In A/C systems, cleanliness is next to godliness!!! If the rear unit has been sitting open for a while, remove the orifice tube and flush the evaporator coil and all related lines. Where I work, we use some stuff called Ken-Sol, which is a mixture of hexane and alcohol. It is a good degreaser and will rid the system of any traces of mineral oil, which is compatible with R-12 only.
Don't skimp on the condensor size, especially with R-134a. R-134a is not as efficient as R-12 and needs all the help it can get in getting rid of the heat, so go as big as you can with the condensor.
Also, on any crimp fitting, if you can turn the hose collar and hose on the 'stem', that is a spot that will leak. The crimp has to be tight to hold R-134a.
Applying vaccuum to an A/C system is a pretty good test for leaks, but it's real purpose is to 'boil' the water vapor out of the system. Water vapor combined with R-12 made hydrochloric acid, which eats aluminum. The little pieces of aluminum particles then float around in the system and plug the expansion valve (orifice tube) and damage the compressor. I assume the same is true for R-134a with respect to moisture. Ditto Koolkar on pulling a vaccuum for AT LEAST 1/2 hour.
Hope this helps--I guess I really didn't say anything that wasn't mentioned before. Good luck!!
