Often the original MC has a built in residual pressure valve as part of the piston or outlet fitting. See if yours did, if so then a new MC with residual valves or separate ones in line would be a good idea. The idea is to keep a little pressure (usually 10psi for drums) in the line to prevent drain back to the MC and a soft pedal, but not so much pressure the shoe return springs can't overcome and retract them.
I'd also use the more modern factory GM brass proportioning valve as Jerry describes, one for drums and drums if that is what you have. I've only used them for disc and disc, a similar looking but different valve I think.
