Remove some of the coolant, sounds overcharged.

Thanks.
Does this car's compressor have a pressure release valve? Is that what the hissing was?

I don't know, you may have blown the seals. When you remove some coolant, if it has no pressure then a seal is blown.

          Ed

I had a similar experience with my V6 - the belt was definitely slipping.  I vented some refrigerant and it works ok now, but the belt must have got polished, because it slips in damp weather.  Going to replace the belts this week.

I think the pressure indicators on these cheap kits are unreliable.