(See my previous post for my brake woes.)
After obtaining a decent pedal with the engine off only to loose it all when starting and running the motor, I took many of your's advice and installed speed bleeders all the way around today. I just knew this would solve my problems. I pumped and pumped, refilling the m/c when needed, and the pedal firmness went completely away. I knocked on the calipers, m/c, and p-valve with a wooden mallet (as suggested), only to be further disillusioned. Plus now my p-valve keeps shifting, illuminating the brake light on the dash. (No problem recentering it, though.) Although I find it hard to rationalize, I think that I have to agree with some of the thoughts posted in the past, of lip type seals in the calipers developing a flat spot that won't leak fluid, but will suck air when retracting into position. I do have new calipers on the front, and feel very confident that they are not at issue, as I have always (within reason) got clear fluid from them when bleeding. However, I still have the EOM rear calipers still on, and have just found out that the car was parked for a couple of years prior to me purchasing it last May. It seems every time I bleed the rears, I do get a trace of air bubbles. (HInt, hint!!) So.....I'll be replacing the rear calipers as soon as they come in and time will permit. I did get a firm pedal back when hooking up the pressure bleeder to where it was when I started this portion today, and not having the brake light come on at all. If this doesn't cure the brake ills, I'll have to up my insurance coverage and "arrange" for the car to be disposed of without my knowledge. JK!!!!!!
Thanks, guys, for all the great feedback and tips. This has been a very perplexing problem that many of you have already experienced. Time will tell!!!!!

 

When I went though this, as a lot of people have, it was suggested to me to use padded vise grips to crimp off some of the rubber brake lines in order to isolate the problem to front/rear. Just something to think about......

 

Just FYI, it's normal for the pedal on a PB car to be stiff with the engine off, then drop when the engine is started. Now I read your other post where you said that the pedal went to the floor, but if there's air in the system I don't think you could get a pedal with the engine off (hydraulically there should be no difference with the engine on or off). C3's with PB have a VERY soft pedal and a fairly low pedal position-have you tried driving the car? Or is the engine off/on test the only test you've done?

 

Yes, I have put about 200 mile on the car and the pedal gets progressively worse. On a back road, I got to 50mph, pushed as hard as I could on the brakes, trying to lock them up (pedal completely to the floor), and could not lock them up. The car came to a slow, steady stop, but not quick enough in an emergency. The four other C3's that I have owned in years past all had PB systems, and never caused any problems.

I agree with you that there shouldn't be any appriciable difference with the engine on or off, except for the "normal" differences sitting statically. This problem has me totally baffeled. After playing with cars for more than 40 years and having a mechanical engineering background, plus owning and operating a machine shop that specialized in the manufacturing and repairing of hydraulic cylinders for 18 years, I feel like a rookie trying to diagnose this brain teaser.

Anybody else have any thoughts or ideas? Since I do have a relatively decent pedal now with the engine off, I might try clamping the rear hoses to eliminate those calipers and see if it makes any difference when I fire up the engine. Who knows what will happen, as I am grasping at straws now.

Thanks for your reply, and to all that have expressed their thoughts too.

 

Was your rod between the booster and master adjustable? If so did you adjust it?If your pedal gets worse as you drive you have rotor runout and one or more calipers is sucking air.

 

Let me ask you this:

How many bleed nipples are on your rear calipers? Because I went through this when I had my `81 (same problem you are having), and they only had one bleed nipple. When I got my replacement calipers they had 2 (perhaps even 3?).

I just did my `76 brakes, and the rear have 2 bleeders each, and the possibility of a third with the lower unused hole (opposite from where the brake line connects) being plugged. Both the ports at the top part of the calipers have bleed nipples in them. I think perhaps it is not easy to move trapped air from one side of the caliper to the other when you only have a single bleed point.

Hope that made sense, its early here. Having been through this though, I feel your pain.

 

When I replaced the m/c, I didn't pull the rod out of the booster. So I really don't know if it is a fixed length or adjustable.

Negative on the rotor runout. I have .003 to .005 TIR on all four rotors. I am still leaning to the rear caliper(s) sucking air.....just like this problem sucks!! LOL!!!! Still find it hard to believe that a lip seal will suck air but not leak fluid. However, I am fully aware that an old dog can learn new tricks. And BTW: No surging feel in the pedal when driving that warped rotors usually transmit.

Hmmmmmmm.........

 

Thanks for sharing my pain!!! My rears have three bleeders each, and the fronts only one. As stated, each bleeder now has a speed bleeder, which are really neat to use, especially if you are a one man shade tree mechanic. LOL. How did you finally get your brakes to a "normal" feel and working properly? (Other than bleed, bleed, bleed, and then bleed again!) Been there, done that, and am tired of doing that, but will do if itwill do any good!

 

have you tried diconnecting the lines frome the rear calipers and capping them off ( after bleeding the line). this could isolate your problem......just thinking out loud...........

 

IIRC correctly the new calipers solved the problem.

I also thought the "sucking air but not leaking fluid" thing was a bunch of malarky, but it seems to be true- this problem is endemic to these cars.

I would take a close look at the play in your rear bearings, if they have never been rebuilt. There is a side to side freeplay spec and I dont remember exactly what it is, but its damn small- something like .005. Anyone feel free to correct me on that number. If too large-You may not be able to solve this problem until that is addressed.

 

What I meant was, if there's air in the system, whether the engine is on or off you should not be able to get a pedal. Air in the system will lead to a squishy pedal, this has nothing to do with the engine running or not. The pedal dropping about half way (but not to the floor) when the engine is started is a NORMAL characteristic of a functioning brake booster-so there will be a difference in the pedal feel engine on/off. When you shut the engine off you should be able to pump the pedal and have it get progressively stiffer as vacuum is bled off from the booster.

You mentioned the pedal got progressively worse and went to floor after 200 miles. I think your first post said it went to floor when you started the engine-which case is correct? It can't get any worse than being on the floor to begin with. If it starts off about half way down (engine running) and gets worse as you drive it has to be a caliper pumping problem.

The rear calipers should have two bleeders each-at the top of the caliper. The third port at the bottom should be a solid plug.

Happy Thanksgiving!

 

The 4 pistons calipers dont show a pedal surge until the rotor thickness varies.If the runout is high but both sides are parallel it just makes the pistons go in and out and the fluid is drawn to the other side via the fluid passage in the caliper casting...thats where the "sucking" comes from.The fluid in the line going to the caliper remains still.