Automotive Air Conditioning Information Forum

People,

I heard that the dust caps are a primary device to prevent gas from escaping from a schrader valve, and not a secondary device (the valve being the primary). Is that true? Sometimes, when I work on my auto ac I unscrew the cap and notice the psssssst. That should mean that the valve is compromised, and thankfully the cap held in the gas from escaping, at least somewhat.

Also, people, can someone confirm if all caps are standard size/thread for either the low side and high side, meaning, all caps for the low side fit ONLY the low side valve, and all caps for the high side valve thread fit all of those. I need to order some.

Noquacks wrote: Tue Oct 21, 2025 3:26 pm I heard that the dust caps are a primary device to prevent gas from escaping from a schrader valve, and not a secondary device (the valve being the primary). Is that true?

I have learned from the experts on this site that the valve caps are very important and are the primary seal.

Noquacks wrote: Tue Oct 21, 2025 3:26 pm Sometimes, when I work on my auto ac I unscrew the cap and notice the psssssst. That should mean that the valve is compromised, and thankfully the cap held in the gas from escaping, at least somewhat.

In that instance, I would also install a brand new schraeder valve there. There are also tools that enable removing and replacing a schraeder without losing refrigerant.

Noquacks wrote: Tue Oct 21, 2025 3:26 pm can someone confirm if all caps are standard size/thread for either the low side and high side, meaning, all caps for the low side fit ONLY the low side valve, and all caps for the high side valve thread fit all of those. I need to order some.

No, valve caps can be different. I have found that my older Nissan Frontiers have different R134a valve caps than some others.