Service brake problems in wet weather

[Chamberpains]

Have you had anybody crawl underneath your truck and listen to the brake Chambers while someone holds down the brake pedal? Internally leaking parking brake Chambers will backfeed air into your braking system causing all kinds of weird breaking problems.

As far as replicating your problem, forget about the buckets of water get yourself a garden hose and soak piece by piece everything you can. Meaning start at the windshield dump the hood over and soak the windshield and the firewall go for a drive soak the drums go for a drive soak the valves go for a drive and so on. It's a slim chance that you will be able to replicate it. But every now and again this method shows something. Especially if you say it only happens when it rains.

It has me racking my brain because you said the brake pedal gets really stiff. That kind of sounds like an exhaust port on a valve getting plugged. Some valves are designed to exhaust a little pressure, very little pressure, to allow an internal cylinder to slide. If those exhaust ports get plugged a little they completely stop working.
I had a manifold valve on the firewall get plugged once with dirt. It caused the trailer service line to cross feed into the parking lines. I know it sounds really weird and I never took the time to find out why it caused this but it did. I cleared the obstruction and all was fixed.