brake experts question for you

did a 68 charger for a guy. They put a kit on the front disc brake kit from I believe it was primoclassics on here I bet over 8+ years ago. It was suppose to be a factory replacement kit with everything from master cylinder to spindles and all other parts. It assembled good. He also wanted rear disc brakes on the car so we bought a kit from speedway motors that welded the caliper brackets on and mounted the brakes. A friend put them on his pro street dart and they worked out great. Well car has never had solid feeling brakes and now hes complaining that even with line lock on all it does is slides the front wheels. They don't hold and will slightly roll forward while applying the brakes and gas. Not solid enough. Any ideas? Car doesn't get driven a lot but he wants good brake naturally. Car is a 440 auto car. Could it be not enough vacuum? Or portioning valve issue? would an adjustable one work? If so who makes the best?

If it's sliding the front wheels then it is doing what it should and he needs to back of the gas. If the wheels are turning then that's something different.

wheels are slightly rolling forward he said

Or get beefier front tires.

I'm hearin that it ain't been right since day 1. I'd (1) set the pushrod clearance to ~.020 (2) take off the MC lines & cap the MC ports with a pair of brass male inverted flare plugs from the Edelman cabinet at your parts house (they're cheap & you will reuse em in the future) (3) eng idling, if the MC is good & bled out & booster functioning, the pedal should be rock hard with virtually no travel. that'd be a start (& the MC/booster is the beginning)