Disc brakes are inherently instant acting as the pads are always virtually in contact with the rotor. Drum brake shoes operate with a clearance and are self energizing. Hydraulics must work against spring pressure, take up that clearance and then the brakes self energize. The hold off valve holds off front pressure until a certain amount of pressure has built up in the rear brake circuit.
Good description is here and backs up Larry's explanation: https://www.musclecarresearch.com/content/what-metering-valve
Been working on cars and restoring most of my life and I just learned something new. Thanks a lot fellas. Sorry I kind of hijacked your post Mart,but we kept it fresh for you
