1 inch on the master for a drum drum. On the pressure residual valve, the original single pot master had a pressure residual valve built in. The pressure residual valve kept pressure on the wheel cylinder pistons and rubber cups, and to a lesser extent the brake shoe springs, to keep the wheel cylinders and shoes from fully retracting and causing excessive brake pedal travel, and to keep a little pressure against the cups to prevent leaks.
When the dual pot masters came on the scene, the original GM units has removable brass seats on each port. Behind those brass seats were a spring a rubber "duck bill" pressure residual valve. One each for drum drum masters, one in the drum portion of a disc drum master. The rebuilders for years would install the duck bill residual valves as part of the rebuild, and then they didn't. And so the rebuilders, presumably because its cheaper have not installed the valves for some time now. But they can be retrofitted. The last time I purchased a rebuild kit, a Raybestos No. 472, the valves and new brass seats were included.
The new manufactured masters do not have removable seats, all cast integrally with the master so no retrofitting.
You may also see a lot of "proportioning" valves on a lot of installations. Its not exactly an accurate term. A common master cylinder to use for a non-power conversion is a 67-69 Camaro master. For drum drum applications, GM used a "distribution" valve at the master. The distribution valve was used to trigger the dash brake failure warning light.
A "proportioning" valve was used on certain Camaros, not many, to delay pressure to the rear drum brakes to allow the fronts to take on the heavy braking so that the rears did not lock up prematurely. Only certain Camaros had these, AC cars, cars with 12 bolt rears.
But the "proportioning" valve everyone refers to, the brass type, is more accurately called a combination valve and is used in disc/drum applications and handles the warning light feature, distribution to two separate circuits, and a hold off or proportioning valve so that the disc brakes hit first before the rear brakes hit. There is no pressure residual function.
So if you want to replicate the pressure residual function on your dual master conversion, you will want to consider adding two 10 pound red Wilwood inline pressure residual valves. One each side, close to the Master.
