Normally they should still work, you just need to press HARD on the pedal.
I was hoping that if the brakes locked up without even driving (put it in neutral and see if it rolls), disconnecting the hose should take the booster out of the picture.
The booster has 2 chambers separated by a rubber diaphragm and a central moving "piston" with internal air valves in it. Normally, the rear valve is closed, and the front valve is open, keeping the same vacuum front and back. When you press the pedal, the movement closes the front valve (sealing the front chamber from the rear chamber), then opens the rear valve, allowing air to enter the rear chamber. The pressure difference moves the "piston" assembly forward and against the master cylinder.
It's possible (though rare) that the internal valves could mess up, opening or closing when they shouldn't, letting the "piston" move and apply the brakes without any help from your foot.
If you can catch it the next time they lock up, try pulling the hose AND the check valve off the booster, and see if the brakes release with no vacuum signal at all.