I believe it has something to do with the factory Distribution block "Bleeding Back to the Rear line inside the Block" Last car I had this problem with was a 70 Swinger. There are SEVERAL different kinds of Distribution blocks. Maybe it is only on a certain block?? I had the line lock right where yours is. Pump it up, hold the line lock, let off brake pedal, start burn out and the car would push right through the braking. The longer you did burn out it got progressively worse.
The not holding the car is a standard problem. I am not the only one this has happened too. This is why a lot of race car guys plumb it into the back & AFTER the Block. You will never have that problem when installed this way. 4spd guys... Well, you better be on your game with some fancy foot work. :D
I challenge you to go out set your line lock and give it enough throttle to load the car on the front brakes. Hold it there giving it more throttle to just before the car starts to spin. You may be surprises as your car too may push right through the front brakes as well.
