I had a T350 that wouldn't hit high. Found a tiny leak where one cooler line fitting wasn't tight, just barely dripped. Tighten that sucker up and it worked like new! My GM mech buddy said the pressures must have been marginal and that leak was enough to stop it engaging Drive. I dunno, sounded weird, but the fitting loosened off again a year later, I slid under and cranked it up tight and away she went! Go figure!