I don't know about the size, but looking very closely at the photos, I'm pretty sure I recognize bearing caps for through the spindle lubrication, commonly mistaken for bearing buddies, but a whole different thing.  If you pry off that rubber cap (I'm pretty sure that's what it is, not a "pressure membrane", I believe you'll find a grease zerk in the end of the spindle.

The idea here is that fresh grease goes in the grease zerk, through a passage in the center of the spindle, and then back out toward the front through the bearing assemblies.  Here's a link to a cutaway diagram:

8_60da3e23-a157-4fac-b535-73752ab7fd89.jpg (634×761) (shopify.com)

One commonly sees this arrangement on large boat trailers and and almost universally on commercial trailers.  jIn my opinion, it's the way to go for keeping you boat trailer bearings healthy over a long period of time.  This was how the Load-Rite trailer for our 2900# Menger 19 catboat was equipped, as well as my old Sea Pearl 21.

In over 15 seasons with thousands of miles of trailering to distant venues and many dunkings, I never had to disassemble the catboat's wheel bearings.  I carried a little grease gun (I like the kind which use the small grease cartridges with a short, rigid spout on the squirty end) as part of my kit and work some fresh grease in through the bearings from time to time.  That rubber cap is just to keep any loose grease from getting on your boat as you go and to keep water and gunk away from the spindle, which it does well.  Never had any evidence of water intrusion into the bearing assemblies, and never had to replace the bearings.  Of course, that might be because, on a long trip, I always had everything I'd need to completely replace the bearings should the need arise, thus never needing to do that.

My experience with bearing buddies on an older boat trailer with regular bearings was less happy.  Before I got talked into trying the bearing buddies, I'd disassemble, inspect, and repack the bearings annually.  Never found any evidence of water incursion using double lipped seals and Teflon plumber's tape in the joints when I put things back together.

After installing the bearing buddies, I would soon find grease working its way out the rear seals and onto my boat (an old Highlander), no mater how careful I was to avoid over greasing the thing.  Worse, when I disassembled the bearings at the end of the season, I did find evidence of water incursion, probably because of messing up the rear seals by becoming less effective and maybe slightly dislodged over time.

I left the bearing buddies off in favor of new dust caps when I reassembled things...though I did keep the heavy things handy on my tool caddy in the garage for possible use as anti-predator missiles in case of predator incursion when I was working with the garage door open, though the need never arose.  Didn't have a lot of junkies looking for stuff to steal in my neighborhood.  I think those old bearing buddies would have worked better for that purpose than they did for the boat trailer, though.  <;-)

Your milage may vary, of course, but that was mine.  I was very please to discover that my new '89 Sea Pearl came with a trailer equipped with through the spindle lubrication.  When we were working with Bill Menger to build our catboat, he mentioned that the Load-Rite trailer would have "bearing buddies", I replied, "Please tell me you mean through the spindle lubrication."  "Um...let me check," he replied.  A quick phone call to his Load-Rite rep soon resolved his confusion.  It is a common confusion.

Anyway, gently pop those rubber pieces off the dust caps, and see if you don't find grease zerks in the end of the spindles.  If so, count your blessings.  If not, well, "pay no attention to the man behind the curtain."

.....Michael

   You, sir, are correct!  I took one off and it's exactly that.  Hurray, one less task.  I will want to get a small grease gun like you said, but it still looks very fresh in there, considering the thing has been back and forth around MD a lot and then up and down New England already.

