Although many techs know (or suspect) this, it may surprise DIYers to know that service port caps are THE primary seal for a vehicle’s A/C system’s service ports. Many erroneously believe the Schrader valve provides the seal for the ports, but in fact the valve merely provides access to the system and still requires the cap to provide the seal.
When diagnosing a system, the first and most important step should always be to ensure that the service port caps are in place and properly installed (with o-ring inside and tightly screwed on). Likewise, it is imperative the cap is properly replaced after servicing the system to ensure the charge is held (assuming there are no other system leaks).
This needn’t only be checked when performing A/C service. Routine inspections to confirm the service port caps are in place is good practice. If one or both is missing in a customer’s vehicle or your own, you should install new caps. This will help to prevent refrigerant leakage and also help keep the A/C system performing at optimal performance by preventing debris and rain entering and damaging the system.
FJC offers high and low side service port caps—Part #s 2614 and 2615. Every shop should have an adequate supply on hand.