If you have a system that has been converted to 134a then you probably won't see any improvement by adding MORE refrigerant. R134 systems have to be larger in capacity because 134 isn't as effecient as R12.
I was talking today to the Independent Porsche shop that buys Duracool from me and He said that systems that he installs Duracool in run COLDER when he is just a tad undercharged.
The advantage to Duracool is that it is a more effecient refrigerant that 134a. It runs at much lower High and Low side pressures. Lower pressures take less power to drive the compressor. Duracool also has a larger molecule structure and doesn't seep through the rubber lines like freon does. My Porsche Indy says that he couldn't keep Freeze 12 in the Porsches. They have LONG hoses and the cars would lose the charge AND not cool as good. He has been using it now for 3 years.
To LEGALLY install Duracool in a R12 system, you need to change the fittings on the High and Low side lines. They sell adapter fittings that screw on and seal onto the existing R12 fittings.

There alot of nay sayers on the site about using a hydrocarbon based refrigerant like Duracool. I have worked on cars for 40 years. I tried it... it works.... If they don't want to use it..... NO PROBLEM. JUST don't knock something if you haven't used it. My $0.02

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.