It is very possible that you have a leak in the system. You really need to have a proper diagnosis performed before you refill it. No point in charging a system that does not hold pressure. It really depends on how well the conversion to R134A was done. Usually ones that were not performed with shortcuts will usually result in marginal results or a component failure. As well, it sounds as though the heater valve may be acting up. You may want to do a search to see if you can carry out more tests on it.

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1989 300CE