Allan R

Is the old MC leaking? If it is there are rebuild kits you can buy. Chances are you need a new proportioning valve. From what I see these are the same unit, just different prices.
Parts Place
Inline Tube

72455

Allan, the old MC had a small trickle where it bolts to the booster. I installed a new MC and the leak is still there...same trickle, same place. I didn't change anything else or do anything other than the swap and bleed the brakes.. I didn't bench bleed the MC before I put it on, but I did take it back off and bench bled it. My stepson actually did most of the work, and while I don't doubt his skills, is it possible the bench bleed doesn't get all the air out?

Again, I just don't understand how it could be the prop valve when everything was fine (except the leak) before I installed the new MC.

Another theory my buddy has is that it could possibly be the throw rod on the booster. He thinks that even though it's a new MC (not reman, but new) that the rod is too short and not giving me enough travel to engage the rear brakes. Is that possible?

Oh, and I already have a new prop valve, but as I stated above, we can't break the fittings loose on the old one. If I install that, it's gonna be with all new lines, front to back. I'm just looking to get back where I was for now.

Thanks,
Dave

Vintage Chief

It's NOT a proportional valve - it is a COMBINATION VALVE. Argh.
The COMBINATION VALVE (as Joe indicated in Post #29) serves several 'functions' - only one of which is to proportion fluid. COMBINATION VALVES are designed for DISC/DRUM, DISC/DISC systems.
The COMBINATION VALVE is composed of a metering valve, a pressure differential valve & a proportioning valve. The proportioning valve (inside the COMBINATION VALVE) does absolutely nothing more than prevent rear disc lockup during an emergency slam the brakes to the floor and say a Hail Mary event.

I 'suspect' (as Allan suggests) you have a bad COMBINATION VALVE; albeit, you CANNOT bleed your rear brakes because, IMO, the pressure differential valve is NOT in the centered position (it's shuttled) during the brake bleeding process and air can freely flow through the system. I might also suggest you might have a bad terminal switch (which sits on top the pressure differential valve) because in theory you should see a brake light lamp illuminating condition when the pressure differential valve is NOT in the centered position. The pressure differential valve may be welded (rusted) in place and may not even be able to move. Are you certain you had functioning rear brakes before replacing the MC? I believe the ratio is nearly 80:20 (80% front braking:20% rear braking) for a DISC/DRUM vehicle. IOW, 80% of your vehicle's weight is stopped by the front brakes.

It's possible the fluid could trickle out the rear when bleeding (not always totally shut off) if the pressure differential valve is shuttled.

I worked on a system very similar to a COMBINATION VALVE while serving in the USAF. The cockpit of an aircraft contains a California Valve (sic), aptly named because it blows, sucks & relieves all at the exact same time to maintain pressure at altitude.

Be sure you center the pressure differential valve inside the COMBINATION VALVE when you R&R the COMBINATION VALVE or you'll never be able to bleed the brakes.

72455

Hey Norm, so lemme rephrase that comment about my rear brakes working before I replaced the MC.
No, I'm not 100% positive that I had rear brakes before, but I DO know that I had more pedal and it took less effort to whoa it down. Basically, I have about 1/2 pedal before I feel the grab, whereas before, it was about 1/4 and it started whoaing down sooner and felt more secure.

Vintage Chief

Hi Dave. Understand and point taken, that’s why I said I ‘suspect’ the combination valve may be suspect. I think a brake system delivers what - 1200psi to 1800 psi in the brake lines? Rust, oxidized metal, contaminants are most easily introduced into normally closed systems whenever we open them up and parts can sometimes move into positions in which they normally don’t belong. JMO

72455

I get that, but if it's the combo valve, then why did I not have this issue when I filled the rear chamber and bled the brakes the first time (last summer)...that's what I don't get. And the difference was like night and day...way better pedal.

I should also add that when the car is shut off, I have a great pedal..barely have to press before I feel the pressure.

Vintage Chief

Well, the brake booster is working. LOL Sorry, couldn’t help myself since that’s exactly why you have a brake booster - to make the brakes easier to push down on when the car is ‘on’ because the booster has a vacuum being pulled on it when the car is on.

99% of the time (IMO) brake pedals go the floor is because of air entering and remaining inside a hydraulic system. Air is easily compressed, hydraulic (brake) fluid is not.

