1991 850 woes

I own a 1991 850. I suddenly noticed a thumping sound near the porer steering reservoir. My mechanic said that it was the power steering pump;so I had it rebuilt and reinstalled after flushing the system. The thumping noise continues until I step on the brake pedal. It stops but if I continue to put pressure on the brake pedal,it sinks down to the floor.I do not want to buya a new master cylinder because it is hit and miss as I already wasted $$$ on a reman p.s. pump.Someone suggested that it could be the accumulator. Anyone have any ideas about this thumping as I feel that it is related to the braking system. Please help any suggestions are welcome.
Many thanks,
Algee 608