I have an 08 silverado ext.cab that came on stock 20s after i put on my 24s it started to vibrate when i pushed the breaks.The rotors look big enough to handle i was just wondering if anyone else is having this problem?  

24s are significantly heavier than 20s. Your rotors could be warped, or the wheels/tires might not have been balanced correctly when they were installed.  

yea i figured maybe the balance but the rotors?I just got the truck 4days ago imma check balance soon thanx for the reply. but if anyone else expiriencing same problem please post would like a tip if theres one to fixing my prob.  

PROBLEM FIXED...some one said torque.On my 04 it was 90ft lbs on the 08 140ft lbs so my mistake for not reading specs on new truck figured itd be the same.  

I have an 07 z71 4x4 and I am completely baffled over the same problem. My truck came with (31''}  265/65r/18's on 18x9 wheels. I put in a 2'' spacer lift up front to level the truck with no problems. Next I added (33'') 285/70r/17's on 17x9 wheels and instantly began getting severe braking vibration. Long story short, turned the rotors, replaced the pads, replaced the rotors, nothing changed the vibration wich by the way is at all speeds but is much more violent between 50 & 30 mph. Put the stock wheels and tires back on and she brakes as smooth as the day I bought her. I might try the torque wrench but I installed them with a breakover bar and put my 285 lbs on them pretty good. hope somebody has the answer.  

ya man my 07 chevy came stock with the z71 18s, and when i put 20s on mine it started to vibrate just like yours. the problem was the brake clips that hold on your rear drums. they come on the truck out of the factory like that for production reasons, and your factory wheels have indentions in the wheels to go over the clips. but your after market wheels don't. causing your wheel not to set flush with the drum, causing your vibration. just take off your rear wheels and pop off those clips and it will be just fine.  

aspringer, thanks for the tip! Sometimes it's the little things that make a big difference.    

And I thought this internet thing was just for nerds not gearheads.  Aspringer nailed it, problem solved. Thanks for the tip that none of the pro's at the tire and brake shops could think of. Those things get tossed on the first brake job, and I don't think I have ever put custom wheels on a vehicle before the first brake job. Thanks again.