Need some electrical help with my ‘68 Plymouth Valiant 100

Concur with @halifaxhops for ignition items and clarification of your setup.

You may find with your brakes that the cylinders or rear lines are just tired. The rubber wears out over time.

Once you get into the service manual you'll have more to assess.

If you can find an old Motors Manual on eBay, they're super helpful as well.

1969 Motor's Auto Repair Manual 32nd Edition 1st Printing Covers 1963-1969 | eBay