Purchased an additional new 7/8" bore master cylinder just in case the older new one I bought doesn't work like it is suppose to when I install it on the El Camino. When I was bench bleeding the older, new one, the front port for the rear brakes would suck the fluid back into the master cylinder after the piston was released. The rear port for the front brakes didn't have this issue.
Is this normal? I don't know, but I will find out from the new master cylinder that I bought.
I also baught a rebuilt, aluminum master cylinder, with a 1" bore, from a 1979 Buick Riviera with the optional rear disc brakes. I did this because the reservior is larger, it can be retro fitted to the 7/8" bore manual brake master cylinder, and it matched the angle of the firewall. You don't need it for rear drum brakes, but I like the extra fluid capacity.