BMW Repair: BMW 328i Auto 4 Door - 1999 - Air con problem, screw cap, soapy solution

Question
Hi Don, I have owned my car for 4 years and the air con has not been
charged during that time.  The air con was working fine until a couple of
months ago but now has a weird low hissing sound when the air conditioning
is activated and the air is not blowing cool at all.  I was hoping it just needed
a recharge.  There is a black screw cap over a fitting towards the front of the
engine where the coolant is recharged but when the cap is removed it makes
a hissing sound.  Is this normal or does it indicate a leak?   Your advice is
much appreciated! Thanks Ray.

Answer
It sounds like a leak.A soapy solution sprayed on it will let you know for sure.If it is(bubbles)the schreader valve needs to be tighten or replaced.One must be careful if replacing the valve as there is freon at that point and there is alot of pressure and it will escape.