on my 70 Chevelle SS 396 my brake pedal seems to work if I push it easy but if I hit it hard there is like a brick under the pedal,i took out the proportion valve,same..changed booster..same,mc ..same the pedal will go down past the spot where it stops while bleeding..i really have to push hard hard to brake torque the car, fronts not holding enough..might get willwood calipers next.

 

The stock D-52 GM single cup calipers are real had to beat.. Have you checked the hoses?

A power brake booster will slow down the action of the brake system during panic stops.. The manual brake systems action is faster..

 

If you bleed the brakes, the failure warning switch valve may go "over center" and have to be reset for the brakes to work properly.
JAck the car up and have someone apply the brakes. Can you turn any of the wheels with the brakes on?