My Latio 4AT Air Con required Gas Top Up after every 4 weeks of driving, since Dec 2014.
From June 2105 onwards, need to top up gas every 2-3 weeks. Need to pay Mechanic $20-$30 for every gas top up.
Once my Mechanic saw me coming back for gas top up within a month, he suspect Latio Cooling Coil is leaking out gas slowly and may require > S$400 to replace (excluding labour).
As my car is approaching the scrap date in Mar 2016, No point in paying so much money to replace the Cooling Coil.
Also not wanting to pay $20 for every Gas top up in 2-3 weeks frequency, I bought a R134A refrigerant tank (S$110) as well as relevant Low valve R134A connectors with Gauge.
Now I do my own DIY Gas Top Up once my air-con starts to blow out less cold air.
Don't consider this DIY a good investment, but more like $$ damage limitation

. Until my car is scrapped

.
My symptom may not be the same as yours. But some info sharing:
- Air Con not as cold as it used to be. After half an hour drive, you are better off winding down the windows.
- Measured Pressure on the "L"ow pressure Valve, with air con on (and high fan speed), only got 20-25psi.
- Pump in R134A until "L" pressure reads 40 psi.