Newbie here, why would the low pressure port spew oily foam when I remove the cap (to add 134A)? The engine was running with the A/C on. It hasn't been the same since I had to replace the compressor last year. Keeps me comfortable (blows about 55 degrees) but it's not what it should be (consistently 45 before comp change). New evap and dryer. Thought it might be over charged or have too much oil. Thanks