GHEN,
Both are correct. I missed it. Your post(s) had mentioned R12.
Retrofit red/blue caps will snap on the retrofit stems that screw over the old R12 stems.
If the compressor clutch works after the low pressure switch test, the next step is to buy a set of AC hoses and dual guage at Harbor Freight.
With the guages you will be able it fill with R134a 1/2 can or so and watch/look for leaks.
Walmart+R134a about $10 a can, get 3 or more. (to be discontinued)
Does the compressor have an oil leak on the front coil/pully leaking down the compressor?
If you cannot see or hear a leak, you might just try to complete the refill.
A vacuum would be better but some have been lucky just to add up to 2 cans of freon and a litttle compressor oil/freon mix. Maybe a freon/oil and leak sealer detector can ???
You can always go back and vacuum and do it correctly if it does not work. Its only money.
GL
John