Well, on my Baer conversion, I can't seem to lock them up, either (but it brings me right up to the edge on dry roads, as in it will leave tread marks, but the wheels don't lock). Of course, I only drove the car a couple hundred miles with the brakes before the engine took a crap on me, so I haven't really stood on them as hard as I can, but I'd think it should be easier than the effort I've applied so far. I have a stock 91 booster, btw. Certainly changing the booster will help, but as you know, it's a PITA to do.