I have changed a fitting my (87 cougar) a/c that has been converted over to R134A a few years ago but the guts in the other connection are leaking/bubbling. Are the guts the same as a tire valve stem or are they different?

I put 2 12oz cans and stopped because the gauge that came in the kit showed that I was in the correct level. Today on the way home from work, the a/c didn't work very well. It blew cold and then changed to warm air and back and forth. I think that I need to add more. I didn't use a vacuum pump to evacuate the system because I don't have access to one.

Yes, the schrader valve is like the one on tires. The core can be replaced, the autostores have kits in the A/C section. There are even tools that allow the core to be changed without discharge the system. I suspect they cost more than the refrigerant.

The R12 - R134a adapter, on the other hand, is an extension only and not sealed. They also are adjustable.

I think my core replacement tools that let you change out the valve core on a fully charged system ran around $50. While the schrader valves are "like" the ones on tires, they are a slightly different size. Larger.