One thing I read was someone with a similar problem thought the rear brakes were engaging before the fronts built up enough pressure to apply fully. Sound plausible?
That exact scenario was going on with another members SSBC kit in a B body.
He used the pressure gauge and could see the system just wouldn’t generate enough pressure to the calipers.
The problem turned out to be the adjustable proportioning valve.
He had it set to recommended starting point(1/2 way?), and once it shut the flow off to the rear, the pedal wouldn’t travel anymore, so it wouldn’t generate any more pressure to the calipers.
The first “test” after that discovery was to back off the proportioning valve all the way, which allowed increased line pressure both front and rear, and breaking was greatly improved.
He was told that it was “impossible” that the proportioning valve adjustment would have any affect on the front brake line pressure.
But with the P-valve set to half way the most pressure he could get on the gauge at the caliper, using both feet and giving it all he had on the pedal was 600psi.
With the P-valve backed off it was 1200psi.
Before I went any further I’d figure out a way to get a gauge fitting in there if I were you, and see what the pressure is at the caliper.
