Yes we all have cars with air conditioning that do not cool as well as they used to. Why? Car air conditioning compressors are not hermetically sealed. They are open type. Fugitive leaks occur on even the best of car compressors over time. Also leaks on mechanical fittings and Schrader valves will cause poor cooling performance.
A significant leak needs to be fixed first before attempting anything else. For properly working air conditioning units that don't cool as well, read on for some tips.
You will need to consult your car service manual to know the weight of refrigerant to use on an evacuated system or the expected operating pressures.
Also a handy tip is that the typical small can is 12oz. My car uses 23oz total. Don't overfill else your compressor will go bye bye. For Topping up 1 can usually suffices.
For this job I used
1. Manifold gauge set for r134a refrigerant.
1. Can 12oz of r134a.
1. Piercing head for the r134a can.
Don't even attempt a job like this without being very competent in handling live refrigerant systems. Also for the love of life don't even connect the high pressure port and open the high pressure valve on the manifold gauge set! Wearing safety glasses is an excellent idea plus gloves are handy too (don't worry about my bare hand in the pics, I had taken off the gloves to take the pics).
Now open the piercing head a bit to allow some pressure into the line.
Turn on the blue (low pressure) valve a bit (not fully open) to get refrigerant into the blue hose. Don't worry none will flow out yet.
Unscrew the protective cap on the car's air conditioning low pressure port. It will have a capital L on it.
Take the fitting on the end and firmly press it into the low pressure valve port. Never connect this line to the high pressure port even by accidentally screwing on the high pressure port adapter fitting!
Step 3: Begin Topping Up.
For my car the correct maximum operating pressure is 22psig at the low pressure port. Yours may be different. All refrigeration systems have a narrow pressure range for which it is supposed to work properly. I chose the upper limit since my car had oem factory refrigerant charge since 1999.
The temperature in my country is warm enough so just leaving the can standing, it will let refrigerant flow easily.
Step 4: Correcting Accidentally Over Pressure.
Due to a major distraction at home, I accidentally let too much refrigerant into the system (yeah my bad!). The compressor began cycling on and off quickly. I immediately shut off the AC switch and closed low pressure valve on the manifold gauge set. I removed the R134a can and slowly opened the low pressure valve to release some refrigerant. I closed the valve and turned back on the compressor. I repeated this process until the low pressure value was 22psig with the compressor clutch engaged.
Now my car cools far better than before. I actually did 3 cars that evening. All 3 cars' air conditioning systems are working excellent! For the future I may invest in one of those digital manifolds that will alert when the correct pressure is achieved (of course those are quite expensive).
For major works like compressor, evaporator or condenser replacement you should take it to a reputable shop.