[D911SC]

Check the condition of the cable guide sleeve where it enters the valve. If it is perished it can affect the opening/closing of the valve. The pic in Justin's link will give you the perfect view of the valve and its corresponding parts.

I just replaced my guide tubes to fix the same problem you are having. If this is the case order the part from the host - it's very easy to replace as long as you have some floor jacks or similar and can secure the car well enough to get underneath it!