Originally Posted by BigBenzWV

thanks for everyone's inputs. Here are some general questions.

Is it more common to have the low pressure side leaks or the high side?

No...

Isn't the low side has a vacuum rather than positive pressure?


No, just lower pressure.

How long does one shot of dye last in the system and outside the leak?
Do I need to inject dye at every recharge?


Forever.

Would I see the dye leaked out from a year ago? Under which condition does it leak faster, - A/C on or A/C off?


A leak is a leak. Depends on the seal that's leaking...ie = compressor shaft can leak on or off.

This guy told me that he vacuumed to the max and waited 20 minutes w/o seeing any drop. But my system is empty within 40 days.


The O rings work to seal with and without pressure...any seal cut/hardened/nicked/old may not roll into place like a new one.
Often a system will hold a low pressure like vac' but not high pressure like under load of 400 psi.(Compressor pulse, output hose O ring.)

Last time it was teh low side o ring on the compressor. This time I could see a trace of dye using my powerful research lab black lamp at the low side (per picture above).


Lamp intensity will help locate leaks ...the 'brighter' the better. After the lamp has found it you need to check with a 'sniffer'.

The small light (battery powered) the shop is using could not see these dye spots.