As BBmarine noted, this was not a part of the OP question but since BB commented I will also. Not everyone will agree but on any trailer that I've even owned where I've used Original Bearing Buddies I have always completely filled the hubs with grease. The intent is to fill all cavities with grease so water can't get in. This means getting all the air out of the hub and replacing it with grease. Bearing buddies are designed to do this. I couldn't attach the link but the information down below is a copy and paste from Bearing Buddy web site.
One thing I also do and may not be necessary but when I pump grease into my hubs, (when new and also as they age) is I jack the wheel and spin it as I very slowly pump in the grease. I find this help in removing the air, especially when new. When the air is removed and the hubs are full there's nowhere for the water to penetrate until your rear seal starts to wear and grease starts to seep out. When topping up the hubs as time goes by, it only take a couple of pumps. If you overfill, it will either squeeze out around the seal (making a mess on the inside of the wheel) or will squeeze out the front of the bearing buddy. I've heard guys say that they have popped the rear seal by adding grease. Either they have had a poorly fitted seal or they have pumped in too much grease and too fast as Bearing Buddys compensate for a bit of overpreasuring. Same goes if the grease expands due to heat but I think that if your hubs get hot enough to expand and pop your seal,,,you have other issues. (Just my 2 cents)
Copied from Website:
How Genuine Bearing Buddy® Works
The axle hub is filled with grease until the grease forces the Bearing Buddy® piston outward about 1/8 inch. Because the piston is spring loaded, the piston exerts a slight (3 PSI) pressure against the grease, which maintains a slight pressure between the inside of the hub and the outside environment. When the hub is submerged, water cannot enter the hub because of this pressure. An automatic pressure relief feature prevents over-filling and over pressurization. See Bearing Buddy diagram: Without this feature, the inner seal will be damaged.
Grease can be added to the hub through an easily accessible grease fitting located in the center of the piston. Lubricant level (and pressure) can be checked quickly by pressing on the edge of the moveable piston. If you can rock or move the piston, the hub is properly filled. In addition to packing the bearings in the traditional manner, it's best to get as much grease as possible inside the hub cavity between the bearings and then, while the Bearing Buddy® is off, fill it with grease as well from the backside. The air pockets inside the hub cavity and Bearing Buddy® will work their way out past the piston (or blue ring) on the Bearing Buddy®.
You should see the piston extend out or the blue ring past the front of the Bearing Buddy® when it's properly filled. However, as air escapes the piston (or blue ring) is going to collapse inward. This is normal. After driving several miles, check the piston (or blue ring) to see if it has moved inward. If it has, add a little more grease to get the piston to move outward or the blue ring to move about 1/8" past the front of the Bearing Buddy®. You may have to do this a couple of times before all the air pockets are gone and the entire hub cavity is filled with grease.
Bearing Buddy® will last the life of your trailer. The outer barrel is made of steel and is triple chrome plated. Internal Bearing Buddy® parts are made of stainless steel. Bearing Buddy® is also available with a stainless steel barrel for maximum corrosion protection.
