This is your answer. Although, I am not sure what the remedy is. The fact that you can "pump them up" means something. Are you sure the rear shoes are adjusted correctly? They should have a slight drag when you turn the wheels.
@Gearhed73 walked me through a couple things and he said it could be the crush washer on the fitting to the proportioning valve is preventing the inverted flare from seating all the way causing a slight leak.