Sudden Loss of Freon

The a/c in my 1977 450slc a/c was not functioning. There was a definite lack of refrigerant. No leak die was located. The previous owner had had the car repainted
the month before I bought it and it was suggested that the freon was recovered before the paint job.
The system was fully charged with R-12 (with Leak dye). All was great but the gas escaped again. NO leak dye can be seen.
This time it was suggested that the seals may have dried out a bit after the system was discharged, So the car had spent more time under a vacuum to make sure there were no glaring leaks and no moisture, Seal conditioner was introduced as were drying agents and a full charge of R-12.
That was about two months ago. The a/c was blowing cold enough to hurt if you had your hand near the vent.
Today No a/c. The compressor is running and there is a reduction of humidity out of the vents but obviously not enough gas to cool. No dye can be seen.
There was no warning. Either time. The a/c was producing ICE cold air one day and then nothing the next. No decrease in cooling was noticed either time. It worked great when I shut the car off and then it put out nothing after sitting a day or two.
The first charge lasted only a few weeks. This last charge a few months. Don't have to mention that a.c. work is pricey, and R-12 is outragous. I am way over buget on car repairs now so i am hoping that I can trouble shoot this myself.
Is there any component that could allow a sudden decompression of the system? This does not appear to be a slow leak. No decrease in cooling was noticed. I am hoping I can replace suspect components before I take it back for service. But again this stuff is not cheap so I dont want to just replace things blindly. And being a 30 year old sysem I want to mess with it as little as possible.
Any input would be welcomed. Going without a/c here is out of the question. Should mention. I've tried to clean the window on the receiver/dryer, It had paint overspray on it. I might be nuts but I think that the paint is on the INSIDE of the glass. Is that possible? What then?
Thanks all.
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I think it is a good possiblity that your leak is at the compressor... or the expansion valve. You need UV light to see those leak easily... Yeah I know... it is green... how could you not see it?
The truth is my eyes are not that good and with the UV light plug the yellow goggle, it pops out like a sore thumb... purple glowing reflection. That's how I was able to find the leak.
There is another possibility and that is the evaporator and to see that leak, you'd have to shine it on the windshield to see if any came out of the defrost vent.
The fact your mechanic did the job repeatedly...tells me he sucks!
There is a possibility that your leak is also at the schraeder valves... strong possibility and on R12 system, it is so easy to change those... just like tire valve. Make sure the cap is on after charging... you cannot have a system without cap as gas will escape at the shraeder valve... both high and low side. Just change it regardless of what mechanic said. Although the system has to be evacuated to do so unless he got the special tool to do so without evacuating.
I don't care for the sight glass... it is nice but a gauge is much better way to tell how much system loss gas... I would buy a gauge... cheap one and put a R134 adapter on it just to use that cheap gauge. You ONLY put the gauge on when the compressor is running... once running, R12 system should have anywhere around 60 PSI... I'd have to refer to chart to see the exact temp... but it is around there.
Lastly, if you got a buddy in AC business, doesn't matter car or home or commercial HVAC, he can help you out. Have him evacuate the gass... put it under vacuum for to 25 vacuum... and then shut off the vacuum... you should be able to maintain 22 vacuum for 5 minutes... if not, you got a major leak. He can also use the gas detector to find the leak...
If you can hold that vacuum, then you can use this sealer http://www.cliplight.com/automotive/products.php?catID=4
Super sealer premium to finish the job...
Last thing considering the age of the car, it is possible the hose is simply bad... old hose are notorious for letting gas escape because the density of the rubber allows them to escape. You can have the hose replaced by a local hydraulic hose shop... they will reuse the fitting but just change the hose
I know the above is too much information... but hey, I am trying to save you a buck... If you have a buddy who has HVAC experience... he is the one to go... serious! If I were him, I'd change the schraeder valve... looks for leak... sniff foe leak with detector... check vacuum holding power... apply sealer as I listed above if passed vacuum test... and then charge up with R134a... add some oil and send you off.
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There's a blow-out plug that you should check. It's in the center of a brass plug on the top of the receiver-dryer. The plug is a piece of wire that's soldered into the brass plug. My '80 300SD once had the same problem - too soon after a recharge. Ouch!
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On May 14, 9:40 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

BINGO! thanks. didn't see dye because it sprayed into aux fan. Will change schrader valves too.
ONE MORE QUESTION? My receiver/dryer takes both the pressure switch and the temp switch. I Know the pressure switch is good because compresser runs. Do I HAVE to change this with new dryer? $80 part, would be nice to reuse if that wouldn't cause problems later on. (Will change temp switch $15)
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Why change the Receiver Dryer and Schrader Valves? (unless you're abandoning R12)
The A/C shop that fixed my A/C blowout plug simply removed the brass plug, resoldered the plug (wire) into the brass plug and recharged the system.
That was at least 5 - 7 years ago and it still cools!
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The Hi/Lo switch? It is only $9 The dier? $20 The temp switch? $15...
All available at autohausaz.com
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Receiver dryer is original to car so after 30 years and a few leaks it should be time. Found 'em dirt cheap.
QUESTIONS about the prices listed above. drier yes, $20 and temp switch maybe i could find for 15, But, and this is what i cant get a straight answer on...The inexpensive black plastic pressure switch with the male fitting...is this what i need to buy for a new receiver?? Currently i have the large metal unit that offers a female thread. much higher in price. I notice that there are different threads on the temp switches too. old style 6mm and a new 10mm. my temp switch is the red metal 6mm i see listed for $20.
My temp switch is flakey so i want to replace it anyway, and if i need the newer type of pressure switch for a new drier okay. I just don't want to end up with the wrong stuff and have to schvitz my schmekle off while i wait for wrong parts to go back and new parts to arrive. temps heading into the 90's this week. summer's comming.
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Call them up and they will help you. Yes, you should change the drier as it is old... and again, I don't trust old devices because why do the job twice if it was cheap?
As for the Hi/Lo switch, yes you have to buy new one as we don't know even if your old one will fit the new drier. AutohausAZ ship their own products out... They don't seem to be dropshipper as others are. So call them up and they will match everything for you.
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Thanks for the tip on autohaus. Went a little nuts (as usual), their prices are better than many of my sources. In case anyone wants to know, the temp and pressure switch are dependant on the receiver drier, not the car.
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Receiver dryer is original to car so after 30 years and a few leaks it should be time. Found 'em dirt cheap.
QUESTIONS about the prices listed above. drier yes, $20 and temp switch maybe i could find for 15, But, and this is what i cant get a straight answer on...The inexpensive black plastic pressure switch with the male fitting...is this what i need to buy for a new receiver?? Currently i have the large metal unit that offers a female thread. much higher in price. I notice that there are different threads on the temp switches too. old style 6mm and a new 10mm. my temp switch is the red metal 6mm i see listed for $20.
My temp switch is flakey so i want to replace it anyway, and if i need the newer type of pressure switch for a new drier okay. I just don't want to end up with the wrong stuff and have to schvitz my schmekle off while i wait for wrong parts to go back and new parts to arrive. temps heading into the 90's this week. summer's comming.
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On May 14, 9:40 am, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

BINGO! thanks. didn't see dye because it sprayed into aux fan. Will change schrader valves too.
ONE MORE QUESTION? My receiver/dryer takes both the pressure switch and the temp switch. I Know the pressure switch is good because compresser runs. Do I HAVE to change this with new dryer? $80 part, would be nice to reuse if that wouldn't cause problems later on. (Will change temp switch $15)
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