Originally Posted by Arthur Dalton

The service valves on 134a are not the tire type valves as R12 uses.. They are BALL check valves .. The most common reason they leak is dirt/debries gets on the surface of the Ball.. an easy remedy is to charge the system and then turn the car OFF so the low side pressure comes up to equalize.
Higher the ambient , the higher the pressure will be.
You then take a small dowel of wood and push the ball in a few quick jabs.. The refrigerant you release is under high pressure and that will clean of the ball and allow it to fully seat. Then put the cap back on.
I have fixed many a leaking charge port valve in 2 seconds by simply pushing the ball in once after a charge to make it seat.................

I do this with all valves, even R 12 type and tire valves. It is the inside pressure that helps seal the valves.

Ever notice a tire leak at the valve stem and all you had to do was push the valve pin IN once and the leak stopped ????
Same deal.............................
And you know how you spit on the tire valve to make sure that is not leaking....well, you do the same on the R valve, but you just use a little refrig oil..if they leak, you will see it in the air bubble from the oil you covered the valve ball with............