Our piston can be opened to access and change the seal inside. Plastic pistons are factory sealed and can only be replaced. We now use TEFLON SEALS on the piston they last longer perform better and withstand the heat very well. So many of these pumps are using steel metal piston sleeves & as you know compressed air carries water in it.
The water in the air will corrode this sleeve, tear the seals, and seize the piston. This sleeve corrosion renders the pump dead as the cost to replace (If it is even available as a part) is very high. Our pump has an aluminum sleeve with a proprietary anti corrosion coating. Although excessive water will ruin any pump our sleeve will last a very long time. As the pump pushes oil out it must inhale air into the reservoir. These plastic plugs are constant.