If your system is intact and fully charged I suspect your problem is with the aux cooling fans. These are more important than some might think. If not working, the high side pressure will climb off the scale at idle and on very hot days. The high pressure does two bad things. It reduces the ability for an effective heat exchange (hence the poor cooling), but also submits the entire system to a lot of high pressure stress which ultimately will cause component failure (ie: hose couplings, sensors, etc.) So if it isn't leaking now, and the fans are not operating properly, it will probably leak eventually. Not a good thing with an R12 system.

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Bart

2002 ML500, 2009 CLK350 Cabriolet
93 300D 2.5 Turbo 265,000 Miles (Sold - bad idea)