I don't know, I had that exact problem and it was the 28 year old accumulator. Replaced it and problem solved, I never had to troubleshoot further. I'm sure the experts on here will chime in with useful info that I'm unaware of. I can only relate my experience. However, check your pressure to see if that's your problem. You have to start somewhere and that's the place I'd start.
Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***
"Worrying about depreciation on your car and keeping mileage down is like not ****ing your girlfriend so her next boyfriend finds her more appealing"