Hi Mick,

Something just occurred to me about your situation. If the evaporator
was restricted (not blocked or perforated)like a crimp in the evaporator
"tailpipe" or outlet, the following might occur:

1) The high side would NEVER be high because the compressor could not
get the necessary R12 vapor quantity due to the blockage.

2) The low side would NEVER be low because the vacuum of the compressor
could never act fully on the refrigerant vapor due to the blockage.

3) "Massive" amounts of refrigerant may be needed to counter the
evaporator blockage and get any cool down performance.

Think about this Mick and talk with your technician (if you haven't
choked him yet). I may have just thought myself out of a trip to
Hawaii....................

Regards,   John