Scotty, I have a 2007 Toyota Avalon that has an AC problem. The low suction side schrader port has a  small hiss when I initially  take off the service cap. I tried to check for a leaking schrader valve with soap and water but I do not see any sign of bubbles or oil leaks. The ac blows cold air for about 4 months before the cabin air temp starts to go up and the system pressures go down. Is this hiss normal? I just replaced the valve evacuated and refilled 1 week ago and I can hear a short hiss when the cap is removed.  I was told that the schrader valve is leaking and you should not hear anything when you remove the cap and the schrader should be replaced. Others have said that the plastic cap is at fault since it seals the port. I have replaced this valve and cap 3 times and each time the results are the same. I do not hear anything from the high side when it's cap is removed. I have checked everything except for the evaporator for leaks and no dye can be seen anywhere. I was thinking about replacing the suction line  but need to know if this short hiss is normal or an indication that the schrader should be replaced?

 

Thank You

Wilton Itamoto Hawaii