For 17 years my car air conditioning has worked great but on each valve when I removed the screw cap I would hear a brief hiss (more like a 'pffft'). Obviously there is some fugitive leaking going on but I don't want to change the valve yet.
Here is how I did a simple and really low cost way to help with this minor leak.
Step 1: Accessing the Valve.
Once the cap is off you can see the valve stem in the second pic with a hint of oil on the threads.
Step 2: Pag46 Oil.
This is the oil my car uses in its air conditioning system. The 8oz bottle is about 6USD on Amazon.com.
Step 3: Adding a Little Oil.
Using a tube I took a little oil by capping the free end with my finger. No need to suck siphon... Ewwww.
You can see the volume I put into the valve. This oil will keep the valve rubber seal a little flexible instead of being hard. I actually did this on the low pressure valve and there is no more hissing when I open the cap so this is what inspired me to do the same on the high pressure valve. Both valves show no telltale signs of bubbles in the oil that is a giveaway for a slow leak.
Not bad for a 2 minute fix on a 1999 manufactured car!