I bought an Accucraft pop valve for my K-27 that did the same thing. I replaced the ball with a precision stainless steel ball and the problem went away. I can't remember whether it was a 3 or a 5 mm ball, but I got it from Quisenberry Station. Another thing I did was once I found the sweet spot on the adjustment for 60 psi, I put a small drop of paint on the edge of the screw in thingy so it stayed in place. I noticed that over time, it moved slightly with each pop, reducing the top pressure. Screwing it in raises the pressure and screwing it out lowers the pressure.