Above is a sketch of a new /good accumulator that hasn't been pressurized.
There is roughly 750 psi (pounds per square inch) of nitrogen on the top side of the bladder when new.
Without brake pressure coming in from the bottom, the nitrogen pushes the bladder down to totally evacuate brake fluid from the accumulator.
The sketch above is of a new/good accumulator under pressure.
When the boost pump starts, it builds up pressure and forces brake fluid to compress the bladder. As brake fluid enters the accumulator, it displaces the bladder further compressing the initial (750 psi) nitrogen charge. The pump turns off around 2600 psi.
On a very good system, you would have that amount of pressure in reserve to assist in the boost (Power Brakes). The factory literature indicates this could be up to 25 presses on the brake pedal, however I have never worked on one with more than 6-8. This would indicate that with age, there is leak down in the system.
This sketch represents the other extreme... an accumulator with most or all of the nitrogen charge depleted.
This happens with age and there is presently no way of measuring the actual condition other than the brake test that have been developed.
This sketch shows the bladder completely compressed at the top of the accumulator. As the nitrogen charge diminishes, it is much easier for the boost pump to force brake fluid into the accumulator.
The pressure switch that turn on the pump comes on at 2000 psi and shut off at 2600. The ABS light comes on at 1500 psi and the Red Brake lamp comes on at 1300 psi.
Since there is no nitrogen reserve, the accumulator is always full of brake fluid and since you cannot compress a solid, the pressure will fall and climb very quickly, this is what happens when you push the pedal once and the pump comes on.
Also in this condition, the fluid level in the reservoir will hardly budge.