Trying to figure out whats going on with the power brakes on my 74. I have had the car for a short time, only 31k miles. I bled the brakes when I first bought it and I've never seen brake fluid that nasty. They were working absolutely great until now, they worked great even before I bled them.

I took off the left rear caliper/rotor to help me get a look at the parking brake mechanism. After putting it back together and bleeding the air the brakes were soft and the pedal contacts the floor. It stops, but terribly.

So, I assumed it was the Master Cylinder not liking when I compressed the caliper a bit to put it back on. I figured some crud got lodged, or dislodged or something.... I ordered the Master Cylinder rebuild kit, Once it arrived I removed the M.C. and found it to be in terrible shape. Pitted, corroded inside and with thick black sludge. Sooooo, I ordered a brand new ACDelco Master Cylinder (# 18M72). I bench bled, installed it, then bled all 4 lines in proper order. Things seemed to go perfect, but still crappy squishy brakes just as before.

One of the main things I noticed is that when bleeding the wheel where it all began (left rear), I never stopped getting bubbles. I get straight clear fluid with no air bubbles through the other 3.

My thoughts are that I should replace the caliper... but at the same time that doesn't seem right. Where is the pressure going? There are no leaks. If the caliper is sucking in air and expelling when bleeding what is happening when I try to use the brakes? The problem seems to be coming from this corner between the caliper and 'junction block'

So, just curious if any of you with c3 experience have any ideas. I've heard mention of a bad 'proportioning valve', but there are no leaks and all 4 corners get plenty of fluid. Maybe the Caliper, but again no fluid leaks.

thanks for reading...

 

I would try gravity bleeding. Try the trouble one first. Put hose on bleeder screw and insert into bottle withe bit of fluid and let it bleed itself slowly. Add fluid as needed. Problem bleeders sometimes need this treatment all around. I replaced all the lines, calipers and rotors and this was how I got a good pedal

 

Rarely is it the proportioning valve... even then you just remove it and clean it out real good with brake cleaner and you should be good to go.

I suspect you have air bubbles trapped in the caliper. Have you tried bleeding the brakes and tapping the calipers with a hammer while you do this? If not, try that.

I always use a pressure bleeder when we do this job in the shop and we rarely have any issues. The pressure bleeder will push the air bubbles out of the system every time (and yes while bleeding we still tap the calipers with a hammer). If you try it by tapping the calipers and still don't have any brakes I'd try to find a shop that has a good quality pressure bleeder and let them give it a go.

IMHO,

Willcox

 

1. you have examined all 16 caliper piston dirt seals and see no brake fluid moisture?
2. Even with no moisture you have already stated that the master was a corroded mess, I would assume all the calipers are the same way and that all the seals are letting air in when the rotor is turning and causing slight movements of the pistons due to minor or major run out.
3. When you removed the rotor in the left rear did you mark it so it went on the same way you took it off ?
4. Replace all 4 calipers with stainless sleeved calipers with O ring seals.

 

Rumor has it, that hundreds of C3 owners are residing in insane assylums walking the halls with a bleeder wrench in one hand while sipping a bottle of brake fluid with the other. Don't be that person.

Those crazy C3 calipers as you know are two halves bolted together. You could have internal air seepage but not a drop of fluid on the floor. I have also found that some replacement bleeder screws don't seem as tight as GMs. So a few bubbles slip past the threads when you have your hose on there. Just barely crack open the screw may help with false bubbles so to speak.

I have had the best luck with vacuum bleeding method. Others swear by Motive Pressure method. Gravity works, but avoid the pedal method as you will trip the proportioning valve. Good luck, stay sane.

 

Thanks as always guys!!

henrikse, out in the garage all afternoon with a different project, it will only take me a couple of minutes to set this up and it can't hurt. I'll let you know how it goes.

wilcox, going to tap if gravity doesn't work. If nothing else my pop will be in town next week. His first car was a Vette and he's a lifelong mechanic. Wanted to take him for spins instead of get his hands dirty if possible.

MelWff. Waiting for someone to point out the obvious. If the master cylinder was wrecked, so are the calipers.... right? originals from 74 I assume. btw, what do you mean by mark it? i did not. It sort of sat right there not to far from being in place... Stainless lines are in the mail, just trying not to pop for 4 calipers if possible. Recommendation for replacement calipers if I have to?

HeadsU.P. (from a fellow Michigander!) When you say trip the proportioning valve do you mean trigger the 'brake' light on the dash?

 

Yes, the light. And it can be tough to get the little piston in the prop valve to reposition itself, dead center. One way is to jab the pedal fast and hard to reset it. There is also a tool ($15) that holds it centered for people that bleed using the pedal method. I have a picture of it installed in my profile photo album.

The prop valve is the GFCI in the brake system so to speak. It detects a fault and reroutes the fluid pressure a different path and triggers the warning light. And it has to be reset after the problem issue is resolved.

 

don't the back have 2 bleeder valves???

 

Yes. Inboard, outboard.

 

i agree, after pumping or sucking a lot of clean fluid thru
the lines, a least a large bottle worth,
let gravity be your friend
good luck

 

From the factory the rear rotors are riveted to the hub. Did you have to drill out the rivets to remove the rotor? If not, someone else has already been there.

 

You know the problem is the left rear caliper. It started when you messed with it and continues to push air. The other suggestions of basically replacing the entire braking system seem a little overboard to me.

You have the mechanical skill to rebuild your master cylinder so you clearly have the skill to rebuild a caliper. The kit is $10. Rebuild it and be done with it. Splurge on a new bleeder for $1

If you open it up and find the pistons are shot, replace them or the entire caliper. Either way, this will solve your issue.

 

bought my 75 and really had a scare trying to stop the car. all new rotors, calipers, master cylinder and vacuum booster. I feel better, the car stops on a dime. No leaks, no Insurance claims. the car had 75k original miles, with the last 20k put on it over 20 years. It was time.

 

I have to say, of all the funny posts I've ever read, this one is in the top 5%!

Sorry to revive this thread, but I'm having some brake issues (I'll post somewhere more appropriate or start a new thread) and did a 'brake issue' search on the forum. Came across this thread and just had to comment!