Brakes / pedal goes soft after applying pressure - Check my sanity

[Siia109]

'97 D1 with 127K miles on it. The brake pedal stops the car but as you sit at a light and don't move your foot the brakes will slowly release. Press down more and it stops.
My thought is the master cylinder is not holding pressure. I see no leaks and I have just replaced the rear rotors and pads the front are 2/3 good still. This car did sit idle for a while (years) in the Tx heat so could that have dried out the rubber o-rings so starting with a rebuild kit might solve the problem? Assuming the bore is not scored / scratched.

Sounds logical? or should I try something else to verify.

Thanks in advance.

 

[German Gruner]

Im afraid you can have trouble with vacuum servo. As You may know, if there is a vacuum leak, by still depressing brake pedal you can feel it losses final break power.
Check for vacuum leak: You can depress pedal it should find a limit. When turning on engine you should feel brake pedal goes lower a little bit. This is sign there is vacuum. With running engine you can cut engine and with open ears you may depress brake pedal. If you feel at start air blowing inside drivers footwell, it is signal of a diaphragm puncture.

Regards

 

[kris812]

Air in the lines, or bad booster. To note there's an o-ring between the master and the booster. That should seal well. Take the vacuum line off the intake that goes to the booster and start sucking away and see if it holds.

 

[Siia109]

Thank you both - time to learn about Vacuum on brakes....

On the D90 one the signs of a master cylinder failing is the leaking brake fluid in the footwell. This is not same on the D1 due to the different set up - correct?

 

[kris812]

Germans, test is actually a pretty good one. Hold your foot down on the brake, and then start the truck. The pedal should go down some. Second test while holding the pedal down and the truck running, turn the key off, the pedal should come up some.

If either of these fail, your brake booster diaphragm is bad, or the vacuum line leading to the engine has a crack.

Bleeding brakes is good preventative maintenance anyhow, so might be worth bleeding to see if any air bubbles come out.

If you are leaking fluid anywhere, that means air is getting back into the system.

Worth checking the flexible lines that feed the brake calipers. My front left one was soft squishy and expanded when breaking. Noticed when I was lifting and installing braided extended lines.

 

[p m]

Regardless of whether the booster is good or bad, the master cylinder can be bad. A common cause for this is supposedly the old-school bleeding process using full travel of brake pedal in the vehicle.
In regular use, the piston in the master cylinder only travels to a certain point, and wears the master cylinder walls creating a tiny ridge near the end of its normal travel. When the piston moves past that ridge (during energetic brake pumping during bleeding), the o-rings get shaved a little. Eventually, it causes a slow internal leak in the master cylinder.