Yes, under the floor. I installed a 2 psi residual pressure valve for the front brakes. I do not know how to check, but my research has me believe that the disc/drum master cylinder would have the 10 psi one for the rear. I believe also that the combination valve also includes the 10 psi residual pressure.

Are my thoughts correct? How can I check?

Thanks!

1950 GMC 100 Panel Truck, 1958 270,
3-spd Saginaw with BW OD,
3.73 12 Bolt Differential from a 1966 C10,
Front Disc Brakes
Hill Holder
Bought it in pieces 9-1976, driving since 6-1978
Follow in the DITY