brakes will not disengage while motor is running
 

Hi Can anyone tell me if free play can on my brake master cylinder be adjusted on a series 2. I have just replaced my master brake cylinder and now when I push the brake pedal the brakes stick on. Thanks

Maybe posting more information might trigger a reply. Why was the master cylinder replaced, Does the pedal return after the brakes are applied, etc.

Sounds like the servo booster cylinder or vacuum servo valve could be faulty.

You do need the Master cylinder to stop the pedal and NOT any other mechanical means.
There is a small hole in the cylinder-to-reservoir area of the cylinder (to allow for fluid expansion and filling the system) that needs to remain OPEN when the pedal is NOT depressed. Covering the hole will allow pressure to build up and actuate the brakes. Pressing the brake pedal a few millimeters covers the hole.

Also having engine VACUUM on the booster cylinder will actuate the cylinder and apply the brakes. The vacuum servo valve on the MASTER cylinder needs to operate properly. The vacuum hoses need to be routed correctly and the valve functioning properly.

More diagnosis and an understanding of how the system operates will lead you to a successful repair.

bob gauff

What model jag? early series 1 vs later has totally different system but from clinical description, valve is malfunctioning and brakes release when vacuum releases.

Bob, I, for one, could use a tutorial on how the (later) boost system works. I looked at the manual and even did some repairs (and it worked!) but I really can't get my head around the actual operation.

My E-type experience is pretty much limited to 'dabbling' at this point.

Cheers
DD

Hi Thanks it Is a 1970 xke series 2---

Did you figure out why the brakes are malfunctioning??

bob gauff

I have not as of this time. I have removed the vacuum servo valve? ( from the front end of the master brake cylinder) it was new and cleaned the end of the master cylinder. No help. Thanks

There needs to be vacuum on BOTH sides of the booster cylinder diaphragm (the lower cylinder) when the pedal is NOT depressed. When the pedal is depressed, the vacuum is disabled on one side of the booster and the brakes are actuated. That is how the brakes are vacuum assisted.

In the end of the Master cylinder there is a small piston with a seal and that is what moves the plastic vacuum actuator to control vacuum.

The vacuum lines also need to be routed correctly.

bob gauff

brakes stuck on
 

I too have a 1970 e type, the plastic small diaphram at the end of your new master cylinder controls the vacuumm to the main booster/master if this is not shutting off completly you will have vacuumm at booster and brakes on all the time!

Their is a great diagram on how all this works in the Haynes E-Type owners workshop manual.

Look at my post under xke brake activation?