Background info: 5.3, bought about a month ago, lifted, and has a programmer in it. The guy i bought it from said he was getting about 19 mpg, and a little better on the high way. I dont expect to get that good, and figured it was just a claim to get people interested in it. I am getting between 12-14 right now. anyway- at around 10,000 feet I have had two separate incidences where i have been going down hill, no more than 15mph, and my brakes will not take me to a complete stop! They will let me roll around 3-4 mph hour. I had the pedal all the way to the floor. Around denver they work fine, and yesterday with the really cold weather they worked great. I just put all new pads on it, and had a brake place look at everything else. The guy i bought it from said he thinks it might be a vacuum leak which would cause the brakes not to stop, along with the really bad mileage compared to what he was supposedly getting. Anybody hear of this before, could he be correct with the vacuum leak causing poor mileage?