since most folks do not have the Kent-Moore tee and gauge setup to check system pressure, operating and cycling, there is a basic bench test to confirm the accumulator has lost charge.
Brake fluid deteriorates the membrane within the accumulator, causing it too gradually loose its charge pressure over time (years). there is an internal metal plate covering part of the membrane. From the accumulator threaded port, insert a finish nail and you will hear a metal on metal sound when tapping the nail against the plate once you drain the fluid from the accumulator.
you have to work a little bit to drain the brake fluid in the accumulator once removed. shake it in a container as draining something without a separate vent takes a little time as you shake out the fluid.
The OE all black accumulators made by Singer (sewing machine co) and the 2nd version with yellow warning print on end (with the hex nut on the outside end) have the metal plate. I expect the Hydac accumulators to have it well as accumulator construction is pretty basic.
secure the nail in a vice. once you check the internal distance to the metal plate from the outside of the accumulator, if you can use your weight to push the accumulator down about a 1/4 inch to the surface of the vice, the internal pressure of the accumulator is too low. A replacement should be purchased.
Vendors and ebay have the Hydac accumulator. check around to learn who has what in stock and how much.