I have a 94 WhiteGMC with a volvo engine and trans. We have had an ongoing problem Building air for months. It will build air to 30 lbs. then stop building completely before the high/low selector engages. last fall it started doing this nearly every day. but if you let it run for ten to fifteen minutes it would finally click in the high/low selector then build all the way. now it just stays at 30 lbs and will not build. compressor is unloading fine, changed governor, air is going in and out of air dryer fine. does anyone have an idea of what my problem may be??