On our last trip out west I noticed on the TPMS that the left dolly wheel was running warmer than the right and there was a little oil on the wheel. I also noticed the left side "Vault" hub piston didn't appear to be poking out of the hub, the right side looked OK.
When we got home I pulled the wheels to inspect the brakes. WOAH. The outer pad on the left side was worn almost completely out. the inner pad was worn a little, but was heat stressed and cracked. The pads on the right side looked almost new.
It appeared that the oil was leaking out of the axle from the rear seal, and the oil was causing the brake to bind.
I contacted UFP (The folks that make the axle for Acme) and their tech walked me through how to replace the brakes and rebuild the seals on the axle.
He also gave me a couple pointers on how to make the job easier, i.e. drill a hole in the lower frame so you can reach the lower caliper bolt with a socket wrench.
Also suggestions on how to make sure the leak doesn't happen again.
They sent me new seals and grease for the both hubs at no charge under their 5 year warranty.
I wrote up the entire process, with photos on my sire:
Very good customer support from UFP.