My GMS2 LSJ made the correct 15 PSI but over the summer it slowly started dropping untill it was only 9-10PSI. Bought a new valve and made a hot 4. Adjusted and got to 10-11. had a buddy adjust it and now it sits at 12-12.5 and cant get it higher. What gives?

 

have you checked the s/c belt for slippage? i thought adjusting the bypass valve would only adjust how soon or late boost hits (coming from the wbody with the l67 but with the m90)? sometimes weather/DA plays a factor on boost.

 

I Have not nor did i even think about that. How does one check for belt slippage
Apparently having the bypass valve in the wrong spot can ever so slightly hold the flap on the bottom of the blower open, Category V Port Worx said there's a special tool for it but its not offered for sale anywhere