Oil is found under the brass cap of a service valve stem on a rooftop unit that loses a small amount of charge each season. What is leaking there?
- AThe brazed joint where the valve body meets the liquid line
A brazed joint at the valve body would show oil at the joint itself, on the outside of the valve, rather than collecting inside the stem cap.
- The packing around the valve stem, which seals against the stem shaft
- CThe Schrader core in the gauge port on the side of the valve body
A leaking gauge port core shows oil at the port, under the separate port cap. That is a different cap on a different part of the valve.
- DThe threads of the cap itself, which are not a sealing surface
Cap threads are not a pressure joint on their own, but the cap with its gasket is the secondary seal, which is why oil is found trapped beneath it.
Why B is correct
The stem is a rotating shaft passing through the pressure boundary, sealed by packing that wears and loosens with every use. Oil appearing under the cap is refrigerant and oil working past that packing. The valve cap is the secondary seal that catches it, which is why the cap must be refitted and tightened after every service call.
What this question is testing
This tests the ability to locate a leak precisely from where the oil is found, and it reinforces why the small hardware on a service valve is treated as part of the pressure boundary rather than as an accessory.
On the job
Service valves get operated on every call, and stem packing is the part of a rooftop unit most directly worn by the technician's own work. Backing a stem out fully to the back seat and then giving it a quarter turn in is the habit that keeps the packing sealed during operation. The caps themselves are frequently missing on old equipment, and putting proper caps back on is a real reduction in seasonal losses across a site.
Memory technique
The cap is not a hat, it is a second door. Oil behind it means the first door leaks.
Exam tip
Oil under a cap means the cap was doing its job as a seal. The leak is at whatever the cap covers.
Where to look it up
Service valve stem packing seals a rotating shaft and weeps with wear; the cap is the secondary seal.