S3 - new owner - what does this little valve do????

[MuzzaS3]

Just got myself an 2000 S3 and love it! still getting to know the car, can anyone tell me what this little valve does?? I was messing about and cleaning earlier today and i took the little black cap off it, does it matter how tight i put it back on?? Its located at the front left of the engine bay. Any help much appreciated!
Cheers

[dan.taylor.1]

its one of the connectors for the aircon system used in refilling etc. theres a big one and a small one, one on the high and one on the low pressure side, cant remember which is which mind!

Dan

[MuzzaS3]

great, cheers for the help!
Murray