I recently replaced my brake master cylinder with a new one on my 1970 Jaguar XKE Series 2. After bleeding the brake lines and testing the brakes, for some reason the pistons in the brake calipers engage after applying pressure to the brake pedal but they don't release completely. This was not a problem prior to replacing the master cylinder. Any thoughts?

Frank

 

A problem like this is typically an issue with the master cylinder or brake calipers. Since it only started when you replaced the master cylinder I'd guess you either installed it wrong, it is defective, or you got some contaminant in the system. These things are incredibly sensitive to dirt so if a contaminant got into the cylinder during installation that could be the cause. I hated rebuilding master cylinders for that very reason, invariably I'd miss some dirt and have to do the whole thing over. (Likely because I was rushing). Good luck!

 

Concur with previous post... but (always a but) be aware that the gap between "released" brake pads and the disc is really small... if you are looking for the pads to open up widely around the disc - this is not how it works,
Take it for a short drive and see/hear/feel what's happening before you lose another afternoon on ripping the whole thing out.

 

Hi Frank
I had that problem,it turned out to be brake lines that looked good on the outside but had swollen on the inside,one would apply the brakes they worked but would take some time to release.

Regards Dan Gardner

 

Since this happened after you installed a new master, the likely problem is with the reaction valve mechanism. There is a small piston at the front of the master that's driven forward when you brake, this triggers the reaction valve. The reaction valve releases vacuum pressure in the rear of the vacuum servo, this drives it forward and engages the slave cylinder. If there's an air leak in the reaction system, the brakes lock.

What I would suggest is to remove the reaction valve (the white thing at the front of the master.) replace the hose that connects it to the vacuum servo, taking care to tighten the clamps well. Then carefully align the seal with the backing plate and screw the reaction valve down. If that doesn't do it, we can look at something else.

 

Just to be Captain obvious.... Did you make sure that the push rod has plenty of play in it so the brakes can fully retract? Herb