You are correct in stating there will be a 500psi pressure drop across the separator piston in this instant only, because the piston is being restrained.Normally the piston is fully floating so the hydraulic pressure is the same as the N2. This Stop would be part of the accumulator design. It could be a change in diameter of the acc bore,shape of the piston or the accumulator end cap design.
The method of pre charging accumulators requires referring to a pressure\ temperature graph and charging the N2 with the parking /emergency brake system hydraulically depressurised. The N2 Precharge then forces thr separator piston fully towards the hydraulic end creating a known volume into which a known pressure is added. On some aicraft types a small temperature change will make a large difference in the precharge pressure.Most aicraft I have maintained have a precharge of around 1300psi with a variation for temperature of around +/-100psi.
I have referred to the type of accumulator used by Boeing and others.Airbus tend to use a round accumulator with a synthetic rubber membrane separating N2 and hydraulic oil.