sorry, not in my experience.

It's a free country, do your bleeding however you choose to, I know what has worked for me for decades. Having worked in more than a few dealerships and garages I can tell you that the majority of mechanics gravity bleed brakes, because they can't always find someone to pump the pedal, and not everyone has a pressure bleeder or a vacuum bleeder handy. You finish the repair, fill the master cylinder and let it bleed while you're cleaning up and putting tools away. Close the bleeders, test the pedal and top off the system. Do a road test, and you're done.

I'm finished with the dispute, if ya'll wanna argue more, feel free.

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Old 06-07-2020, 10:20 PM
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Interesting discussion but I'm also having a hard time understanding how gravity-driven fluid can flow past a spring-loaded check valve that is capable of holding residual pressure. Even if the bleeder is open at the wheel cylinder and residual pressure relieved, the spring will still be holding the check valve shut at the master. As I interpret, only when the master pressure exceeds the spring "closing" pressure will fluid flow. For gravity bleeding to work, it seems to me the fluid is either flowing a different path or the residual pressure valves are leaking. Perhaps they leak without the back-pressure (residual pressure)?

dmorg1 - The photos are great, thank you. What is the actual sealing surface inside these check valves? Is it metal on metal or was there an elastomeric seal or ???

Scratching my shiny head on this one...

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Old 06-08-2020, 06:35 AM
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Shiny,

You have nailed it: the fluid cannot pass the check valve under gravity. The design of CVs varies as per brand, but the easiest to visualise & understand is the GM dual m/c where drum brakes are used. The CV is a simple rubber tip with a small spring that pushes the rubber into the brass outlet fitting on the m/c. No fluid exits until the spring pressure is over come, by some force on the push rod.


Pontiac Jim.

Later cars with disk brakes do not use a check valve [ & even some drum braked cars ], so these can be gravity bled.

  #25  
Old 06-08-2020, 10:14 AM
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Quote:
Originally Posted by Geoff View Post
Shiny,

You have nailed it: the fluid cannot pass the check valve under gravity. The design of CVs varies as per brand, but the easiest to visualise & understand is the GM dual m/c where drum brakes are used. The CV is a simple rubber tip with a small spring that pushes the rubber into the brass outlet fitting on the m/c. No fluid exits until the spring pressure is over come, by some force on the push rod.


Pontiac Jim.

Later cars with disk brakes do not use a check valve [ & even some drum braked cars ], so these can be gravity bled.


Please read how a residual check valve works from the ASE, in their study guide for ASE tests, it's in the last paragraph:

https://www.freeasestudyguides.com/r...eck-valve.html

As I reiterated over and over, and Jim said the exact same thing, when there is zero pressure in the system, gravity bleeding works just fine. My actual experience, over your theory, especially when the theory is inaccurate.

It allows fluid into the system UNTIL the pressure is reached where it shuts off the flow maintaining a slight pressure in the system. With no pressure. the valve is OPEN allowing fluid flow.

Sorry mate, the ASE shoots down your theory. I'd be much more inclined to believe the ASE than you. This explanation from ASE, and my previous experience, "nails it".

If you want an example, just find a system with a residual valve and release the pressure. See if it doesn't flow without pressure in the system. Since I've done it untold times I can tell you what the result will be. Fluid will flow unimpeded, as long as there is zero pressure in the system, which is what happens with gravity bleeding. If you leave the system open it will empty the master cylinder completely.

Just as I asked you in previous posts, "If I cut a brake line on a residual valve system, will it empty the master cylinder"? You never answered the question, but posted shop manual explanation that said with pressure bleeding the valve would have to be overcome. Well apples and oranges because I wasn't talking about pressure bleeding, so the shop manual (which never acknowledges gravity bleeding) says you have to overcome the pressure activated residual check valve..............

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  #26  
Old 06-09-2020, 05:17 AM
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Sirrotica,
The ASE link doesn't shoot down any theory & omits a crucial part of the explanation.

There is no pressure build up in a CV system, until the pedal is pushed to build up enough pressure to overcome the CV pressure [ say it is 10 psi ]. When the pedal is released, 10 psi remains in the system. That pressure is retained because the CV is closed, blocking fluid return to the m/c. The fluid cannot drain out of the m/c because the CV is closed. For bleeding , when more than 10 psi is generated [ foot on pedal or pressure bleeder ], the CV will open & allow fluid into the lines.

Maybe systems you have bled did not have CVs & bled ok with gravity.
Motors Manual, which I referred to in an earlier post lists the above method for bleeding brakes. It is a mechanics' guide; it would not recommend a two-man procedure if one man gravity bleeding was sufficient. That manual is designed to save time & provide the most efficient way for a mechanic to do the job.

