ABS Pressure Accumulator for VW Passat 35i

"On my Passat, manufactured in 1992, with its older ABS system, I've recently been experiencing the following problem:"
"When I apply the brakes, the ABS warning light illuminates and the pressure pump activates. It appears that there isn't enough pressure reserve in the pressure accumulator to generate the necessary braking pressure. My suspicion is that the initial pressure in the accumulator has leaked out. I would be very grateful if someone could confirm this or offer another suggestion."