TR6 Brake Issue

[Bob Legate]

1974 TR6, just installed new master cylinder and servo from Moss. Bled the brakes, pedal is firm and high - just as it should be. Once I start the engine and vacuum is supplied to the servo, the pedal almost goes to the floor. Turn the engine off and the pedal is firm and high again. I installed a new hose between the non=return valve and the servo. Any ideas? Thx, Bob

 

[poolboy]

It's a sign that the servo is working when the vacuum from the manifold makes the pedal drop when the engine first starts...but not almost to the floor.
My "guess'' is air in the system..
I have heard of defective "classic gold" master cylinders though. Do you have one of those ?

 

[sqbsprite]

Sounds like air to me as well. Did you "bench bleed" the master cylinder before installing it? You might want to use a vacuum bleeding system and go bleed it at each wheel again until you are sure there is no air.

 

[Bob Legate]

I re-bled the system and there was a substantial amount of air in the left front line. However, when I test drove it, the pedal, once again, dropped almost to the floor. I did a little research and discovered that there is an adjustment on the front of the servo. I lengthened the "ball" on the shaft that comes out of the front of the servo so it pushed a little more on the back of the piston in the master cylinder. That kept the pedal much higher. After another test drive, I believe that solved the problem. However, I'm going to bleed the system again after a few days to insure that all air has been removed. Thanks for the input guys. I'm now confident with the brakes and can drive her once again!

 

[rooster]

I lengthened the "ball" on the shaft that comes out of the front of the servo so it pushed a little more on the back of the piston in the master cylinder.

Hi Bob,

I've made adjustments to that ball in the past...I've found it to be very sensitive an adjustment to make. If you have it putting a little too much pressure on the brake master cylinder then when you are driving after a while the front brakes will heat up to the point that they will slowly overheat and seize, even when you are not using the brakes much if at all. If this happens just back off on the adjustment a little bit.

Good luck