I have also bled brakes countless times. Not sure of the first time, but it was in the late 1960s.

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Old 06-09-2020, 08:09 AM
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Shiny your welcome.

Here are a few pics of the Residual Valve. This valve consists of only two components, the spring and the rubber piece. Lets call the rubber piece the stopper.

There may be other designs but this is the only type I am familiar with.

The spring and the stopper are fitted into the recess of the master cylinder port and trapped in place by the inverted flare seat.



Fluid flows through the inside of the stopper to activate the wheel cylinders and around the outside of stopper to return to the reservoir.


Note the features of the stopper.


You will see dimples on the large end. They are to prevent the stopper from fully sealing against the surface of the master cylinder port when fluid is flowing back to reservoir.

On the other end you will see a rectangular feature. The rectangular portion fits into the inverted flare seat. In the center of the rectangular area there is a slit. This slit behaves similar to the nipple on a feeding bottle. The slit will allow for some seepage when there is no pressure on either side but it will not allow a flow.

The spring is going to hold the stopper shoulder against the flare seat not allowing fluid back into the reservoir until this spring pressure is overcome.

Flow created by the piston moving in the master cylinder overcomes the static resistance of the slit in the stopper allowing the fluid to move to the wheel cylinders.

When the pedal is released pressure on the line side is now greater causing the the stopper to lift from its seat allowing fluid to flow around the outside of the stopper and back to the reservoir. Once the relief value of the spring has been satisfied the stopper moves back onto the flare seat creating the residual line pressure.

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Old 06-09-2020, 10:09 AM
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While this is an interesting read...

I have always gravity bled followed by the pedal pump method. I have used pressure bleeders and suction bleeders. Gravity bleeding in my opinion does not get all the air out. It gives you a head start. Again, in my opinion, you need to follow up with pressure to get a good bleed. I have bled literally 100's of cars(all makes and models) and have dealt with some stubborn ones that didn't want to bleed as well as ones that have in no time.

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Old 06-09-2020, 10:45 AM
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Thanks again dmorg1

I wrongly assumed it was a simple spring-actuated check valve until I saw your diagram and others online. I couldn't figure out why the spring was on the "wrong" side of the "stopper". If it was a simple check valve, the seat would be on the reservoir side and the spring would be on the line side. Not so.....:



This drove me nuts and I clicked for an hour until I found an explanation for how the valve works. This is definitely NOT a simple check valve. Here is the best explanation of "how it works" that I found. This is very consistent with dmorg1's great explanation:



Unfortunately, the diagram is still missing the state when the valve is holding residual pressure. The bottom diagram shows line pressure exceeding the spring force and allowing fluid flow back into the master. Once the pressure reaches the "set point" or desired residual pressure, the spring will push the valve back into the tubing seat and the flaps in the rubber valve will close. So this diagram is missing an illustration like the upper diagram, only with the flapper CLOSED. This would be the state with residual pressure in the line.

The other question remains what happens when there is no residual pressure in the line. The valve will be seated against the tubing seat by the spring but not clear if the flapper valve will leak enough to allow gravity bleeding. I suspect it is possible and accept Sirrotica's experience that it does. As he said, cutting the line and looking for flow would be a compelling confirmation but I don't have a master with a check valve knocking around.
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  #30  
Old 06-09-2020, 04:56 PM
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Quote:
Originally Posted by PontiacJim1959 View Post
Yep, the pump works good. They sell them in the Adam & Eve stores and they too will give you an extra inch.
Lol, this comment seems to have slipped under the radar amid the more spirited discussion.

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  #31  
Old 06-10-2020, 06:27 AM
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The thing is that when I entered the automotive repair field (1970 upon graduating from Vo tech and high school) there were tons of these 4 wheel drum brake cars on the roads at the time. One of the things my Vo tech instructor showed us was how to gravity bleed brakes.........

I worked on them daily, plus owned my fair share of them. It was a novelty to see a car with front disc brakes in 1970, that's how common the 4 wheel drum brake cars were at the time. GM is far from the only manufacturer to have used residual valves in their drum brake cars, I know ford used them too, maybe some others I can't recall specifically from 50 years ago.

As I said, I've never had any brake bleeding assist tools, most times I worked by myself. The only way to bleed these systems was to gravity bleed them, which I did frequently in the course of my job. I had zero problems with any of these cars, despite what the opposition wants you to believe. My father, which started working on automobiles as a mechanic in 1937, showed me how to gravity bleed brakes, in that time frame there were no disc brake cars, and back when he started "Juice brakes" were just starting to show up over the mechanically activated brakes.

