Glider Handling on Tow

We tow with a Pawnee at our club as well. We were also having issues with the tow speeds showing approximately 5-10 knots (10-15 mph) slower on the glider's ASI than on the Pawnee's. This spread worsened when the Pawnee's side window was open. ( we had a window latch that was loose and occasionally would open during takeoff or tow ) There was a very apparent change in the pitch attitude required to maintain the proper indicated but wrong actual airspeed and the glider pilots would request a faster tow speed. I found the static line broken inside the fuselage, hidden between the fuselage fabric and steel tubing next to a bracket near the throttle. This leak obviously induced large errors into the static system especially with the window open. I imagine this could be an issue with many of the Pawnees around. The hard static line is very close to vibrating fabric and can also be damaged by ingress/egress from the cockpit if the fabric is pressed into the line. The line eventually work hardened and broke in two.