Why we should do maintenance
When I got this motor home (93 Beaver on a Gillig chassis) I knew the chassis was really rough, but the cabinetry was nearly pristine. i've changed a lot of parts, I've made a lot of parts but after 3 years it's getting close to being done. The air dryer was really slow to purge, and I figured it was original. It's a Bendix AD4, parts are nearly non existent for a rebuild so I found an AD9 for a decent price and installed it instead.
FWIW it's pretty much a drop in. The AD4 had 1" inlet and outlet openings with reducer bushings for the 1/2" fittings used. The AD9 had 1/2" openings so they screwed right in. The governor line was originally attached to an elbow close to the inlet T. It was too close to reuse the elbow on the AD9, I replaced it with a straight fitting.
I was really curious about the condition of the AD4 so pulled the purge valve out. I wasn't disappointed, I have no doubt all of it was original but am amazed it operated. I imagine the desiccant looks as bad.
Diesels are designed for a lot longer life, one of the big reasons is a lot of the components require maintenance. Two, maybe three, POs of this one apparently didn't know that (or care).
Don't do the maintenance and everything works downstream to where it can become a problem that's a pain/expensive to repair.