OK, I changed the front brake lines(to stainless ones) and replaced the master cylinder. Also replaced all the brake bleeders, new brake pads on front and ran three quarts of brake fluid through the system. Rear pads like new. Checked booster and vacuum. has 18 " at all times. I have a better pedal but still can't lock up the brakes. At 45 mph it glides to a stop. It has 4WDB with GM calipers and a new combination valve for disc/disc. Checked the calipers when on the jack stands. Push pedal and can't turn wheels. I have done everything you suggested. If I need to panic stop I'm up a creek. Any more ideas?