The front brakes are hydrostatic, they do not need pressure in the system to be bled. The rear brakes do not work if there is no pressure in the system and cannot be bled without it. Your problem seems to be that pressure is not being retained in the accumulator. Obviously when the brake is pressed pressure from the accumulator is used. When the brake is released fluid returns to the tank until after several applications, the pump starts again to recharge the accumulator. If the pump never stops the pressure is being lost somewhere. The fault has to be either the pump NRV or a valve in the ABS unit stuck open allowing fluid to return to the tank. You can bleed the system to your hearts content but it will not cure the pressure loss. Does your car have rear wheel traction control with a TC lamp that comes on with ignition? If so this lamp should go out when minimum working pressure is achieved but the pump should still run for a while after this lamp goes out before full pressure is gained and it stops. You could try fitting a blanking plug in the high pressure feed from pump to ABS unit. If with this plug fitted the pump stops the problem is with the ABS unit. If it does not stop the problem will be with the pump NRV.