Klaus,
Quite a story, she didn't leave you stranded though.
There is a high pressure switch in the system that is supposed to turn the compressor off if it gets too high.
where is the "high pressure valve?" There is no mention if it in any AC service information that I have.

From service info.
A/C PRESSURE SWITCHES
The A/C compressor is connected in series with the low pressure
switch, high-pressure switch, and safety switch. The high pressure
and safety switch cut power to the A/C compressor if pressure
in the A/C high-pressure circuit becomes excessive, supplying a signal
to ECM to start cooling fan.
The low-pressure switch (pressostat) turns A/C compressor on
and off to maintain pressure within limits.
--
Tom 69-1800S, 72-1800ES, 96-850T,2000-S70 GLT-SE