Triumph Repair: brake problem, wheel cylinders, bleeder valve

Question
Howard,
I have a TVR 2500 M (TR6 brakes) and have a problem very like one reported previously (lock-up after about 8 miles with a completely rebuilt system). However I have found that it is not distance but rather time the car is running.  The brakes just start locking up sitting at idle after about 20 minutes.  Could the booster be causing this just from the vacuum pulling on it?  Heat somewhere causing fluid to expand and a valve malfunction not releasing pressure?

Answer
Hi Jim,
When a brake system has the wheels locked up, the first thing to do is to open a bleeder valve to see if it is hydraulic pressure that is causing the lock up. If Opening the bleeder relieves the lock up then the problem is something to do with the master cylinder (which includes the booster) When your foot is off of the brake there is suppose to be a open and free travel of fluid from the calipers and wheel cylinders all the way to the reservoir. When the brake pedal is applied you push the master cylinder piston in which closes off that free passage and locks fluid in between that piston and all of the wheel pistons. If there is pressure in the system when your foot is off the pedal, then that piston in the master cylinder is covering the hole that allows the free passage of fluid to the reservoir. Yes, a triggered or sticking booster can be holding the master cylinder piston in, but keep in mind that a simple thing like a sticking brake pedal can do the same and a sticking master cylinder piston can do the same.
If you can relieve the pressure by opening a bleeder, do this, close the bleeder and pump the brakes up again and see if you can get the wheels to drag again and this time don't open a bleeder but quickly loosen the two nuts holding the master cylinder to the booster and see if that makes the wheels free up. I have run into several Triumphs that required a thicker shim /gasket between the booster and the master cylinder. Also check for a pedal return spring too.
Let me know,
Howard