BTW, Having owned my own automotive repair business as well as working for dealerships and independent garages, I'm very, very, familiar with MOTORS manuals as well as Chiltons manuals, I had stacks of them from 1966-1985, labor guides, as well as the technical guides. I have found numerous errors in their information over the years. If you want to take their words as gospel, that's your choice. I've personally found Chilton and MOTORS not be a 100% error free publications. Most of the time they can be trusted, but not always. Many times I've also found their step by step instructions aren't always the best way to repair problems, and leave something to be desired.

Just because information is printed in a book, it's still written by a human being, I've never found a 100% error free human being, and then there are typographical errors that aren't caught.

For anyone that doesn't know, the residual valves have a two fold function. Keep a slight pressure on the system so air doesn't enter around the wheel cylinder cups by keeping them expanded, as well as shorten the stroke of the brake pedal by keeping a slight pressure in the system so there isn't as much travel before the brakes contact the drum. Typically 2-10 lbs, depending on the manufacturer, after the system is closed and pressurized, zero until then. The earliest vehicle I've heard of having residual valves in was a 1947 chevy, master cylinder under the floor so they used them because of the uphill fluid travel during pressurization of the system.

Really, in my course of repairing everything mechanical, I've run into quite a few people that wanted to tell me how a certain system works, and how to repair it. My reply has been, if you know how to do it, why did you bring it to me? Save yourself some money, and do it yourself. I've then heard the excuses of why they weren't taking on the problem themselves. It's so simple, why isn't everyone doing it?............

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  #32  
Old 06-10-2020, 07:46 PM
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Sirrotica, you are just banging your head against a wall.

A wise man once said, you can fix ignorance, but you can't fix stupid.

How did we EVER fix cars without the internet and manuals to tell us how to do our repairs and that the way we did it was the WRONG way - and when it worked and nobody got killed.

This is one of the biggest reasons why today's generation has to throw out points, and go electronics, and throw out the Pontiac engine and go LS. If it requires a "manual" touch, they are lost - they gotta be able to plug it into a computer to tell them what you and I can hear, sense, or feel by the seat of our pants and then repair it using basic tools. And then they have the nerve to still call it a GTO when under the skin there is nothing that left the factory still in place. Sad, but their loss and they don't even know it.

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Old 06-11-2020, 05:12 AM
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If people want to understand why you cannot gravity bleed a system with a WORKING check valve, the last 4 lines of D. Morgan's #27 post explains it well. No flow until the piston moves, which means somebody pushing on the pedal. Nothing stupid about that.

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Old 06-11-2020, 10:48 AM
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I personally don't think it's clear.

Comes down to whether the pressure differential by gravity is sufficient to allow any leakage at all through the rubber "nipple".

By design, a "reverse" pressure of only a few psi will force the nipple to seal tight. Otherwise the valve wouldn't hold residual pressure.

Also by design, the pressure developed by pushing the brake pedal is clearly enough to allow flow through the nipple. Again, this is obvious by design and the fact the brakes work.

But with only the pressure of gravity and no residual "back pressure" acting to seal the nipple, can any fluid leak through the valve? Sirrotica's experience would say yes. Obviously the valve will restrict flow but how much isn't clear...

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Old 06-11-2020, 12:07 PM
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Shiny thanks for sharing the information you found. Looks like you have a good understanding of the workings of the residual valve now. Maybe this will post will clear things up for you.


I am not quoting any publications/manuals or hearsay. This is my personal experience and I am confidant that anyone who has rebuilt a master cylinder that has included replacement/installation of the residual valve or valves will confirm this.


The pic below is the master cylinder in which the residual valve from post 27 was installed. This master cylinder happens to be for a manual disc brake 68 so only one residual valve is required and placed in the drum port.



The master cylinder then received a bench bleed. Upon completion of the bench bleed the flexible tubing was removed from the hard lines and the hard lines were clocked to the 6 o’clock position.



The line to the forward chamber which feeds the calipers and has no residual valve achieved a flow that would easily produce gravity bleeding success. While I did not time how long it took to deplete most of the forward reservoir, I am going to estimate approximately 2 minutes.


The line to the rear chamber which has the residual valve and feeds the wheel cylinders did not flow. Other than the fluid that remained in the hard line from the bench bleed no measurable amount of fluid amount flowed from the rear chamber. Unfortunately I do not have a pic of this.


I contribute this lack of flow to the designed resistance of the slit in the center of the stopper. The six ounces or so of fluid in the reservoir is simply not enough to overcome this resistance on a new valve.


Possibly if the reservoir held a gallon or so there would be enough volume to overcome the stopper slit resistance and allow a flow of fluid.




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  #36  
Old 06-11-2020, 12:38 PM
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Or if it were in the car, the weight of the column of fluid in the lines will pull fluid from the reservoir as in a siphon. There is an approximate 2 foot drop from the firewall to a hanging differential with the wheel cylinder.

