[kyle_blake]

oh hey,

I'll spare you a re-read of my old brake post and just ask the new question outright.

Just wondering if anyone knows if the pv68 valve is suppose to move back and forth when i press the brake pedal? my assumption is no.

Probably only suppose to move under a massive hard braking event? Does anyone know?

The master cylinder shouldn't be pressing more pressure than normal. it's he right model.

it's leaking fluid out of the back of the valve every time i hit brake pedal mildly. thank god i didn't drive it out of driveway.

I'll call right stuff tomorrow and ask.
Here is a picture and a video.

https://youtu.be/6hr39LvrKJo

really appreciate any help. I phoned the muscle car shop that sold me this and all they could say is they gravity bleed those and never an issue. I couldn't even send fluid through it without pressing the button . so i dunno all things lead to this being defective. of course. feeling bummed.

[chuckies76ta]

I used this unit on my wife's 68 Firebird and the brakes worked very well. That valve is called a Proportioning Valve. There should be no leakage from any component on a breaking system. Sorry I'm not much help to you. Here is a brief description of the valve.

https://piratejack.net/template/img/...vqfsFdAC2yRiDz

This is a post from someone that had issues with a leaking unit.

Direct fit for a 1969 Firebird 400. No issues, installed as OE, appears OE, works like OE. The old one leaked out the front "button" under the rubber cap. Do not fear, although the service manual says to "press" the button when bleeding, this is a misprint in the service manual. Install it and have someone press the brake pedal hard and the button will move forward. Impossible to press it in as at rest its already in. This replacement works perfect and often repairs the issue of air getting into the brakes from the old button seal. Pull the rubber cap, see if its wet under there, if so replace. See in button moves forward when you press the brakes hard, if not, also replace it. This same part was cheaper than other Pontiac parts suppliers for the same part.

[kyle_blake]

Appreciate the reply a lot ! Just been never ending and so frustrating.

Vendor said they gravity bleed and it just works.
As we know I couldn't even get fluid through it with two man bleed.
So pressed button as I was told to do for 2 man bleed.
Did button press for whole bleed process, no bubbles then take button off.

Mine leaks out the back of the valve and as the video shows my button moves out, not in like the "other post" with the 69 firebird suggests.

Vendor is like we've never had one go bad.

So I dunno , it's been 4 years of figuring it out when i wish i just started with the valve. I can't think of anything else I've done incorrectly. It's same distribution block down below.

I'm sure we'll get through it, really appreciate PY for everyone's help.

[chiphead]

Do you want it to look correct or just work? I'd replace that older setup with the 71-newer combination valve. That's what I did on my '67 LeMans with disc brake conversion. I removed the brass distribution block on the frame and replaced the lines to use the combo valve under the master cylinder. There is one for disc/disc and a different one for disc/drum setups.

https://www.summitracing.com/parts/r...iABEgKJafD_BwE

https://mpbrakes.com/how-to-series-w...g4cl-4oqYxGlov

https://forums.maxperformanceinc.com...d.php?t=821965

[kyle_blake]

Thanks, I was hoping to convert to that system when i change out my exhaust to ram air manifolds and have access "space" to the distribution block on the frame. Right now, I'd have to drop the headers and all that jazz in the garage and I just thought I could buy the hold off valve and use my car. Everything else on the car brake wise is brand new and replaced. every hose caliper , piston pad, shoe , springs , fluid. It didn't leak anything last time I drove without but the front brakes held and the car would not roll forward on a incline. So the valve must be required. the rod in the booster is same size as the day i got it. booster works fine.

I agree I think the new valve is a better idea and I have all the stuff to hook it up waiting but I'd like to use my car. I've been having some shoulder problems so I can't do the header job myself.

I read the last link where shurkey mentioned that that you need more than 10 psi to bleed it with a power bleeder.

How the vendor says he just gravity bleeds them and it works , I don't know. Maybe he was using the "inline tube" and not pv 68.

I hope the next valve works . Feel like hooking a line up when I pick it up and putting dot 3 in it and seeing if it gravity bleeds through before even taking it home.

I mean it had to be defective , i'm sure pressing the pedal car not running and the valve not letting anything through is more than 10 psi. should of just returned it then.

have a good long weekend!

[Schurkey]

Originally Posted by chuckies76ta

I used this unit on my wife's 68 Firebird and the brakes worked very well. That valve is called a Proportioning Valve.

NO. The guy who wrote the sheet you've linked-to is WRONG. It is NOT a "proportioning valve", it's a Metering valve, or Holdoff valve.

