Most repair parts are made offshore, with little or no quality control, so it's likely you've encountered a bad run of new parts. I'd try rebuilding the new wheel cylinders with good quality USA-made repair kits, after making sure the pistons don't have excessive clearance. There's no such thing as "reducing pressure" to the rear brakes, no matter what kind of proportioning valve is used. The rate of pressure rise can be delayed, but the only way to actually change the pressure would be to have a master cylinder with two different diameter pistons. Most disc/drum systems have a spring loaded valve that delays pressure to the front brakes to allow for a little bit of rear brake to be applied in slippery conditions such as ice, snow, or mud without locking up the front wheels and losing steering control. Some pickup trucks use a proportioning valve that monitors rear spring squat to prevent rear wheel lock when running with a light load.
Most hotrodders don't have the engineering expertise to redesign their brakes, so they're at the mercy of the "kit" builders who are more interested in having a healthy bank account than actually providing their customers with a safe, reliable conversion. I have a tendency to be a little suspicious of anyone who promises to solve all my problems by selling me something! They might be selling "snake oil"!
Jerry
