'81 - Pedal was going to the floor at low RPM and firming up with higher RPM.
Found front drivers caliper leaking. Replaced with new and bled the front two calipers. Problem still exists.
Could it be some kind of vacuum leak? Brakes seem to work at higher RPM, but to the floor at idle.
One suggested Master Cylinder.
One says proportion valve.
Another Brake Booster.
Don't know how to pinpoint the issue.
Don't want to keep throwing parts (Master Cylinder next) to try and find issue.
Any brake experts who have seen this before?
Thanks for help.

 

Have you bled the brakes, (possibly repeatedly), with either a pressure bleeder, or a reverse bleeder?

If you replaced any rotors, did you zero out the rotor runout?

I'm not suggesting that any of your other suggestions are wrong, or even suggesting an order to try to fix things, just trying to see what else has been done.

A runout tool is cheap, and you can use it for a TKX swap, too.

 

No rotor change, just new caliper with the relatively new pads that were on the old caliper. Almost new pad thickness.

Thank you

 

Low RPM=High vacuum. Higher RPM = low vacuum.
pedal to the floor when vacuum high? Sounds like 2 problems at once. (Which will often drive you nuts). First, the booster should be able to hold vacuum, so you have a bad check valve or a leaky booster. And the pedal goes to the floor under high vacuum shows you still have air in your lines. Highly recommend reverse bleeding.

 

Thank you.

Ran and then turned off. Pulled the check valve and had a strong woosh of air. Booster appears to be holding vacuum.
Will re-bleed all 4 tomorrow. See if that works.

 

Yes. Once you get air in a caliper, you have air in the system and must repeat the entire process. Its tedious work but patience pays dividends!

 

A pressure bleeder makes bleeding the brakes easier. Remember to start at the passenger rear outboard and work in and do furthest from the master to closest...

 

When Idling if you keep stepping on the brake pedal and it pumps up and stays its air in the calipers or air in the master if it wasnt bleed correctly. If you step on the pedal and it doesnt come back up it generally indicates a bad booster. Bad boosters are the hardest to determine.

 

Sounds like you have a vacuum leak or, poor vacuum, or bad check valve or bad brake booster. With engine off, you should have high pedal after bleeding off vacuum with several pedal pushes. This is normal. With high vacuum at idle and low pedal, you have a hydraulic brake issue...air in system, master cylinder issue, etc.

Idle vacuum should be approx. 16-22 in. mercury. Check valve is in 90* vacuum connector on brake booster. Wait a few minutes after shutting off engine and pull off the connector at brake booster (natural or black plastic). You should hear a hissing noise after pulling it off. That should indicate the brake booster holds vacuum. Run same test a couple more times, increasing the wait time after shut off to make sure.

Using 4 hose pinch off tools, pinch off each brake caliper hose, one at a time, start engine and check to see if brake pedal still goes to floor. If all 4 clamps are on and pedal still goes to floor, the brake issue does not involve calipers. If brake pedal stays up after attaching a clamp, remove other clamps one at a time to see if pedal still stays up, Then that clamp isolates the issue to that caliper(s). You can also isolate front or rear brake system by attaching 2 clams to only the front caliper hoses , then to only the rear calipers.

 

I made my own pressure bleeder with a piece of aluminum plate, an old master cylinder gasket, a C clamp, a hose fitting, a bit of tubing and a bicycle tire pump. Worked great and didn't cost a dime.

JT

 

When the vacuum system is working properly on my 1968 C3 it will hold vacuum for weeks. I have come out literally weeks later and when I raised the headlights and they would "pop" right up. My brake pedal would also stay firm. My system held vacuum for so long because the complete replacement of every vacuum hose in the C3. The vacuum hoses were installed by a professional and they have never had any problems after that. The Vacuum Hose set was from DocRebuild.com. DocRebuild had the best quality hose kits available and several restoration companies use his parts all the time. He also sells the "correct" check valves and other parts for the vacuum system.

How old are the hoses in your vacuum system? Have any of the components of the vacuum system been replaced or worked on recently? Do you hear any vacuum leaks while the car is running?

For testing a vacuum system I first check to be sure that the hoses are in fact on the appropriate connections. Then check to see if the hose connection is tight, if it is too loose then it will leak. If you have the room to do it, I would cut off the ends of the vacuum lines and put the fresh end of hose on the connector to assure a good solid connection. If space is limited then try some Die-Electric grease on the connectors and slide the rubber vacuum hose over it. The grease will seal the connection a bit better.

In the recent past I have heard of Vacuum reservoirs developing leaks. This would be easy to test if you can isolate the reservoir and pull a vacuum and see if it will hold. The vacuum reservoirs are made of Steel in the early C3's and are "mostly" repairable. A little bit of JB-WELD and the tank will be ready for another few years. I would only pull the reservoir IF it was leaking.

I am very hesitant to clamp the original rubber hoses used to connect the chassis to the calipers as those original hoses are old and frequently not in the best shape. I would clamp the SS braided lines I am using in place of the old rubber lines because they are new compared to the originals. If you have the original hoses in this application you might want to consider replacing them. The Department of Transportation suggest that the rubber hoses be replaced at 10-15 years of usage.

If the brake booster is causing your ailments then ship it off to the following: https://www.brakematerialsandparts.com/ These guys will rebuild your brake booster to better than new condition! I would call them and explain the symptoms and see what they come up with. With their experience they might know just what was happening which could help you.

Master Cylinders can fail in many ways. I had one that was "Super-Sensitive" and if you pushed the brake pedal hard it could throw you forward. It was so sensitive and nobody could explain how it did what it was doing. Then one day I am cruising along at 70+ mph on a highway and I needed to slow down to exit. When I pushed the pedal it didn't budge, it was like there was a brick on the floor holding the pedal up. As I entered the cloverleaf exit ramp I was doing close to three times the suggested speed. Fortunately I have Four speed with a functional Emergency brake so I was able to drive the rest of the way home safely. I replaced the master cylinder and the car now had a normal brake pedal feel to it. This was a first for me as the master cylinder just locked up, kind of scary.