Jaguar Repair: XK120 Brakes not releaseing, wheel cylinder, hydraulic problem

Question
After I have been driving my car for about 20 minutes or so, the front brakes seem to hand up (not release).  If the car sits for a while (2 - 4 hours) they release.  What should I do the fix?

Answer
Hi Tom,
As I remember the 120 had a two stage master cylinder. Two different sized bores in one cylinder on the earlier model. To confirm that it is a hydraulic problem you need to drive it until the it is binding and while it is binding open a bleeder valve to see if it quickly releases. If it does then you need to first look at the rod going into the master cylinder to see if it is in a bind and check to see that the brake pedal is not sticking or holding the master cylinder operating rod down a little, this closes off the ports to the reservoir and holds pressure on the system. One of the pistons in the master cylinder could also be sticking in the bore holding pressure on the brakes.

If opening the bleeder did not make the brakes release then there is a mechanical problem in the brakes themselves. Wheel cylinder pistons can stick or such. This would usually cause one wheel to not release. while it is possible for more than one wheel to have pistons to stick, it is less likely.

There were two different brake systems used on the 120. I believe the two stage master cylinder was used on the earlier one.

Howard