[wwest]

Believe it or no, Porsche designed your system knowing the result would mean a slow leakage of the R-12 refrigerant. The owners manual for my '88, as likely does yours, states so in no uncertain terms.

It appears, according to the engineering technical white papers available from quite reputable sources, that "curing" the A/C hoses with the previous use of r-12 will actually prevent R-134a refrigerant from leaking.

So you may not be looking for an actual leakage fault as it is highly likely that the R-12 charge was lost by design.