Rally need some help. I have been working on my restoration of a 66 OTS. I am closing in on the mechanics. My problem is that when the engine is running and I step on the brake the pedal gets hard as a rock and the brakes lock up. I can only release them by removing vacuum to the braking system. Welsh Jaguar told me it was the reaction valve on the master cylinder. The MC is new but I bought and installed one which did not work. I suspect the brake booster since it is the only thing that I did not replace with a new part. Any help would be greatly appreciated.