The single pot master cylinders all had a pressure residual valve built into them. When dual masters came on the the market, they had these rubber duck bill pressure residual valves installed on the drum side of the master, behind a removable brass seat on the master. so one valve for a disc drum and two valves for a drum drum. These pressure residual valves kept a little pressure on the back of the cups of the wheel cylinders for sealing and operation, and also helped keep the piston itself from retracting too far into the cylinder. the rebuilders replaced these valves and the seats when rebuilding. Somewhere along the line the rebuilders stopped installing the valves. Then, the parts scene got worse with all the chineseium master cylinders coming onto the market. these new manufactured masters no longer hand a removable brass seat. the seat for the brake line was now cast integral with the main casting so now way to install the valves and no way to retro fit the valves. So, to mimic the correct residual pressure, you install inline valves.
It is possible that you have an internal leak in the master, i.e., as soon as the pedal gets so far, it loses pressure in the master around the piston part. But before we go there, can you shoot a couple of photos and show how you isolated the front and rear system and how your lines are plumbed in the master? Also, where exactly are the residual valves installed? Can you shoot a photo of your master cylinder also?
Last edited by Dragsix; 10/26/2023 6:35 PM.
