[PontGuy]

Mike,

Have you had a chance to hold pressure on the system and look for leaks yet? My main attention is now on getting the engine installed, but I am still bleeding and leak checking the brakes as I work along.

I finally have all the fittings on the front system leak-free and I am getting absolutely no air at all when bleeding at the calipers.

The rears are working and the fittings seem to be leak-free, but I still have air in the system. The pedal goes all the way to floor when bleeding the fronts, and the combo valve switch is still engaged. But I am not getting big bubbles when bleeding the rears anymore, instead there are extremely fine bubbles evenly mixed in with the fluid- it almost looks "milky". Seems to me I've seen this before and it indicates leakage at the wheel cylinder? Also I can see a very small amount of fluid collecting at the bottom lip of the backing plates when I hold pressure on the system, but haven't figured out how it is getting there. Without pressure all the fittings are staying dry.

Anyone out there- any advice on where to look given the above?

Walt

[Shiny]

Good news on the caliper seals. Engine install sounds like a great way to forget about the brakes for a while.

I have not been back. Was away yesterday and woke up to 4 inches of snow today.

If you are seeing fluid, and it is not from the fitting, it has to be bad cylinders. Lift the rubber caps. I would think any fluid getting past the pistons would be obvious.

I don't think your switch will center until you get rid of all air in the back and build pressure.

I am curious about the pedal. If the front is good, but you have air in the rear, what does the pedal do?

[PontGuy]

4" of snow already! Sounds like you have a short driving season there. It has just now begun to cool off a bit here in Georgia, have not yet needed to wear a jacket even at night.

I checked this morning and there is fluid at inside bottoms of both backing plates and the fittings seem dry. So it sounds like my brand-new cylinders may be leaking, if so chalk ANOTHER one up for the local auto parts store. Will pull one side apart and chase down the source.

If the rear system is air-free then the pedal should go down when bleeding the fronts until it reaches the limit of the multi-valve, then there should should be some pedal at the end when the rears tighten up against the drums. I think. Isn't that the safety aspect of the dual-system, you are supposed to still have some brakes if one side goes out?

Yeah after all the car work this year, putting the engine in the bay is pretty exciting stuff. Hopefully I will be able to change my PY signature by Sunday evening!

Walt

[Shiny]

Had some time to try again and still have not fixed this PITA.

I loosened and re-tightened every fitting in my rear system, then pulled the rear line from the combo valve. I stuck a wire into the port and there was nothing stopping it, so I conclude there is no residual pressure valve in the aftermarket combo valve.

I bled the rear system again, held pressure on the pedal, and saw no leaks.. I tightened up both rear adjusters a little more until I could tell the shoes were causing drag on the drums.

After bleeding, I had decent pedal again with the engine off (no booster). After pressing about 10 times, though, the pedal started sneaking toward the floor again.

I am still stuck.

Any more ideas?

I guess I can buy new rear cylinders and a residual pressure valve. If that doesn't fix it, I guess I will have to pay somebody to figure it out.

Walt - did you end up replacing your rear cylinders?

[GStage1]

If you think about this for a minute....if you are applying pressure to the brake pedal...there are no leaks.....there is only one problem....the master cylinder plunger set-up is loosing pressure.....it is not maintaining pressure due to the fact that it is internally bypassing itself.....the fluid is leaking past the input plunger. I had this happen several times with rebuilt masters and went to new ones....TRW/Raybestos and never had a problem again. If you have a new master.....you may have a defective new master cyl and need to return it and get another one.

[PontGuy]

I haven't worked on my brakes except to try tightening the connections at the wheel cylinders one more time. Have not had a chance to do any bleeding or to put pressure on the system. I did notice the day after my last post that fluid had dripped down the face of the booster under the master cylinder. I noticed this before but thought it may have been from a little spillage when filling the master cylinder. But now I think it may be leaking past the rear seal and if so then I will need another master.

All my attention has been getting everything ready to start up the new engine. I hope that it goes a bit easier than the brake system job!

If you are sure that everything is tight but the pedal does not stay up then I agree that it may just be time for you to try another master cylinder.

Walt

[Shiny]

Thanks again for hanging with me on this and continuing to make suggestions.

GStage1 - After you suggested a bad master earlier, I called Inline Tube and was told to check for bypassing by plugging the outlets of the master. I fabricated some plugs and plugged both front and rear outlet ports on the master. When I pushed the pedal, it was hard as a rock, so I concluded my master was OK and not bypassing. I guess what you suggest could still happen if there is some internal defect that only happens when the piston actually travels a little ?

