This O-ring seals the connection between the hydraulic lift cylinder and its mounting bore on the host tractor. It sits in the groove of the lift cylinder port, preventing pressurized oil from leaking past the joint during implement raise and lower cycles. A degraded or nicked O-ring allows external oil seepage and can introduce air into the circuit, causing spongy hitch response. The part is manufactured to the original cross-section and durometer specification, which maintains the correct compression set in the groove so the seal does not extrude under peak system pressure. On replacement, clean the groove thoroughly and lubricate the new O-ring with hydraulic fluid before installation.