Impossible. All vehicles sold in the US since 1993 use R134a. Either your truck is older, or you are mistaken as to the refrigerant.
neither of the 2
>shops I trust has any left to re-charge the system, but one still has
>the equipment to recover and re-use the R-12.
A shop that does not possess recovery equippment is not qualified to do A/C repairs. Also, it cannot legally attempt refrigeration system repairs. It can do electrical A/C work.
The A/C worked last
>week after having it to one of the shops and they jumped the
>compressor to test it for operation, it worked but they didn't want to >go farther.
This means that they are smart enough to stay away from work that they are not qualified to do.
The A/C worked when I went to pick the truck up, so I was
>happy.
>
>2 days later, it stops working again, jiggle the connection on the
>pressure switch in the accumulator and the compressor kicks on, GREAT!
>Not working, AGAIN. I jumped the 2 contacts on the lead for that
>pressure switch and the compressor came on, the accumulator got nice
>and cold and the air coming out of the vents was cold. Would this
>mean the pressure switch is bad
That is one possibility. There are others.
and I can take this to the shop with
>the recovery equipment to have them replace the switch?
You can do whatever you wish. I'd suggest that you find a shop that knows how to do A/C work. For what it's worth, the low-pressure switch should be mounted on a Schrader valve, and would not require discharging the system for replacement. A competent A/C shop would know this.
>BTW, I'm not spending $250 to have the most-trusted shop disassemble
>my A/C system and do a proper conversion...
Cold air costs money. How comfortable can you afford to be?