A few things I learned about trailers, mostly from my uncles, who were all mechanics one way or another, and my little brother who was a mechanic in the Army...but none of whom had anything to do with boats (I come from the longest line of landlubbers you could envision and have no idea where all of this urge to mess about in boats came from!):

1. On a long drive, take a few minutes to check the trailer over whenever you stop for a break, before you do anything else.
2. Gently feel the hubs with the back of your hand to see if they are hot.  If they are too hot to touch, bearing trouble lurks around the corner.
3. Check the tires in the same way.  If they are very hot, let 'em cool down and then check the air pressure, which you will likely find to be low.  You'll want to deal with that soon (I always carried a tire pump).  Underinflated tires overheating and blowing up at speed is the commonest cause of spoiling a good trailer sailing adventure.
4. Once you've established that the tires aren't overheating enough to burn your hands, grab the wheels at the tops and give 'em some firm tugs in and out.  There should be a tiny bit of play in the wheel bearings, but any real slop indicates bearing trouble lurking around the other corner.
5. Walk around checking for any shifting of your load (a boat, in this case), checking the straps as you go and looking for any chafing, either of the straps of the boat.
6. Check that the hitch ain't fixin' to come loose.
7. Might as well check the trailer lights while you're at it.
8. Check anything else where screws and bolts might be loose.  I believe that the best way to find out if any assembly will come apart is to strap it down to a boat trailer and drive it 500 miles.  If your assembly hasn't shaken itself apart by then, it probably won't.

After all that, then go get something to eat, hit the head, or whatever else it was you stopped for.  <;-)

.....Michael

P.S., I wish you all joy of carefree boat trailering!

My Trailex SUT-250 has a Dexter E-Z Lube spindle, I believe. The manufacturer has some how-to videos on YouTube. 

Just got off the phone with the helpful folks at Trailex. Thought I'd pass this on. Might be helpful, if like me, you're new zerks, nipples, and alemites. You pull the little rubber cover off the axel (spindle? hub?) and undernearth is a standard zerk (the thing into which you squirt the grease). Buy a standard grease gun. Any standard gun should be compatible with the zerk. Don't need anything fancy, such as high PSI. The bearings are shipped with non-sythetic marine grease. So don't squirt in any synthetic because they don't mix. "Don't let the people in the store try to convince you to buy synthetic!" Apparently nowadays sythetic is better, but you'd have to swap it all out to do it properly. So I've been told.

   John,

Good to know about the grease type, thanks.  My big grease gun still has chassis grease in it so I'll just get one of the compact ones w/ the right stuff.

BTW, I found one of the aluminum angles holding the bunk board had cracked and broken right at the corner, right before I was to take the trailer for an inspection to transfer the title back into MD (we are back in Chesapeake country!).  So I ran out to Lowes and grabbed what I could to fix it.  I'll order some better metal at my leisure.  The brackets are just standard square edge profile aluminum angle, and road shock fatigued it right where you'd expect.  Just a heads up to check those bunks because it was invisible until I yanked and wiggled the bunk.

So Michael (Gramps), from that linked illustration you put here earlier is it prudent to infer a Trailex owner whose rig is so equipped pull the dust caps off prior to regreasing hubs so as to ensure they're not already packed full of old grease?

Seems counter-intuitive to have new grease go in via hollow spindles when there's no room on the outside to accept what gets displaced?

And yes on the synthetic vs. traditional lubes - same applies to what gets added to your engine's sump when topping off/changing oil & filter. Best to stick with what they started with rather than changing formulations.

The way I used the through-the-spindle lube was to pump in new grease until I saw a little starting to push through the front around the front.  If that stuff looked okay (no water or junk), the assumption is that the bearing is well and truly "packed" and good to go.  I'd scoop out any excess or grotty lookin' stuff with a finger (keep some DL Hand Cleaner and paper towels in in your kit), put the rubber cap back on the dust cap, and drive off with a smile on my face.

.....Michael

Meant to type "...until I saw a little starting to push through the front around the spindle."  Might not have gotten enough coffee in me this morning.  <;-)

.....Michael

...and, I ought to have mentioned, those little rubber caps will degrade over time, like tires and most everything else on a boat trailer, and the prudent trailer sailor would pick up some spares the next time he was in a good trailer supply shop, like maybe to get replacement bearing components to have so he/she won't need 'em, and have them handy to replace a lost or damaged rubber cap.

.....Michael