Automatic Transmission Seals
Automatic transmission seals are used to contain pressure and prevent leakage.
These seals are made of metal, synthetic rubber, and Teflon. They can be either positive or non-positive. A positive seal contains all fluid while a non-positive seal allows a small amount of fluid past for lubrication of moving parts.
Rubber Seals
Lip Seals: Lip seals are ideal for both 
rotational and axial movement. They are used to apply pressure. Piston seals are lip seals vital to transmission performance. This type of lip seal has the lip on the outside of its diameter. As the high pressure fluid pushes on one side of the lip it forces the seal firmly against the cylinder wall forcing the piston forward. They are also used to contain fluid from leaking past the transmission and axle shafts. This type of lip seal has the lip on the inside of the diameter supported by a garter spring. This spring is used to apply constant force as the seal wears.
O-rings: O-rings are usually used for static sealing. A static seal is a seal between two non-moving parts. A dynamic seal a seal between two moving parts like the lip seal. They are great for creating a positive seal against fluid leakage.
Square Cut: Both the square cut and the O-ring seals are installed in a groove cut into the part it is being fit into. O-rings are mostly used in static applications because they tend to roll in the groove when the parts are moved against each other. Square cut seals have less movement in this groove due to the square cut design. They allow for more axial movement than the O-ring.
Due to time and wear these rubber parts will harden or become mushy. This will greatly decrease the transmissions performance. If the transmission is loosing pressure and beginning to slip an overhaul will be necessary.
Ring Seals
Metal Ring Seals: Metal seals are usually used around shafts to provide non-positive sealing. These seals will fit into a groove in a shaft and will have a gap between the ends of the seal like a washer. Teflon seals are used for the same purpose and offer less wear on the related parts. Sometimes there is a small fastening hook on each end. These seals also come in one piece and require a special installation tool.
