Losing Brake Pedal While Driving

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ChasingSand

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In the last year I have been fighting many issues on my car and the last one I cannot figure out is why my brake pedal continues to go soft on me after bleeding.

I will bleed it in the garage then while duning it will become very soft and I have to pump it back up.

So far I have replaced the following. New Front Spindles/Brakes (calipers/Pads/Bearings/Brakes Lines, New Master Cylinder and In the Rear, new hubs, brakes (Calipers and Pads). 

The old on the car: Turning Brakes (early 2000's and proportioning Valve)

What is strange is that when I pull the turning brake for the driver side it is firm, but the passenger side will firm up then the arm will continue to move. 

Not sure if there is any correlation between the two?

 
It sounds like you have replaced everything except the turning brake.  if thats true, that would be my next thing. 

 
Sounds to me like the turning brake is not bled properly. You could have some air in the passenger side cylinder that hasn't bled out. That could cause the issue.

 
You might need to add a 2PSI check valve. It helps keep a little pressure on the system and keeps the pedal firm. 

 
Mike E said:
Check valve threads into the master. 

Did you change rotors? Rotor runout (warp) can cause this issue. 
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I did. All the brake parts were brand new from Jamar. I went with complete brake kits from them on the front and rear of the car.

 
What are you running for rear bearing assemblies? If the rear bearings have any play this can be part of the issue, allowing the brakes bleed fine in a static situation and lose pedal in motion. The bearings that are used on the micro-stub set-ups (pressed assemblies) that bolt in are known to wear out. If they are worn the rotor and hub assembly will knock the pads back when the car moves 

 
Mike E said:
What are you running for rear bearing assemblies? If the rear bearings have any play this can be part of the issue, allowing the brakes bleed fine in a static situation and lose pedal in motion. The bearings that are used on the micro-stub set-ups (pressed assemblies) that bolt in are known to wear out. If they are worn the rotor and hub assembly will knock the pads back when the car moves 
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I swapped the bearings when I installed the new rear brake kit, but the problem was there before the new brakes and remained after.

 
I had a similar problem, and it was the CnC turning brake.  Installed a Wilwood 340-1477 and fixed the problem.

 
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Did you make sure there is a continual uphill run to either the master or caliper? If there’s a high point between, you’ll never get them bled. Because of how my lines run (basically flat with the turning brake as the high point in the middle) I have to remove my calipers, hold them up at a high spot, and put a similar thickness piece of steel between the pads to get them to bleed properly. Since I have a 2 seater, bonus is I can hold the calipers while in the seat and bleed my brakes myself… :biggrin:

 
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Just ran into this issue in camp. Tried and tried to bleed and it had a soft pedal. The turning brake was mounted at an angle allowing an air pocket to stay where the plungers are. Unbolted it and tilted. Then bled. Fixed. Post a pic of your turning brake and the angle. 

 
it does sound like the wheel bearings are loose and bumping the pistons back in the caliper.

 
I have had this same issue in the past on a car that I installed a complete new brake system on, And for some reason the cutting brake would be stiff on the drivers side but when i hit the passenger side it was mush . The same issue you are explaining is what i had . My fix was installing residual valves and bleeding & bleeding over and over again. If you can borrow a power bleeder it helps a ton, My issue was that the air was getting trapped in the cutting brake. Good luck .

 
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