Originally Posted by chuckies76ta

There should be no leakage from any component on a breaking system.

Correct. That leaking valve assembly is defective. GM says they're non-repairable, but I figure (almost) anything that can be made, can be taken apart and re-made. (If you can get repair parts.)

Originally Posted by chuckies76ta

Here is a brief description of the valve.

https://piratejack.net/template/img/...vqfsFdAC2yRiDz

The description is generally correct except for the bold print claiming it's a "PROPORTIONING VALVE".

Originally Posted by chuckies76ta

Do not fear, although the service manual says to "press" the button when bleeding, this is a misprint in the service manual.

NO, that is NOT a misprint. The button is to be pressed--holding it in place--when the FRONT brakes are being bled using moderate pressure as might be encountered with a pressure bleeder. You would not need to dick with the metering valve when bleeding with zero pressure as in gravity bleeding, or with high pressure as with "pump-the-brake-pedal" bleeding.

This part functions in a three-phase system. With zero or very slight pressure, the valve does not block fluid flow. The brake system beyond the valve is allowed to vent into the master cylinder reservoir based on temperature change. The brake fluid expands when hot, it has to vent to the reservoir to avoid applying the brakes especially disc brakes.

As you apply the brake pedal, the rear drums need enough pressure to overcome the tension of the shoe return springs. This valve blocks low pressure to the front brakes, so that the rears can develop that minimum pressure to move the shoes.

Finally, as the pressure continues to increase, the valve is 'blown open" allowing full pressure to the front brakes so they apply more-or-less simultaneously with the rears.

As a result, trying to bleed brakes using that somewhat-low pressure that is enough to cause the valve to block flow to the front brakes, you'll get no fluid flow--so you'd have to force the valve back to it's "vent" position; or pull it out to it's "blown open" position to get flow.

THAT'S THE THEORY. In actual use, I have NEVER needed to dick with the Metering/holdoff valve to bleed front brakes. I do not own the tool to push or pull the metering valve stem. I pressure bleed at no more than 15 psi, and more-often at about 10 psi. Apparently, that's not enough pressure to cause the Metering valve to block flow.

Originally Posted by kyle_blake

I read the last link where shurkey mentioned that that you need more than 10 psi to bleed it with a power bleeder.

Nope. I bleed at around 10-15 psi when I use a pressure bleeder. HIGHER pressure MIGHT require playing games with that metering valve stem. Most of the time, I gravity-bleed.

Originally Posted by kyle_blake

I hope the next valve works . Feel like hooking a line up when I pick it up and putting dot 3 in it and seeing if it gravity bleeds through before even taking it home.

I mean it had to be defective , i'm sure pressing the pedal car not running and the valve not letting anything through is more than 10 psi. should of just returned it then.

Yup, that valve seems to be doubly-defective.
1. It leaks, and
2. It blocks fluid flow all the time.

[chuckies76ta]

NO. The guy who wrote the sheet you've linked-to is WRONG. It is NOT a "proportioning valve", it's a Metering valve, or Holdoff valve. While I do agree with you, The 1968 GM Service Manual refers to it on the brake system as a Proportioning Valve. GM manual goes on to call it a Metering Valve. If you read the instruction sheet, the first line refers to the valve. The PV68 is a hold off valve.
If you look up the definition of a Proportioning Valve. https://en.wikipedia.org/wiki/Proportioning_valve.

Do not fear, although the service manual says to "press" the button when bleeding, this is a misprint in the service manual. These are not my words. Just info that someone had posted on the brake setup. Like I said in my original post, Sorry I'm not much help to you.

[grivera]

Maybe the metering valve has a broken seal/o-ring or a faulty bore? If the vendor doesn't replace the valve, you may want to take it apart and check the seals. I bought a kit from Musclecar Research LLC and rebuilt the one on my 69 Lemans:

https://www.musclecarresearch.com/gm-3905525-autopsy

https://www.musclecarresearch.com/gm...al-kit-1?v=132

[kyle_blake]

Hi

Vendor is ordering a new one in 2 weeks. My old one from the 90's was way too pitted inside the bore to rebuild but that is cool to know they are available. I have access to a power bleeder from my friends and will try the 10 psi once everything is hooked up. I'll pull the rubber cap to observe what happens out of pure curiosity. Thanks again for clarification.

[gtobird]

Originally Posted by kyle_blake

Hi

Vendor is ordering a new one in 2 weeks. My old one from the 90's was way too pitted inside the bore to rebuild but that is cool to know they are available. I have access to a power bleeder from my friends and will try the 10 psi once everything is hooked up. I'll pull the rubber cap to observe what happens out of pure curiosity. Thanks again for clarification.

