MG Car Repair: MGB Brake sticking, valve piston, fluid cylinder

Question
Brian,
I am restoring a 1976 MGB. Almost there. The brakes are sticking when the car is running. Once the car is turned off the brakes unlock. When I pull the brake peddle out it seems to help. Took off the lid to the peddle box compartment to look at the mechanism. the peddle return spring is in place.The plunger the brake peddle activates and extends into the Servo unit goes in at sticks. That is where it seems to be hanging up. What do you think the problem is?
Brian

Answer
Brian:

I would say the problem is likely the result of sticking in the servo air valve assembly. If the air valve piston sticks in the open position (after brakes are applied)pressure will not be released from the servo and brakes will stick. The reason I think this most probable is that you state once the car is turned off the brakes unlock - this is because the engine is no longer providing a vacuum to the servo.

As to your description of "the plunger the brake peddle activates and extends into the servo unit"...It is my recollection the plunger goes into a separate dual master fluid cylinder - not the servo unit itself. If this is so then the problem would seem to be in the brake fluid master cyl. Could be time for a rebuild with new seals.  You may effect some temporary relief by flushing/bleeding out the system with new, fresh brake fluid.

If it has been a long time since the entire brake system was bled out (like years)and if you are using regular vs silicon brake fluid, then I suggest going over the entire brake system including the calipers and wheel cylinders/ hoses, etc.

Good luck! Let me know how things turn out.
Brian