Oh its from the servo my bad but the master cylinder has been fully replaced --- i just replaced seal nr 20 of the servo with the smaller one from before but will bleed the brakes tomorrow ---> we replaced the master cylinder because it was beyond saving
Just because it’s new doesn’t mean it’s not the problem.
If the brakes only lock on when the engine is running, then something is causing the servo to operate. The valve on the end of the master cylinder is what opens one side of the servo to asmosphere, causing it to apply the brakes. Theres a little pushrod in the end of the master cylinder that operates the valve, that can stick. Also the valve can fail to seal, causing a vacuum leak. Another option is one of your vacuum hoses is collapsing, acting as a one way valve. It needs proper vacuum hose, heater hose won’t do. Do they lock on the moment the car is started, or do they stay on after the first push?
If you search the web for Jaguar sticking reaction valve, there’s several sites explaining how the system works. It should be in the Rover workshop manual too.
What grease have you used on the servo components?
Sounds like the grease is fine, I was just expecting it to be red.
If the brakes lock when the car is first started, it would suggest a problem is not a collapsed hose that’s holding vacuum when it shouldn’t. Having said that, it’s not a bad idea to replace the hoses with the correct stuff if you don’t know how old they are.
I’d look again at the reaction valve on the master cyl. Did you get a new one with the new master cyl? It’s also worth checking if you have enough free play on the brake pedal. If the master cylinder can’t return enough, you get some residual pressure which causes the reaction valve to operate the servo.