Kyle,

I did a lot of research on this topic as I had a leaking metering (hold off) valve on my car. I bought a repop replacement and it leaked too. I also had an older metering valve. I took them all apart and was surprised at the design. First off, all of the ones I have incorporate the same basic design. The "button" in the back is really a rod that is spring loaded. The spring pushes the rod to the front of the valve which pushes a piston like part to the front to block the front port. Pushing on this DOES NOTHING to help bleed the brakes. It's true that if the piston was stuck in the open position, pushing on the rod "might" cause it to go back to the front of the valve, but that would only close off the front port, preventing the brakes from being bled.

Again, I did lots of reading from various vendors, etc. One of them claimed that the original GM design had an entirely different type of valve setup inside and pushing the button did indeed open up the valve so you could bleed the brakes. "If" that is true, I understand the confusion. I'm not sure if it really is true or this is just someone repeating what was in an old repair manual that may have been incorrect.

Also, the valve does nothing to the rear braking system so you should be able to gravity bleed, vacuum bleed or pressure bleed the rear brakes through the pedal or using a pressure bleeder no matter which design you have.

I don't see how you could gravity bleed the front brakes with a properly working valve of the same design that I purchased and others on this forum will claim you can do it. In the normal resting state, the port in front of the metering valve is completely blocked until enough pressure is reached from the master cylinder. Again, that's for the design I have and yes it says PV68 on the valve. I can't rule out that there are older or even newer designs that may allow you to gravity feed the front brakes, but NO WAY can you do it with the ones I have.

I also found that the 67 metering valves were different from the 69 as they were a bit smaller on the outside. 1968 appeared to use both the 67 and 69 styles. I can only guess that GM switched when they ran out of the old design. Again, a guess on my part. Depending on which vendor you buy from, the vendor may claim the 67 design is correct for a 68 and others will say a 69 is correct.

All three of units I have were the 69 design. I recently purchased the 67 version of it from a vendor that claims it's correct for a 68. Again, I can't verify, but if it works, I'm good with it. I have not tried it yet.

Last, but not least, I checked the size of parts inside the units and found all of the units had different tolerances between the piston and wall as well as other dimensional variations. It really makes me wonder if these are cheaply made poor copies of the originals and that's why they leak. I'm using DOT 3 Lucas synthetic brake fluid on a complete flush.

Anyway, please let us know if you solved this one and which unit you used.

Thanks
Old Joe

[kyle_blake]

Hey man. I swore I wrote a reply to this but I must not of hit submit damn it.

- Agreed 100% the stock 69 hold off valve is not meant to button press to bleed. I have 2 of the old valve (mine and my friends) - the only difference is the rubber seal near button ending. My buddy's 69 gto manual car did not have the rubber seal, it was flush.

- Yes rear brakes gravity bled fine, pressure bled fine - separate circuit down to block that splits it up - agreed


I was incorrectly advised by the manufacture and with both valve's I got, well I pressed he button so much on both that I crushed the washer. Until email support from the manufacture said, whoever you spoke to on phone was wrong. So I couldn't return it to my local vendor twice and look like an idiot and blame manufacture .

So I emailed same guy back from manufacture back and said can I use old wavy washer in it's place. He said yes.

I put it back together. I had to use the pedal to advance the fluid (car not running) through the valve with front drivers bleeder valve open. .... Then if you can believe it. Gravity bleed occurred on both front's.

I too had the pv68. I can't really speak to 67,68 myself. interesting though slight variations.

If I ever do a full resto and start with empty lines , I'd repeat my procedure above and verify.

As for driving it, haven't had a chance yet but backing it out of garage and short back and forth in complex seems ok. No fluid is gushing out of back every time i press the pedal. so that is good.

crushed wavy washer picture shown below.

[gtobird]

Originally Posted by kyle_blake

Hey man. I swore I wrote a reply to this but I must not of hit submit damn it.

- Agreed 100% the stock 69 hold off valve is not meant to button press to bleed. I have 2 of the old valve (mine and my friends) - the only difference is the rubber seal near button ending. My buddy's 69 gto manual car did not have the rubber seal, it was flush.

- Yes rear brakes gravity bled fine, pressure bled fine - separate circuit down to block that splits it up - agreed


I was incorrectly advised by the manufacture and with both valve's I got, well I pressed he button so much on both that I crushed the washer. Until email support from the manufacture said, whoever you spoke to on phone was wrong. So I couldn't return it to my local vendor twice and look like an idiot and blame manufacture .

