Technical Weird brake situation. your thoughts?

I got this '64 chevy with a new disc brake conversion that never saw the street, and sat for some years. Took it off the trailer with no brakes. After making some necessary repairs and bleeding it, it had a firm pedal until we started the engine. Pedal went to the floor and would not pump up. After a little troubleshooting work, it turns out someone had put washers on the caliper bolts, essentially locking the front calipers in place, so the pads weren't applying. Took the washers out, lubed the pins, rebled everything and they work fine with the engine running. Problem solved.

SO... What I can't get my head around is why, given the situation, when the booster came into play would the pedal go to the floor and not pump up and then return to a perceived 'normal' with no booster. Thoughts?

 

Since the pistons could not retract, as they were all the way in already, the pedal cold not return.

With the pedal not all the way up, the booster was engaged.

 

Make sure that the rear drums are well-adjusted and go from there. The pedal drop might be a characterization of the booster itself, see how it stops once the rears are adjusted.

 

See, that's the weird thing. The pedal did return on the return spring. It felt like installing a master without bleeding it, or like there was a big leak somewhere. I mean, either way it works now, but I spent a couple of days really puzzled by the whole thing.

 

Like I said, the system is working fine now, I just can't explain what was happening in the moment.

 

The push rod isn't attached to the piston on most all master cylinders that I know of and has no way to pull the piston back. It should relieve the pressure so the combination of fluid and internal springs in the piston assembly cause the piston to return.

 

But the springs inside the master are pretty stiff, as evidenced by the effort required to manually bench bleed.

Chris