Master Cylinder - Weeping Front Seal

On an Auburn , the stop light switch is at the front, that is where I thought in tas leaking.

Curt the weep is at the front of the master cylinder. No leaks occurred when I pressure tested the master cylinder on the bench . The leak only showed up when fitted in the car.
The leak is at the secondary cup Item No 2 in the attached picture

Well then, either the mating surfaces are not flush, the washer is bad or the threads are comprimised. I am unsure what kind of thread locker can be used in that situation.

Thanks Gary and Curt, no I am not using silicone, I'm using DOT 4 brake fluid.
I tried silicone brake fluid and gave up on it. Everything was cleaned properly before using the DOT 4.

Are you using silicone brake fluid ?

Terry what brake fluid are you using? Not sure what the standard master cylinder clearance is supposed to be. I realize the cup stays under a small amount of pressure at all times to help keep the cup open to the bore. Hopefully someone with more information will chime in. Car looks gorgeous.


Gary Parsons

After going through the complete braking system the front seal on the master cylinder is still weeping.
I had honed then polished the master cylinder and wheel cylinders, fitted new rubber seals as well as new pistons. I had made a new master cylinder piston with 0.003" clearance. The push rod had also been aligned. Pressure testing the master cylinder on the bench resulted in no leakage, so after fitting the master cylinder and pressure bleeding the system I am puzzled as to why the front seal is weeping.

Any comments would be appreciated, she is almost ready for the road.