An automatic pressure relief feature prevents over-filling and over pressurization. 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. Bearing Buddy will last the life of your trailer. The outer barrel is made of stainless steel. Internal Bearing Buddy parts are also made of stainless steel for maximum corrosion protection.