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Personally I think you have done a good job of diagnosing, an unknown to you system.
You went through the proper steps to verify that you have lost refrigerant by studying the low pressure switch. When you jumped the switch connector that concluded it takes pressure to operate the magnetic clutch and electrically verified a good ground circuit.
Those relays are sometimes a time delay relay on the cars! Just thought I would throw that in the mix! (:-)
You apparently have not opened the system to know if there was the any gas at all? Or failed to mention that detail.
If there was a slight pressure in the system, that will keep out air or moisture under most weather conditions.
So your dryer portion of the system should have stayed in good condition.
Never pull a vacuum and leave it that way. Always leave a slight pressure if possible.
The evidence of a colored slime is most likely a leak detecting dye. Since its under the that valve cap, protected from the outside environment, the material under there should not be corroded.
The valve under suspicion is a Volvo factory "assembly line" use only valve.
It appears to a special high volume designed port when compared to a Schrader port.
That's it's down fall. The size of the seal/sealing surface area inside, time and the spring.
This thing was made for the assembling line so as to have a "one an only, fast port."
They pull the vacuum, fill the system in one fell "swoop" right into a receiver/accumulator canister to boil off and equalize.
I have not seen an access fitting like the one used before. I am not aware that a female one is offered anywhere on my side of the trade. But then, I was never a professional "automotive service technician" across many brands either.
When "We" need to go that big, we use actual "service valves" that have metal seats with slip on wrench handles for the stems.
I explained my position about this, So this way, I can call it a "unique piece of crap" today!
That thing is probably connected to the GM's think tank.
The place for saving time & labor, all in their favor. A Plus, might come into play for some of those future "special tools" sales.
Volvo likes to present those in manuals as well! Though, I have not seen it!
I am aware that the valve CAN and does leak! Along with an occasional crankshaft seal on these compressors.
These are the only leak points I have ever had on my system.
I super glued a very flat metal slug over the top mine to seal it. If that had not worked for me I was very prepared to just silver braze it shut!
The "O" ringed cap works well but is not infallible. Just like the dust caps on a car tires! There for more or less as a back up.
If you have a bad compressor seal or going bad, there will be a film of oil secreting on the face of the clutch plate or spinning off beside it onto the frame of the car.
Look for some there and clean that area to make sure it's dry before recharging!
It's a ceramic/carbon face sealed with an oil film but all is supported by neoprene.
They leak after long periods of non use as the oil flim evaporates and the carbon sticks in the residue or skips onto a dry spot.
Upon startup it "can" rip a surface path. Most of the time it wears back in but age catches up to everything.
Maintenance, is to exercise the A/C every few months or less! Use it or lose it.
Use it to defrost with heat during winter. Even on foggy, rainy wet shoes days! It helps tremendously to keep the car dryer inside.
Make sure to do a 30 minute leak down system test upon recharge.
I personally, Rotate the clutch face plate shaft by hand a few turns during the waiting period.
This gives me sometime to do, besides pick my ... nose! (:-)
As far as opening a Pandora's box, (:-) this box it has multiple lids!
They are call the Doors, on any car!
Think of the A/C as a cookie jar and you like to have a cookie every once and awhile. (:-)
You know you have to smile some, to get your lips around a whole cookie!
Phil
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