Re: replace hub or buy a berrings kit?
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If trailer hubs are < 10 years old, I would simply remove the hub, clean it up (using old rags and dollar store rubber gloves), then take one hub to a trailer or auto parts store. For example, UAPA type store. With your old hub in hand, they can match new seals for it.
If trailer hubs are > 10 years old with lots of road miles, I would simply remove, clean them up and take to the trailer / auto parts store. I would then replace both hubs. If > 10 years old with low road miles, I'd get new seals for it. And, replace hub at 15 year mark. Unknown to some folks, hubs and leaf spring packs should always be replaced in pairs (on the same axle). Thus, getting matching wear/tear and matching parts on same axle. And, getting same braking ability (if there were brake drum hubs).
Unknown to some, leaf spring packs and hubs should be replaced at 15 year mark. Leaf springs become brittle after 15 years. And after 15 years, the races into the hubs get worn out as well. Some folks push 20 years (depending on miles) but for me, these items get replaced every 15 years. Just like I replace tires every 5-7 years as well.
If uncomfortable getting fingers in grease, simply take to a auto / trailer repair shop. They'll fix it up. But, they will charge you dearly - for this simple DIY task.
Hope this helps.