Obviously my years of experience having successfully done what you allude to as impossible, carries no weight. I couldn't even begin to count how many times I have gravity bled drum brakes successfully, most of the cars likely had residual valves in them since not just GM used them. The readers can make up their own mind as to whether it will work or not.

Just to further confirm, about 3 months ago I broke a rear brake line while changing the rear wheel bearing on my wife's 2008 Vibe, which has a drum disc combination. I had to make a new line and install it. Of course I then had to bleed the rear half of the system by gravity. Hard to believe for the doubters, I successfully bled the system the same way I've been doing it for 50 years. Hands on experience again, the process works just as described.

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  #37  
Old 06-11-2020, 02:37 PM
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Dave,

Thanks again. Yes, I now understand the design and appreciate how clever it is. Your bench bleed experience is definitely credible and makes sense.

Sirrotica,

Your experience is also credible and your comment about the column height from the top of fluid in master down to the wheel cylinder is a good one.

In the bench-bleed case, Dave says the head was too low to force fluid through the slit. He attributes the lack of flow to low volume in the master. I think it is more about the low height of the fluid above the valve and the resulting low differential with the down-wind side of the valve being open to air. The pressure differential will be very low.. maybe 0.1 psi.

A couple feet of fluid in the tubing below the check valve (down to the cylinder) will definitely change the situation.

2 feet of brake fluid below the valve will generate about 0.8 psi of additional pressure differential in the whole fluid column. Is this sufficient differential to cause flow through the slit in the valve?

I dunno….

Doesn't sound like much to me but is definitely a higher differential than in a bench-bleed case.

I wish I had a master with the valve and could go see how much fluid column below the valve is needed to cause flow.

If nothing else, this thread is a good example of how "details" can influence what appears to be a simple situation and assumptions (like my "it's a simple check valve") can lead to suspect conclusions.

Or as my wife correctly suggests, I can over-analyze almost anything and still not have a clue...

Good topic to stimulate some learning!

Dave,

Is there a source for those check valves? If I wanted to buy a new one to go play, where would I get it?

  #38  
Old 06-11-2020, 04:40 PM
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Shiny,


I agree that fluid in the lower lines will have some effect, quite possibly enough to generate a flow. That will have to be determined.


When an entire system is replaced there will be no fluid in any of the lines. If the main run of line is replaced there will be very little if any fluid left in the lines.


The residual valves were included in rebuild kits not sure if they can be purchased alone.


Purchased the kit, Raybestos MK1108 from Rock Auto in 2014 it was $21.00


It doesn't look like they are available thru RA any longer.


I will look thru my collection of leftover stuff. If I have a valve I will get it to you.





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  #39  
Old 06-12-2020, 10:59 AM
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Quote:
Originally Posted by dmorg1 View Post
Shiny,


I agree that fluid in the lower lines will have some effect, quite possibly enough to generate a flow. That will have to be determined.


When an entire system is replaced there will be no fluid in any of the lines. If the main run of line is replaced there will be very little if any fluid left in the lines.


The residual valves were included in rebuild kits not sure if they can be purchased alone.


Purchased the kit, Raybestos MK1108 from Rock Auto in 2014 it was $21.00


It doesn't look like they are available thru RA any longer.


I will look thru my collection of leftover stuff. If I have a valve I will get it to you.





Dave



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I guess it's a little known trick, to fill an empty system, you bump the brake pedal at the top of the travel with the bleeders open, to more rapidly fill the system, when it starts leaking out the bleeders you wait for the bubbles to quit, then tighten the the bleeder. This works twofold by letting air out of the master cylinder at the top of the system. Looking at the reservoir it will bubble as you bump the pedal, your now bleeding air from top, and bottom of the fluid column, rapidly filling the system.

You surely can wait for the system to fill by itself, but if you work flat rate, and it's not dripping by the time you get the car lifted, or crawl under it, you can expedite it by just bumping the pedal slightly. 99% of the time, it's already dripping out the bleeders by the time you fill the cylinder, and either lift the car, or lay on the creeper.

As I've said, never had a car that I couldn't get the air out of the system without the aid of pressurizing the system, or putting a vacuum on it. It's really tough to do a two man job with one man, but by relying on gravity you get rid of the need for the second man. Gravity is also free, no need to invest in more tools, or pester someone to help you bleed brakes.

As my father used to say, "Can't, never got a damn thing done", gotta figure out the problem, and solve it.

People that refuse to try something because they read somewhere that it "CAN'T be done, are doomed to follow like sheep.

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  #40  
Old 06-12-2020, 02:28 PM
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Brad,

Bumping the pedal is a good trick. I will give it a try next time I am in that situation. Thanks for sharing.

Dave

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