Dufrain

Im having a time with the brakes on one of my bucket trucks.Its an 85 f600 with the hydromax/lucas girling system.I just put on a new booster and master cylinder.I still have a hard pedal and the pump groans like its low on fluid sometimes.I have to engage the parking brake to get the brakes to work.They work for one stroke and then hard pedal again.The service manual say maybe too much pressure from the pump.It sounds weird that the pump would go bad and put out more pressure.Anyone have experience with these systems.