And if the primary (rear) piston cup was bypassing, would I see fluid getting past the piston or could it bypass internally without leaking out ? I have never had a master apart, but the diagram I have shows what looks to be an o-ring or similar seal on the end of the piston opposite the cup. Is this seal good enough to keep bypassed fluid from leaking out ?

This is the second master cylinder I have tried and it still happens. I guess I can try a 3rd.. It will be cheaper than having my car towed to a shop and paying somebody, I guess... but that option is starting to be very appealing.

I just have a hard time believing I had two bad masters. Even if the quality is so bad that 1 of a hundred is crap, the odds of getting two bad ones would be 1 in ten thousand. I suppose I could be that unlucky, though...


PontGuy - Yesterday, I wiped up a little fluid off the combo valve bracket I have mounted on the booster. I thought it was just something I spilled since I have been opening that lid so often. I will pull the master off the booster again and check for signs of a leak. It sounds like I should replace the master again anyway.

[PontGuy]

I forgot that you have already plugged the master cylinder and was solid. Sounds really unlikely to me but when you are grasping at straws...

I am going to hold the pedal down hard to put pressure on my system again to see if my master cylinder is really leaking before jumping and buying another. In my case I know that I still have air in the rear system and small leaks at the wheel cylinders.

Walt

[Shiny]

Here is info I found on ECI's web site: http://www.ecihotrodbrakes.com/brake_facts.html

"10 PSI Valves - These valves are used in a drum brake system to prevent air from being ingested into the hydraulic system when you release the brake pedal. Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this. Some disc/drum master cylinders have 10 PSI residual pressure valves installed internally, some don't. If you're not sure, call us and we can tell you how to check. Also, some new style wheel cylinders have cup expanders which negate the need for the residual pressure valve. Either way, if you are not sure whether you have one or not, put one in. They are not cumulative and it won't hurt anything if you have two. Don't worry about brake drag, it takes roughly 75 PSI to overcome the return springs. "

I will call them tomorrow. This might explain my problem and suggests I could fix it by replacing the rear cylinders or by installing a residual pressure valve. My fingers are crossed !!

[Shiny]

Per ECI tech (very nice guy):

- Every GM system prior to 1981 with drum rears has a residual pressure valve somewhere in the system

- After 81, rear cylinders were changed to include spreaders on the cups which eliminated the need for the residual pressure valve

- My drum/disc combo valve should have the residual pressure valve built in

- My symptoms are exactly what he would expect without a residual pressure valve (he laughed when I told him what I have been through trying to fix this)

My conclusion: I either got a combo valve that was defective or I got one for post-81.

This last weekend, I did pull the rear line from the combo valve. Per the tech at MP Brakes (who also told me I was "sucking air" on the return stroke), if my combo valve had a residual pressure valve, it would be in the outlet port and if I pushed a wire in, it would not insert more than a half inch before hiitting the valve. My wire hit nothing for two inches.

I ordered a 10 psi residual pressure valve from ECI ( ) and will install it between master cylinder and combo valve.

[PontGuy]

Mike, I am eager to hear how it works out. I am just one step behind you now.

Which combo valve do you have? I ordered the 1969 GTO/Lemans "factory replacement" distribution valve from Inline Tube. I don't know if there is a residual pressure valve in there or not, but the original that I pulled off the car didn't seem to have one. I blew air through the rear port and it was wide open to the rear brake inlet. I'll try to remember to stick a wire in it tonight.

Walt

[Shiny]

Walt,

You are so far ahead of me...

I ordered the 71-72 combination valve reproduction from Inline Tube.

I am sorry, but I don't know where the residual pressure valve is supposed to be in a 69. If it is for a disc brake car, don't you have separate proportioning and metering valves?

I did not want to go stock for my 70, since the combination valve seemed simpler and cheaper than having to buy two separate parts. Oh, did I step in it, though.

If it was a drum car, the check valves for both front and rear would be in the master cylinder. Let me know if your new valve has something in the outlet port when you do the wire check...

I won't have the valve for a few days, but I'll post what happens.

[PontGuy]

Not expecting to have any problems I ordered what I needed to put my brakes into original configuration, and the new stuff looks exactly like what I pulled off. Yes there is a separate valve at the master cylinder that "slows down" the front brakes. But nothing on the back brakes other than the distribution/combo/whatever-it-does block.

The front brakes seem to be working just fine. And the jury is still out on the rears. I tightened the fittings one more time yesterday and held hard pressure on the pedal for a while- didn't see any leakage so maybe I have this licked. But I need to get the wife back in the drivers seat one evening this week to get remaining air out- up/down/up/down...

