Need some electrical help with my ‘68 Plymouth Valiant 100
So I added more brake fluid, pumped the brakes like crazy, but I don’t think I’m seeing anything leaking out of the back wheels...
Pumping isn't what you need to do.
You need to pump initially, to load as much fluid into the lines as will fit.
Once you get a firm pedal, you need to lean on it, HARD, with all your strength.
If you have power brakes, you need to do it with the engine running, so that the booster is boosting.
With hard, steady pressure, even if there is some air in the lines, you should arrive at a point where everything is as compressed as it can be, and, even if the pedal doesn't get truly hard because of air in the system, the pedal should stay firmly in one position and shouldn't slowly sink down.
If the pedal sinks, you've either got a leak or a bad master cylinder.
Pump it up again, and repeat. After a few cycles, there should be a lot of brake fluid on the ground somewhere. If there isn't, then you have a bad master cylinder.
By the way, how long was this car sitting before you got it?
– Eric