Larry and all other responders,

Thanks for all the advice. We learned a lot from it.

On the R12/R134a issue: too late. We converted from R12 to R134a years
ago, and the system was working fine with R134a for a long time -- now,
no longer. When we installed the last compressor (marked clearly
'R134a' all over it) last month, we evacuated the system and put in a
new dryer, a big accumulator, and of course, gas. The compressor comes
with oil in it, as does the gas, so we didn't add any more.

We put in enough R134a to just barely fill the sight glass -- maybe
that was too much! The expansion valves are on the original factory
settings -- 2 turns closed from fully open. There are 2 low spots
below compressor level in the low-pressure copper tubing: one where the
tube runs under the floorboards from the fridge to the engine
compartment, and one at the bottom of a full loop just before the
compressor.

When we last charged the system, we had 140 psi on the high side, and
something below atmospheric pressure on the low side, when the system
was running and the plates were cold. When we closed the valve on the
high-pressure side, we got 150 psi on the high side and a full vacuum
on the low side. That was the day before the compressor blew. Now it
doesn't draw down to a vacuum (only to about atmospheric pressure) and
doesn't hold the pressure differential at all, after shutoff. So we
figure it's blown.

We didn't hear any knocking sound before the compressor blew but of
course the engine was running at the time and the engine room is
sound-insulated from the cabin. Initially, we did see a light fast
flow of freon across the bottom of the sight glass, then nothing. We
waited 5 minutes, turned off the system, hooked up the gauges, and
discovered we had no pressure -- and a blown compressor.

We don't have a high-pressure valve. We do have an electric clutch,
but in practice it never cuts out because we never run the fridge more
than about an hour at a time, to avoid freezing the lines. We also
check fairly regularly after we've recharged the system with R134a that
we're not frosting the return lines.

So right now I'm concentrating on three possibilities: (1) we may have
overcharged the system, causing fluid to be forced through the
expansion valves and back into the compressor at startup; (2) the
expansion valves might be clogged up and the compressor starved for oil
because of the lack of a return supply; or (3) the new compressor
simply blew -- we discovered that 3 of the nuts on the compressor head
weren't fully tight. But that in itself wouldn't cause the compressor
to fail, would it?

The gunk on the valve filters certainly could have come from
incompatible gasses. We used to get the unit refilled from time to
time by 'professionals' and we're pretty sure at least one of them put
in the wrong gas. That was 2 years ago, but still ...

So, what to do? Apparently, compressors can't be repaired (though it
sure looks like one could replace the head gasket), so we have to buy a
new one. When we do, we'll open up the valves and inspect them,
replace the dryer, vacuum-pump the system, then barely fill it until we
see liquid running across the bottom of the sight glass. What else?
More oil? Lighter-weight oil because of the low operating
temperatures?