Need some electrical help with my ‘68 Plymouth Valiant 100

In order of appearance:

1. Do not buy anything from Classic Industries, unless you own a diamond mine in Botswana.

2. (Yes, I know this is an electrical thread) When you say you have a brake fluid leak, do you mean that you see a puddle, or do you mean that you have observed that brake fluid is missing?
If you have a puddle in the back, then, obviously, you need to replace the line or the bad wheel cylinder before you drive the car on the road.
If you are seeing one half of the master cylinder is low, then you may have a leak somewhere, but you probably have a bad master cylinder.
You said the rear brakes were leaking – If you observed that the rear part of the master cylinder was empty, then that is the FRONT brakes.
When the master cylinder fails, it generally leaks brake fluid out the back, into your carpet.
Sometimes it pumps the fluid from the back into the front, so that the front is high and the back is low.
But if it's the rear reservoir that's empty, then you need to check the FRONT brakes for leaks.

3. You check for water in the oil by looking at the dipstick.
If it is foamy, generally described as a cappuccino-like appearance, then it has water in it, and you can't leave it that way.

– Eric