R134a service ports - thread?

The plastic cap that goes on the service fittings are the "main seal" to the ac system. The schrader valve is not. Are you missing a cap or is it cracked or something? Just buy new caps.

You got that the wrong way around. The cap is just to keep debris out of the service port, there's no way it can keep in the pressure from a leaking Schrader valve. The Schrader valve is the main seal.

Before taking a drastic approach, I'd recommend getting one of the Schrader valve removers used for tire valve stems, and using it to ensure that the Schrader valve is nice and snug.

Then, get a set of new caps, put a wee bit of rubber cement (the kind used for tire repair) in there, and then tighten the caps good and snug.

Good Luck!

If you think that then you better think again. There is a rubber seal in the top of the caps. They "DO" hold ac in the system. Everytime I decap a good system the cap hisses because the schrader valve doesnt do jack in the long term. This is even an ASE question IIRC.

Many of my AC caps have O-rings in the top of the cap and will seal the system. I have one tractor with a damaged schrader valve that has been holding pressure for almost two years with the cap providing the sealing. I have been waiting for the system to need recharging before replacing the bad schrader valve. So far I have been waiting in vain.

Regards,

Ed White