Need some electrical help with my ‘68 Plymouth Valiant 100

snailgarden said

Drum brakes. I checked the brake fluid and the front half of the master cylinder os completely gone. So most likely something is wrong with the rear drums, or at least that’s what I assumed. I filled it with brake fluid and tried pumping the brakes to see if I could find leaking from the rears, but I couldn’t seem to find the leak.

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You will have to bleed them and may have to prime the master cylinder. Google search, there are different ways to do so. I think it is also in the service manual. If there is no evidence of leakage in the carpet or down the firewall, you most likely have a leak in the tubing to the rear, and could be "anyplace from the master on back. By the way the FRONT fitting on the master feeds the REAR brakes.