Dont use a DIY top up kit, there a waste of time & money, you risk damage to your aircon system & yourself, its illegal to vent R134 to the atmosphere, plus there being banned from sale in the U.K. in next 2 weeks anyway.
Take it to an aircon specialist & have the job done the correct way.
The system needs to be vacuumed out for at least 45 minutes & pressure tested for leaks before adding the correct amount of compressor oil, R134A gas & UV dye.
If you do go down the DIY top up route dont expect your aircon system to stay in a working state too much longer.
Lee, June 2007