I’d pull the brake cylinder, disassemble, and see what is in there. It might be gummed up with rusty crud from old brake lines. Wheel cylinder disassembly is not terribly difficult, just messy. No special tools required except for maybe a brake tool for the brake springs, but there are work arounds for that (vise grips).
The rear brake line goes from the wheel cylinder/drum backing plate and snakes down to a junction T on the rear axle. Brake lines are cheap and easily replaced. You can even get the pieces from your FLAPS store at a prebent/ pre-flared length so you don’t have to double flare the line.
If t’were me, I’d be pulling every line from the master cylinder to inspect and the wheel cylinders. The lines are cheap, wheel cylinders not so much (but rebuildable if decent), stopping when needed...priceless.
Going is fun, stopping is critical. I was sure glad I could stop one day when a doe and her fawn jumped over a guardrail in front of me on the approach to a bridge. Tires locked (Hucks), me cringing, two semi trucks behind me locked up because I was “going slow”, and the deer scrambling back to safety. Needless to say, those brakes were thanked, and those semi drivers backed up. WAY BACK.
