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[Tommal]

I installed a new master cylinder (TRW) yesterday in my TR6. While driving it today, the front brakes locked up and would not release. After a tow home, they worked just fine. I found the inside of the cap (holds the gasket in)loose in the bottom of the reservoir. Could this have blocked fluid return somehow causing the brakes to lock? Any other thoughts?

[TR3driver]

I doubt the inside of the cap was the problem.  My guess would be that something is holding the MC slightly depressed all the time; like perhaps the pedal is not returning (binding) or the pushrod in the brake servo is just a bit too long for your new MC.

Try pushing one of the caliper pistons back into its bore.  If it will move, then you may have fixed the problem.   If not, then one test might be to loosen the bolts (or are they nuts?) that hold the MC to the servo and pull the MC out a bit.  If that allows the piston to move, you know the problem is mechanical rather than hydraulic.

If the problem happens on the road again, you can avoid calling the tow truck by loosening the fitting at the MC and letting a few drops of brake fluid out.  Doing that once on my Stag was enough to get us the last 400 miles home.

[Tommal]

Thanks. Checked it all over this evening and everything appears to be fine. I bled the fronts and the fluid was clean, no air or debris. Brakes worked fine on a short test. I'm checking on any issues with the Master.

[Mike V.]

I had the same problem when I changed out the master cylinder. Found out it was a faulty master cylinder. The front brakes would lock up and I would have to loosen the fitting for the front brakes on the master. Replaced it again and it took care of the problem. I have been told that there was a bad batch of master cylinders.