Had a catastrophic failure of the bearings on one hub of my 2007 Extreme trailer. These were "oil bath" hubs, which use oil instead of grease and have a sight glass on the end.
My dilemma was to fix/replace the one wheel or update all the hubs to a newer style (or conventional grease).
Here's what I found:
1) Grease hubs (i.e. "buddy bearing" or caps) PROS: proven with proper maintenance, can run temporarily even if grease is low. CONS: buddy bearings caps will pop off with excessive heat, such as braking on hills. Too much grease also causes over pressurization. More frequent maintenance.
2) Oil Bath hubs PROS: gear oil runs cooler than grease. Relatively low maintenance
CONS: Once they leak, if not caught quickly, will lead to catastrophic failure quickly. Slow, undetected leaks also cause water to displace the oil
3) UFP Vault hubs PROS: Uses a hybrid grease/oil, which acts like grease at cooler temperatures and heats to an oil which keeps the bearings at lower temperature, sealed system that are maintenance free, 5 or 10 year warranty. CONS: Difficult to service, more likely to replace. Still more likely to fail with a leak.
After talking with multiple trailer shops about reliability, and also hearing UFP's pitch, I chose to replace my oil bath hubs with Vault style hubs. Unfortunately the spindles are different vs an oil bath so they also needed to be replaced.
I found the best deal was to buy the kits, at roughly $200 a piece, which included everything: trailing arm, spindle, bearings, hub, rotors, calipers and pads. This means with 2 bolts you can replace everything, without rebuilding or replacing a portion of the setup. Purchasing 4 kits basically gave me a new trailer from the frame down, with the latest and greatest. Swapping the kits are easy.
1) Mark the axle
The trailing arm / spindle fits over the axle and determines the trailer ride height. Before you remove the spindle, it's best to mark the position of the old trailing arm to keep the same height, or understand it's reference. I used a black sharpie, marking where the split is relative to the axle, and then used a chisel with a hammer to make permanent mark. You can also count the number of teeth down from the top center of the axle.
2) Remove the old spindle.
Remove the spindle bolt with a 15/16" wrench and socket/breaker bar. Then you'll need to split the trailing arm bite with a wedge or chisel to open it up. A hammer and chisel is the fastest way vs prying it off. Rust typically develops on the axle forming a tight bond. Once you "open" it up it will be much easier to pull off. You'll also need to remove the brake line from the old caliper. Some trailers have just one line to the caliper, and others use a daisy chain.
3) Clean the axle
Usually there's a fair amount of rust on the axle from years of dunking the trailer in water. It's best to quickly clean the axle with a wire brush, throw some WD40 and perhaps a little grease over the axle prior to putting on the new spindle.
4) Install new trailing arm kit
Line up the split in the trailing arm with the indicator mark you made earlier, and then gently press it on until it slides evenly. You can use a rubber mallet to tap it on further as needed. The axle has a cutout in it for the bolt, so you'll need to push the assembly on far enough for the bolt to slide down evenly. Reassemble the bolt (torque appropriately) and then re-attach the brake line to the new caliper.
5) Bleed the brakes
Bleed your new brakes, starting from furthest wheel out from the master cylinder to the closest wheel.
Replace your wheels and check your height. Your trailer should be sitting at the same height as before but your can of course make adjustments as needed.
Check the hubs for heat as you use the new trailer. Stop often and place your hand on the hub and check.
Enjoy the new hubs. This method worked well for me.