My MkIX brake pedal hardens and all 4 brakes lock up when driving. I replaced the booster with a universal part from XKS unlimited and a rebuild kit for the master cylinder. I even bypassed the hill holder with a new brake line. On the lift, as soon as vacuum is applied to the new booster, the brakes again lock and the only way to release them is to crack the hydraulic line going from the booster to the brakes. There deosn't look tobe another part in the system that could be causeing all 4 brakes to lock at the same time. Could I have received a bad (new) booster from XKS? Any suggestions would be appreciated!!

 

This question should have been posted in Mark V - X 420G forum where members with the same model are more likely see it and be able to offer help and advice.

I'll leave it here for the moment to see if it attracts wider attention.

Graham

 

I figured since the system was used on several different cars, it might be better in the general forum. Sorry!

 

I don't recall the brake line plumbing on the Mk IX but I have seen internal failures on flex lines at the wheel that acted like a check valve & would not allow fluid to return. A flex line between the master cylinder to the junction block could do the same thing

 

I see it to be likely you received a bad booster if after you apply the brakes they stick on.

 

This might be completely irrelevant but years ago I had an old Wolseley ( no power brakes) and the brakes locked themselves on one day, rock hard pedal and drums all hot. Like you I had to crack a nipple to release the pressure. Problem was the little hole in the master cylinder that allows fluid back into the reservoir when the pedal is released was blocked. Might be worth a look.
Good luck