Importance of "dust caps" for schrader valves

[Noquacks]

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.

[Cusser]

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.

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.

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.

[GM Tech]

When I worked for Delphi Thermal, the refrigerant lines and hoses people informed me that the plastic caps are designed to be the primeary seals- thats why they have a rubber element or washer or seal in them. The shrader valves are there to hold in the refrigerant just long enough to get the caps put on which are now the actual refrigerant seals is what hey told me.

[363ls2gto]

I came to this website looking for good information. After seeing this thread I am not so sure.

Please try removing your dust cap for 60 days. Measure before and after pressures with a quality manifold gauge.

Try removing your schrader valve for 60 seconds. Now measure before and after pressures with a quality manifold gauge.

Actually, please don't remove your schrader valve as venting to atmosphere is bad.

Try the same experiment on a tire with 15psi and safety goggles.

Now imagine a similar cap holding back 150psi in your AC system.

Please tell me now if the schrader valve or plastic cap is the primary seal.

[JohnHere]

Caps designed for a tire valve and for a Mobile Vehicle Air Conditioning system's access ports are different, as are the pressures, which has been correctly pointed out. On an MVAC system, the caps are the primary seals while the Schrader valves are the secondary seals.

[Cusser]

When I worked for Delphi Thermal, the refrigerant lines and hoses people informed me that the plastic caps are designed to be the primary seals- that's why they have a rubber element or washer or seal in them. The shraeder valves are there to hold in the refrigerant just long enough to get the caps put on which are now the actual refrigerant seals is what they told me.

GM Tech is the expert of experts.

[363ls2gto]

Forget tire Schrader valves and caps. That example was used so some idiot does not go and remove the schrader valve on a pressurized AC system.

Maybe there is a language difference here.

If you remove the cap from either the high or low side of your AC system you may get a very slow leak at worst.

If you remove the Schrader valve on the high or low side your system will depressurize to atmosphere violently in minutes.

The definition of primary:
1) of chief importance
2) earliest in time or order

Please tell me what you disagree with here.