Brakes won’t ‘hold’

What does the pedal do when this is happening?
I have a line lock for that but I can still easily hold the car with the pedal and light em up on my 73 Dart with front discs and my brakes are not loose in the back.
It feels pretty solid but as soon as tires start to spin it just lurches forward. Doesn’t really shake or anything. On the street with my normal 30ish psi in the tears it’s a similar tale. They start to spin but roll forward pretty quick. Can’t sit still and do a burnout, but can leave about a 9 or 10 ft pair of black marks.