Hi PETE,
With a faulty TX Valve the system most likely should retain the gas as long as all of the O-Rings are o.k.
On mr2y's Evaporator Assembly the green dye appears to be from the joint between the TX Valve & the flange on the Evaporator.
Now seeing the clogging of mr2y's Evaporator & anecdotal evidence of other similar Evaporators, I reckon that needs to be looked at as well.
Smoking in the car with the A/C or Heater on, may be another cause of some of the black gunk.
The symptoms of my faulty TX Valve were that the A/C would work for a short time after switching on, & then it would get weaker & stop on one of the safety cut out pressure switches. Probably the High Pressure one.
I reckon that the TX Valve would close & not pass refrigerant.
It is o.k. now but I reckon that I have too much gas in the system.

I can confirm that the following part numbers do NOT fit a 1993 mr2 turbo....
88515-60210 (Toyota part)
447500-2370 (denso - equivalent to above) - the 6mm bolt holes on one side taper to 5mm so you can't put the holding bolts through.
447500-9052 (denso) - the recess lip for fitting against the plates/orings on the evaporator is not deep enough to form a seal.

The old part number from a mates car is 047500-9071. However the part has been discontinued / superseded. Other part numbers given have been discontinued. Auto shops seem to have little knowledge than looking up a book that gives the above part numbers. I should have it sorted in the next day, swapping parts / ordered other part numbers.... I will post up the new part number once I have it.

My advice after seeing the condition of aircon parts in a mr2, take the time yourself, before re-gassing get the system degassed, replace all the orings with new green orings ($30), raplace the drier ($30), remove the evaporator and clean with soapy warm water / gentle hose (allow hours to do this), clean fan and ducting around evaporator, replace the a/c Tx expansion valve ($50-$75 once I find the right part number), if your super keen remove the a/c compressor for service of new oil ($75), new belt ($15) and then regas (under $200).
So all up, doing it yourself $475 parts/srvice. Labour for me approx 2 hours replacing all o-rings, 30 mins replacing drier, 4-6 hours on evaporator/Tx valve in and out plus clean, 2 hours remove and refit compressor/belt after sevice.

Hi Tony,
If Ashdown-Ingram is nearby, I would suggest that you go there with a TX Valve that you know is the correct shape/size.
While the counter person may not know much technically, they should have stock of valves with which you can compare.

Ashdown-Ingram seem to have all the components and parts of an a/c system. Never heard of them before but will remember now.
I am awaiting a part from super cheap as they didn't want to refund the wrong part that was ordered, wanting to replace with another part. See what happens when I get this next part.
I also found a a/c specialist who has the superseded part but wouldn't give me the part number.
With the link to ashdown-Ingram I am confident this will soon be solved.

TonyMR2
I can confirm that the following part numbers do NOT fit a 1993 mr2 turbo....
88515-60210 (Toyota part)
447500-2370 (denso - equivalent to above) - the 6mm bolt holes on one side taper to 5mm so you can't put the holding bolts through.
447500-9052 (denso) - the recess lip for fitting against the plates/orings on the evaporator is not deep enough to form a seal.

Thanks for your findings Tony. Just to be clear, did your 1993 SW20 come factory with R134a gas? My 1990 GT had R12 and the evaporator is slightly different to the R134a version. I was under the impression that the 447500-9052 is the replacement for the R12 TX valve which converts the R12 system to R134a (see my post further down)

Ok found this information from searching the internet and various catalogues:
 
SW20 12/1991 - 8/1993 - R12 evaporator 210x245x90 - Denso 047500-1730
SW20 9/1993 - 11/1999 - R134a evaporator 207X242X92 - Denso 447500-9052
 
It seems you and I both might have the R12 core (I know I definitely do because I measured it)
So far it looks like autofrost.com.au has been the most helpful:-
 
R12 core TX valve - http://www.autofrost.com.au/store/index.php?route=product/product&product_id=753
R134a core TX valve - http://www.autofrost.com.au/store/index.php?route=product/product&product_id=819
 
Please don't take this as gospel just yet, I'm just reporting my findings...
 
Edit: here are the dimensions of my R12 evaporator core (approximate): 210x245x90
The R134a core is roughly: 207X242X92
(added to the part list above)

Peter
On mr2y's Evaporator Assembly the green dye appears to be from the joint between the TX Valve & the flange on the Evaporator.

 
I'm putting two and two together, I believe I have the recalled R12 TX valve, which I assume jammed and would not return to the open position (I bet the spring corroded and broke inside it, I'll be doing an autopsy soon). With a closed high pressure opening, every time the compressor engages it will get an over pressure condition tripping the pressure switch. But in the meantime that pressure has gone beyond the specifications of the o-rings and sprayed out from the TX valve and at the high pressure end of the compressor. Basically thousands of dollars have been spent because of a broken thermal expansion valve failing to return to the open state. Autopsy will be coming soon ...
 
Edit: I should mention the R134a version of the valve will work at a slightly higher pressure so it's important to get all the seals replaced and working. I don't believe there's a different pressure switch (the one located inside the evaporator box) I think it should work just fine for R134a.
 
Edit2: I just realised the spring in the TX valve pushes it closed (not open), so it's more likely the gas in the pressure bulb has leaked out, so an autopsy wont reveal anything.

Yep have read the Internet, all the listings and unfortunately I have purchased multiple parts that list as a mr2 however they are not - see the pics. My misfortune from buying eBay where it costs more to return than your money back - hence why I am posting my experience so others don't waste money.
The picture attached shows the ugly looking one on the front right (dead out of my car), the front left one out of another club members car who's tried to help, then the other parts purchased in the background, with the left one wrong ridge/lip size, and the right one wrong bolt hole size.
The denso 447500-9052 has the incorrect size ridge / edge for forming a seal / sitting against the evaporator plate/joiner.
Yes you need the Tx expansion valve for the type of gas you are running cause the gas behaves differently. Some of the part numbers are discontinued/superseded because of the change in Australia to only the new gas being sold (legally).
I will upload when I have found the correct replacement part.

Your sacrifice will help many. I'm betting on Denso 047500-1730 for SW20 manufactured before 8/1993 and including the Japanese ones before 12/1991. If I wasn't so broke I'd click buy now, but I'll have to wait a few weeks.
 

Solution- early 1993 mr2 turbo sw20 air condoning replacement Tx expansion valve for leaking air conditioning. This is for Tx a/c expansion valves with a recessed lip and 6mm bolts as per previous pictures. There are different Tx valves for mr2 sw20s.

The 1993 car was designed for the old gas. Toyota recalled the mr2 of 1993 because of faulty Tx expansion valves in high heat / high humidity, so the part was listed as discontinued.

The new a/c gas molucules are smaller, so Tx expansion valve internal seals were redesigned, however unknown part number for the mr2 to this point. Apparently you can still use the old Tx valve designs, there is a possibility the gas at an incredibly small amount can leak past the internal seal. Apparently it's a molucules amount and the gas stays within the system, just that it can go past the internal Tx valve (stressed that it's a molucule amount) and doesn't effect the heating/normal air running past the evaporator.

Supercheap auto assisted me with their supplier of jayair products. The supplier has the original part design (old gas design) denso 047500-9071 aka TX9070. Supercheap auto can order it from the supplier. Cost was $55.

That was my experience in trying to get a fitting part. Should anyone come across the part number of the new gas style valve Tx expansion a/c valve that has a recessed lip, please post up for future readers.