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I am in the process of converting the A/C system from R12 to R134 in my 89 Bentley. The A/C worked fine with R12 but the car sits for extended periods and the system goes flat. Since R12 is difficult to purchase I made the decision to go with R134. I flushed the system, changed the "O" rings at the connections that I could access and changed the expansion valve. I did not change the hoses. When I tried to vacuum the system I could only pull 15 inches of vacuum and the gage goes to zero when the pump is shut off. This gives me the indication that I have a leak somewhere. I tried adding a can of refrigerant with UV dye but cannot find any leak. I understand that it could be the evaporator and I have examined the drainage hoses but there is no evidence of a leak. I do not know how to go about trying to pressure test the system or if it is recommended. I have the Workshop Manuel TSD 4700 which does not provide the procedure on how to remove the Evaporator housing. Hopefully someone has some experience and knowledge on how I should proceed. Thanks. Tom Greasel
 

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BTW You should have flushed the system and changed the receiver-dry. Do not overtighten the new O rings, doing so wil destroy them. UV dye is a PIA, use a sniffer.You need almost a full charge to carry the UV dye throughout the system. Adding external pressure is not recommended.
How long does it take to rise to 0 ?
 

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Discussion Starter · #3 ·
Thanks for responding I did flush the system and changed the receiver dryer. Hopefully I did not over tighten the "O" rings. The vacuum pump pulls 15 inches in a few minutes of operation but immediately goes to zero when stopped. I have a "sniffer" and will add a can of 134 and see if I can locate a leak. Thanks. Tom Greasel
 

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Hi Tom
The garage I took mine to used nitrogen to pressurise the system, this is what you should do as nitrogen is harmless to the environment and is a lot cheaper too!
Mike
 

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Agree nitrogen is the way to find it.
if it worked with r12 then it’s one of the connections you have disturbed-not much help but redo each one one at a time vacuum down each time -don’tfill with 134 it will soon prove expensive
I use a refrigerant friendly paste until I find the bad joint
Liquid Bottle Fluid Font Bottle cap
 

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I am in the process of converting the A/C system from R12 to R134 in my 89 Bentley. The A/C worked fine with R12 but the car sits for extended periods and the system goes flat. Since R12 is difficult to purchase I made the decision to go with R134. I flushed the system, changed the "O" rings at the connections that I could access and changed the expansion valve. I did not change the hoses. When I tried to vacuum the system I could only pull 15 inches of vacuum and the gage goes to zero when the pump is shut off. This gives me the indication that I have a leak somewhere. I tried adding a can of refrigerant with UV dye but cannot find any leak. I understand that it could be the evaporator and I have examined the drainage hoses but there is no evidence of a leak. I do not know how to go about trying to pressure test the system or if it is recommended. I have the Workshop Manuel TSD 4700 which does not provide the procedure on how to remove the Evaporator housing. Hopefully someone has some experience and knowledge on how I should proceed. Thanks. Tom Greasel
R134a has smaller molecules then R12 which allows the R134a to seep thru the old hosed. That is why we had to put barrier type hoses in when we do conversions. We have been able to find most leaks with the dye but use our sniffer sometimes. It is important to remember a leak is usually present at the bottom of fittings so test Slowly to locate leak. We find a lot more leaks at the condensor vs the evap core
 
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