With the conversion to R134a fittings and it worked well last year, the most likely diagnosis for your condition is a shortage of R134.
But to eliminate the unknown possibility of a mixture by anyone , I would simply evacuate , vacuum , and recharge with corrcect amount of R134.

Youi may even get away with a simple topping off of 134a, but a full vac would be best.


No one converted those fittings to r134 to put R12 in there, but they MAY have not vac all the R12 out.....
My bet is the car has been converted from R12 and simply has a slow leak.

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A Dalton