I have a 2wd 89 3/4 ton Suburban with a 454 TBI and TH400. I've had the truck about 4 years and while it's great for towing and hauling, the brakes have always been scary. It stops straight, but even if pushing on the pedal as hard as you can, it's impossible to lock up the tires. Normal stops are ok, but trying to stop quickly is impossible. It actually stops better with the trailer because the trailer brakes haul it down. I've changed the front pads to Hawk Super Duty and replaced the brake hoses. Master cylinder, booster and rear brakes were replaced shortly before I bought it. Adjusting the level sensor to use more rear brake helped, but not a lot. I checked the engine vaccum yesterday and it only has 14" of vacuum at idle, which seems very low, but I can't find a spec for it. Most sources show 18" minumum for proper booster operation. I checked for vacuum leaks and found none, so I'm wondering about a possible plugged exhaust system. As far as I know the engine is completely stock and it won't rev above 4000 rpm (not that it really needs to). It has 2 cats (both seem to be original), one ahead of a Flowmaster 70 muffler and one behind it. What is the normal vacuum level for this engine? Has anyone had a similar problem with the brakes and solved it? Any thoughts or advice would be appreciatted.