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1. Good news: You don't have the horrible 254mm (10") leading-trailing shoe rear drums. You almost certainly have the JB/JN 6, 11.x Duo-Servo rear brakes.
2. Looks to me like the adjuster is fully-retracted. Good chance that the service brakes are WAY out of adjustment. Could be that the adjuster is seized or otherwise malfunctioning. The service brakes have to be properly adjusted BEFORE you adjust the park brake. You MUST retract the park brake lever at the drum assembly BEFORE adjusting the service brakes--the shoes MUST BOTH TOUCH THE ANCHOR PIN at the top of the brake assembly before the service brakes can be properly adjusted.
3. Why is the interior of the brake wet? Leaking wheel cylinder? Or did you spray lubricant on the exposed part of the park brake cable, and get a lot of overspray?
4. When the park brake pedal won't return, it's almost always due to seized cable(s) in the park brake system. I suppose that the park brake pedal assembly could be stiff/seized, but I'm betting that the cables are rusted. You could spend the rest of your life trying to lube them, and maybe you'll have success. When it's me, the seized cables go in the scrapmetal bin, and I install new cables. New aftermarket cables are
surprisingly inexpensive; (in part because they're cheap Chinese) but they're not a perfect match to the OEM cables in terms of the mounting brackets and such.
The short story here--that I'm inferring from a
single photo of
some of
one brake assembly--is that your park brake is perpetually applied because the cables are seized. That is forcing the park brake equalizer bar under the wheel cylinder to spread the shoes apart. And THAT is preventing the service brakes from properly adjusting. THAT is MAYBE causing a wheel cylinder to over-extend and leak. It's also why the park brake pedal can be pressed, and not stop the truck--the park brake is always applied, but the service brake adjuster is so loose that the shoes don't put pressure on the drum.
Given the non-rusted condition of the brake shoes, and the very thick lining material, I'd say the last guy to service the brakes had no idea what he was doing.