As Fla54Chevy3100 said, the drum/drum master cylinders, '67 and later had the residual valves built in. When they switched to disc/drum, they put the residual valve in the combination valve. The factory combination valve has a proportion valve, a metering valve and a residual valve. I believe there is only a residual valve for the rear. When the master is up on the firewall, the discs don't need one because the disc piston does not pull away from the rotor. The after market sells the 2lb valve for the front, because if you mount the master under the floor board, the fluid will drain back. I think most of the aftermarket combo valves have no residual valve so you need one front and 10lb rear.
