I'm in the process of restoring my car and now working on the brakes. I have a disc/drum powerbrake setup with the old school hold off valve (see https://www.v8buick.com/index.php?threads/metering-valve-hold-off-valve-button-function.379880/) I have new lines, new hoses, new rear wheel cilinders, new master cilinder, new power brake unit. I had my hold off valve rebuilt with a rebuild set from musclecarresearch.com and even today opened it up again to extra check. With modest air pressure, the valve opens, seems to work ok (still don't understand the button function though) I have followed the manual for the rear brake setup with almost binding and then 30 notches backing off. I have followed the manual for the parking brake, locking at 3rd click. I have bled my system now 4 times. Brake pedal feel is firm. It does brake in front, but with moderate braking, both rear wheels lock up. My car has no doors, no trunk lid, no bumpers, no window glasses. Not sure if that would make such a difference, but it feels like the rear is locking up way to0 easy. What am I missing?