My 1990 Corvette brake pedal cannot be depressed. I thought that it was due to the power brake booster, but it appears to be fine. Could the master cylinder be locked up? And, if so, is there anything that could possibly be done other than buying a new master cylinder?

 

Appears?

We need a little more info to judge an 'appearance' - pics, diagnostics done so far, etc., ...

 

When I unbolt the master cylinder, the brake pedal depresses perfectly. When the car is running, I still cannot hear any hissing when brake pedal is depressed.

 

Ok, so the pedal action is okay.

Either unplug the 2 MC lines and see if the MC pushrod action is ok, OR, start diags from the other end - put the MC back on, hold the pedal down, and bleed each corner and see what (if anything) comes through.

Third option, to test for collapsed brake lines, and this is a stretch: get the front corners in the air, and see if the fronts are binding. If they are, lines could be collapsed. I had this very thing recently on a '98 GMC, but it didn't cause a hard pedal. None-the-less, if they're binding, pull the MC cap, and carefully see if you can wedge a screwdriver between the pad and the rotor, to force the pistons back into the bores. The should go back slowly and steadily. Use steady increasing pressure. If they don't go back, you likely have collapsed lines, and possibly that's the problem.

GM 90's brake lines (caliper-to-fender liner) were dual layer lined, and when the inner line collapsed, it can act like a check valve. The final test is removed, drained line: can you blow air thru' the line one way but not the other. An old mechanic told me that, and I didn't believe it. Till I saw it myself.