I use an adjustable valve from Speedway Motors, it's mounted under the bed in the rear brake line. I went to a local shopping mall that closed and made a bunch of 30 to 50 mph stops adjusting the valve till it would stop quickly with out the rear coming around. It will still just start to slide the rear tires but it takes a full on the pedal stop to do it. You will notice when driving normal the pedal feels a little softer, but mine still stops when needed. I have a valve pretty much like yours just off the master cylinder, it came off a late '70s early '80s Chevy Malibu, figured the weight and portions were close to my truck. Not sure what it does, but suspect its more for residual pressure than for proportioning.