Vintage Chief

Let’s face it, at one point didn’t you say there was NO brake fluid in the rear reservoir last year until you filled it? If in fact, you really had NO brake fluid in the rear reservoir and you were operating the car for a period of time with NO fluid in the rear reservoir and then you filled the rear reservoir w/ fluid. You would have HAD to have had an enormous amount of air in that system. When you say you bled the brakes last year, I hope you got SHOOTING streams of brake fluid from the bleeder valves. Because if not, you still have mucho air in your brakes. Add to it you just replaced the MC and didn’t bench bleed, then reinstalled after bench bleed, then did a pneumatic check? I’d get a vacuum bleeder system to do a proper brake bleed at this point. 28 times of opening and closing bleeder valves in the correct sequence may still not have removed all your air. JS

72455

Good morning Norm. In post #40 you will see that my buddy did a pneumatic bleed on the whole system yesterday. He initially tried a vacuum bleeder but wasn't getting any fluid from the right rear, so that's why he did the pneumatic bleed. Once he did that, he was able to pull fluid. IRT your comment about mucho air from last summer, I won't debate that, but again, as I have said, once the fluid was filled at that point and the system was bled, I had a good pedal, so I rolled with it. No squishiness, no sponginess, just a good, solid pedal with less stopping effort than before.

Vintage Chief

Yep, you're right. I even knew you did it when I made my last post. It was late, I was on my phone thinking out loud - that's what I get for thinking out loud.

Vintage Chief

I don't know. I've done a MC R&R two times in my life - I'm no expert. The MC's I've replaced were identical to the MC which was on the vehicle to begin with so they were replaced with identical units. I've watched and assisted a couple friends do MC replacements on 'other' cars and they've needed to make an adjustment to the booster push rod so it mated correctly to the MC piston. If the booster push rod has a 'gap' between the booster push rod and the MC piston (that point where you attach the MC to the booster), it won't push the MC piston in right away & you'll have to push further on the pedal to engage the MC piston. If you replaced the MC with a different brand/manufacturer, there's a chance the positioning of the booster push rod to the MC piston is incorrect and needs adjustment so they mate correctly?

oddball

If you have another leak at the back of the master, then your reman master is bad.

That isn't surprising.

There's no such thing as guaranteed quality on anything remanufactured now. I don't trust one brand over another either. I've had ACDelco labeled stuff be completely wrong out of the box and no-name chinese stuff work perfectly.

72455

That's the direction we're heading now, so I'll keep everyone updated.

72455

Thanks for the reply, but it's not a reman, it's a new unit.

oldcutlass

Either way, whether new or reman, its bad if its leaking. Remove and take it back for an exchange.

Allan R

Agreed. My new Master Cylinder that I installed should have been perfect, but wasn't. It leaked after 1 winter. I'm seriously looking at the one that Parts Place has, as it appears to be correct with the 2 bleeder valves too. $69 seems pretty reasonable.

Dave
re: replacing the brake lines. Yes that's certainly an option. But first have a look at the overall condition of the lines you have. If they look relatively good I'd suggest just leaving them. At the proportioning valve connections be sure to use a flare nut to avoid rounding off the nuts. If they are stubborn, use a propane or mapp gas to heat them up. Heat does wonders to release frozen metal parts. I'd try that first before replacing anything.
BTW, I used ILT for the replacement brake lines on my 72 and the fit was really quite good. Everything comes folded in half for shipping but follow the instructions to straighten them and you're golden. The only connection that you might have an issue with is the one to the center brake line that goes up to the rear bulkhead.

72455

If I replace the lines, I am going with ILT. For now, the work is on hold till Wednesday. Going to install another new MC and see what happens.

Fun71

I agree with the others - new does not always mean good. You'd think a new part would work as it's supposed to, but that's not always the case.

72455

Upon further inspection, my buddy discovered that the rod in the booster was corroded, thus not allowing it to move freely in the rubber diaphragm.
Along with that, we think that when the new MC was installed but not bench bled before, I blew the seal because of trapped air. So I picked up another MC and this time, it was bench bled before installation.

I am happy to report that she stops like she's supposed to and no leaks 😀

Thanks to all for their advice and help...now onto the next item on my list

Dave

oldcutlass

Glad to hear you got it fixed.

Allan R

You mean you're planning to change the interior to black??? Or will it be front end cosmetics? Or bumper changes? or restore hood to Original? or replace the missing stainless? I should start a thread with a poll...

Seriously - you've done a lot on this car and it's obvious you show pride of ownership.

72455

Look, Allan has jokes😀 lol. Actually I'm planning on installing those torque braces and replacing the incorrect fender bolts on the pass side and the dome light lens...I noticed the other day that mine is so deteriorated that it's actually crumbling apart...no bueno 😕

72455

Thanks Allan...I'm just enjoying fixing it up and making it "mine", and learning a thing or two in the process 😀