Could you tell me reason that on a xj6 the low brake pressure indicator on dash is on and how to get it removed?

Presuming this is on an early xj40, the brakes system operates at very high pressure as it also operates the self leveling suspension.

Low pressure could be caused by faulty accumulator, esp if accompanied by ticking sound from start up as the pump tries to build pressure.

It should clear by pressing the clear button on the computer, but will reappear if problem persists.

Generally, not much to worry about but due to the safety critical nature i would seek a professional opinion immediately.