Weird Brake Problem

[MisterAl]

After my 92 Lsi had been sitting outside for a month, I found my brakes were locked on. I could barely get the car to move. I removed the left front wheel, as it wouldnt rotate, and had to hammer the caliper off the disc, it was jammed to that extent. No rust on the disc.
I removed the bolt holding the outer moving part and found the pins were free, no corrosion. I used a clamp to depress the pistons and found they moved OK, but when I depressed one the other one moved out. Same when I tried the second one . I tried to compress both together but I couldnt move them. I used heavy clamps with a lot of force, but no movement, and the fluid level in the container didnt move.
I removed the clamps and tried depressing the brake pedal, it went to the floor with no resistance. Same thing with the engine running.
So on the one hand it seems as if the master cylinder has gone because the pedal goes to the floor, but if this is the case why can't I compress the pistons?
I have done many brake jobs on other cars, but haven't met anything like this. Any ideas guys?