Classic/Antique Car Repair: brakes, brake light switch, piston cylinder

Question

master cylinder  

master cylinder   i seen a question on here about a 1940 plymouth p10 front brakes not releasing.I have the same type of car and my rear brakes won't release after driving it just a little ways. and what's odd is that it builds up so much pressure that it activates my brake light switch in front of the master cylinder and holds the brake pedal stiff to the top, but it doesn't seem to effect the front brakes at all. my next question is about the master cylinder, inside of the reservoir is 2 holes, the one closest to the back of the master cylinder goes straight through to the piston cylinder and the second one doesn't go any where. i thought it might be clogged but it doesn't seem to have any channels branching out from it. i've tried to find an exploded view of it but haven't had any luck. any info on any of these questions would be greatly appreciated.

Answer
It sounds like the brake pedal adjustment is not right. There should be about 1 inch of free play felt in the brake pedal when worked by hand before the master cylinder push rod touches the piston. If there is not any free play this pressure build up will occur. As far as the holes in the bottom of the cylinder reservoir they should both go through, although one goes to a chamber in the piston. With the piston removed they both should go through to the bore. Also there should be a one way check valve at the bottom of the bore before the fluid goes to the lines. This is to hold about 14 pounds of hydraulic pressure in the lines when the brakes are released to keep the wheel cylinder cups expanded.