So I emailed same guy back from manufacture back and said can I use old wavy washer in it's place. He said yes.

I put it back together. I had to use the pedal to advance the fluid (car not running) through the valve with front drivers bleeder valve open. .... Then if you can believe it. Gravity bleed occurred on both front's.

I too had the pv68. I can't really speak to 67,68 myself. interesting though slight variations.

If I ever do a full resto and start with empty lines , I'd repeat my procedure above and verify.

As for driving it, haven't had a chance yet but backing it out of garage and short back and forth in complex seems ok. No fluid is gushing out of back every time i press the pedal. so that is good.

crushed wavy washer picture shown below.

Now that is really interesting on gravity bleeding the front brakes after you first pumped them. I've had to do a slight bump on the pedal to start fluid flow then gravity bleeding to the back brakes, but not a regular push like you would use to brake pedal bleed. I'd really like to understand how your method allowed the front brakes to gravity bleed. It sounds like the piston inside the valve somehow locked open from the initial pedal push so you could then gravity bleed. It almost sounds like a spring was left out, but I'll take your word for it that both springs were put back in.

For anyone else that's interested in what's inside of these, here is info from Muscle Car Research with their instructions on how to rebuild them and pictures.

https://musclecarresearch.com/gm-3905525-rebuild

[gtobird]

One last thought on this.... I'm looking at the picture you just posted. When you get some time, can you post a picture that shows the entire body of the piston? I want to see if it looks like mine. Also, I noticed the black stuff on the rod. Is that residue from the rear cup seal that sits inside the valve? I've been wondering if the synthetic DOT 3 brake fluid has any negative side effects on these seals. It's supposed to be entirely friendly to brake systems in normal older brake systems.

[kyle_blake]

Yah, I'm 100% positive all the springs went back in. The black mark, I'm thinking from the button pressing must of angled into something? Not sure.

The actual parts looked pretty good in the pv68 compared to old one.

I got the bleed to work quite awhile ago. I may of put a rod against brake pedal / drivers seat to keep it open then cracked the bleeders? I forgot?

But I have the two videos of both sides gravity leaking so it somehow does?

As the vendor said " Kyle we've never had one returned before and we've always just gravity bleed them".

That's the only other picture I have attached. you can see the chip from the chisel mark on the end of the rod where we all call it a button . lol

[gtobird]

Originally Posted by kyle_blake

Yah, I'm 100% positive all the springs went back in. The black mark, I'm thinking from the button pressing must of angled into something? Not sure.

The actual parts looked pretty good in the pv68 compared to old one.

I got the bleed to work quite awhile ago. I may of put a rod against brake pedal / drivers seat to keep it open then cracked the bleeders? I forgot?

But I have the two videos of both sides gravity leaking so it somehow does?

As the vendor said " Kyle we've never had one returned before and we've always just gravity bleed them".

That's the only other picture I have attached. you can see the chip from the chisel mark on the end of the rod where we all call it a button . lol

No need to reply any further as you probably have better things to do. Thanks for posting the picture. Your piston does look slightly different than mine as it seems to have different spacing for the cutouts that let fluid flow (as best as I can tell). Also, the piston has 6 weep holes on the bottom of it that the black rubber washer (seal) sits against. Those holes look bigger on your piston. I find that all to be interesting. When I measured up 2 of the 3 valves I had, in the "at rest" position, the rubber washer was sandwiched between the piston and the front plug from the spring force, which in theory gave it a tight seal. On the other one, the top of the piston made contact with the lip on the front plug allowing the rubber washer (seal) to sit rather loose in the resting state. I'm wondering if that's how your valve was built. If so, the weep holes may be allowing a small amount of fluid to pass for gravity bleeding, or perhaps the piston allows a small amount to pass.

Anyway, I'm convinced that there are subtle differences in these valves, whether by design or error in copying. I can see how some might allow for gravity bleeding and others not so much. I appreciate the dialog. So many mysteries, so little time...........

Take care

[kyle_blake]

Hi again,

Hey no problem at all, the forum has helped listen to me on many subjects come back and forth. Always here to help out. Here is the 1969 gto judge manual trans car valve ( my buddies opened up, mine us automatic) not that any of that matters really.

I didn't snap any other pics of the new one but here are the old one taken apart just now. This one is complete but clogged holes as you can see.

If I could like Shurkey said, try power bleed 15 psi it should push out the front past the valve. If I can ever borrow it again, I'd try that.