I take it this car is equipped with Teves III power brakes, in which case there would appear to be an issue with the ABS, but is probably caused by the pressure reducing valve being jammed open.
It could also be that the ABS valve block has a stuck closed return valve so no fluid can be released from the rear brakes via the ABS valve.
You should be able to read the fault codes for the Teves III system.
Plough your way through the linked thread and you should have all the information you need to be able to diagnose and work on the Teves III system.
Teves iii braking system failure - XJ-S - Jag-lovers Forums