Alan de Enfield

Ideally, it should be reading somewhere around the 400 PSI, the fall off it power is due to wear or lack of maintenenace and can often be relatively easily be sorted out.

It is possibly valves leaking, or piston rings stuck, both could be due to carbon build up and a good de-coke could solve it. It may be more involved with worn bores etc, but that would be identified when you start the decoke.

Are all cylinders showing the same sort of pressure ?

Good luck

Alan