Walt

[PontGuy]

Oh, and as far as being ahead of you goes.. here is a photo I took a few days ago. I still have a little more to do than get the brakes working, but showing progress.

Walt

[ELKHORNAOG7]

You only need a residual valve if you have disc/drum you don't need a valve if you have all drum or all disc brakes.---BOB

[PontGuy]

I stuck a wire in the back port of the old valve. Nothing in there, wide open to the inlet port. I can't swear for sure that it is original but it sure looks like it was on the car for almost half a century, and it also looks just like the replacement.

Walt

[Shiny]

Walt - that looks awesome... you can be proud of that !!

Thanks for checking the valve. I assume you are talking about the brass body valve and not the cast iron one mounted at the booster?

I think the brass one is the proportioning valve and the cast iron is the metering or hold-off valve.

The comment about 20 years old means your car was original with disc brakes ?

I have no idea where the residual pressure valve would be in yours, or even if it had one.

Here is a blurb from the Inline Tube site that also mentions the residual pressure function. As you can tell, even this adds to the confusion as they describe what is in a "combination valve", yet call it a "proportioning valve":

http://www.inlinetube.com/install%20...op%20Valve.htm

Proportioning valves provide four functions:

Proportion correct pressure to the front and rear braking systems.
Provide 10psi residual pressure in the brake lines.
Is a metering valve to apply pressure to the rear brakes before the front brakes.
Has a brake warning light to detect a loss in pressure.

Bob - thanks for the reply. My drum/drum car had one check valve in both the front and rear systems, and they were both built into the master cylinder. The first pic I scanned from the service manual shows the two check valves (23 & 24). The second pic describes their function.

I have zero confirmation that a disc/drum system ever had a check valve, but it seems necessary to me for the drum system. The service manual makes it clear the valve is not in a disc/drum master cylinder, but it does not talk about the guts of the metering or proportioning valves. I agree a disc would not need a check valve as there is no chance of ingesting air and no significant retraction of the pistons.

I may never know the "truth" about this stuff, but my rear system acts like it is sucking air back in, so I am going to install an aftermarket residual pressure valve. I have nothing to lose and am desperate for a fix. I truly hope no-one else ever has this much grief with their brakes.

[PontGuy]

Yes my car was original with disc brakes. I got the PHS documents when I bought it just to verify stuff like that since it had a very odd/unusual combination of options. It is the only early A-body (or F-body) that I have owned with factory discs, all the rest have all had drums all the way around.

There was one other thing that I have almost mentioned a few times that I find very strange. The drums look original, or at least very old, and they are perfect. No wear marks, and no indication that they have ever been turned. And the adjuster knock-outs in the backing plates have never been knocked out. Also I don't remember ever replacing the rear shoes in the 11 years that I have owned the car. The shoes that I removed were very oil-soaked and I didn't even think to look to see how worn they were.

All of this makes me wonder whether the rear brake system has ever worked properly. Everything else on the car shows its age, and it certainly wasn't babied before I bought it. It has always stopped OK, but maybe it was just working off the front brakes.

If so, the lack of a residual valve may have been a design weakness that affected some of these cars. Just speculating, but interesting, isn't it. How many 40 year old cars still have steel in the adjuster knock-outs? So I am very interested in how your residual valve works for you.

Walt

[PontGuy]

Have you installed the residual valve?

Should have my engine fired up this weekend, and with vacuum on the booster I should be able to tell if I really have brakes or not...

[Shiny]

Hi Walt,

Way to go on the install. You are going to be a rocket man soon.

Did you replace the rear cylinders and shoes? Sounds to me like you never had rear brakes... ???

Yes, I got the residual pressure valve installed wednesday. I think I had to do it 3 times to get the flares and bends correct. While I was working on it, we got over 2 ft of snow which kept me from bleeding, as I need to open the garage door to get at the back end. It was 50 yesterday and melted a lot, so I should be able to get at it in the next few days.

I had a few more email and telephone exchanges with John at Inline Tube. He does believe the residual pressure function is provided by the combo valve, so if this fixes mine, I would conclude my new valve was defective. He does not have the ability to test for it, though. If the aftermarket residual pressure valve corrects my problem, I will probably exchange the combo valve to make sure the metering and proportioning functions are working correctly.

I ran across another post by screamingchief on this forum who also believes every rear drum system has some form of residual pressure valve somewhere in the system to prevent air ingestion on the backstroke:

http://forums.maxperformanceinc.com/...idual+pressure

I am still optimistic and will let you know. Odds of this being a problem for you are very low...