ERRECOMBrilliant - 7.5 ml, Fluorescent UV Dye for Detecting Refrigerant Gas Leaks in Vehicle A/C Systems, Includes R134a Adapter

For those that want to know; assemble the syringe and adapter and with your hand covering the top of the syringe quickly push snap the adapter onto the low pressure valve with the other hand. Note it doesn't fit the high pressure valve, and you NEED to wear safety glasses due to pressurised gas being not good for them. As soon as it's connected push the plunger down until empty. Pull up on the adapter lower part to release and remove it. Clean up any residue around the valve with a paper towel and replace the valve cover. This kit appears to work even if you have already filled the gas prior as I found out just so long as you haven't maxed out the space; use your filling canister valve gauge prior to check pressure is in the green region. Once complete, turn your car on and go for a drive. Take the car somewhere with some shade and with UV safety glasses (they're yellow tinted) and a UV flash light use it to follow all the pipes from the valve around the engine bay until you get back to where you started. Any predator 'blood' found will show your leak. Hope this helps someone.

Vacuum the system, attach to the low side and it'll suck itself into the AC... Refill the system... Easy.