My '02 Avalanche always had crap for brakes. I also discovered when a brake line burst that the '99-04 GM's used a poor material for the brake lines, causing them to rust out prematurely.
My solution, and it has made a HUGE difference in braking:
EBC Yellow Stuff. I had previously wanted to run Hawk Super Duty pads as that is what I was running on the front. Seems Hawk doesn't sell them for the G80 locker rear end (which is what all the Z71's have + other models can be optioned with it). I tried contacting Hawk multiple times by phone and e-mail with no success, so EBC got my business and I'm glad they got it. Had been a great pad so far.
Any decent quality rotor will do. Summit Racing has their own brand of cross drilled and slotted rotors at a great price, so I went that route.
Fronts are all dual piston and if you have a G80 the rear is as well. I just went with Centric calipers as they were cheap. BUT - go with the GMT900 front calipers. They come with brackets and simply bolt on. Just look them up as a 2008 Chevy Tahoe. Make sure to get pads/rotors for an '08 Tahoe as well. **Note - you HAVE to have 17" or bigger wheels for these to fit** The GMT900 front brakes are 1" bigger and a must have upgrade that isn't expensive and very easy to do.
Because mine were rotted, I bought pre-bent stainless steel lines from Custom Tube. I actually got them from Summit as well as they were cheaper that way than buying them direct from Custom tube. I replaced the flex lines with Russell braided lines, as the flex line connectors were all very rusty and a whole new set of flex lines were almost the same cost as braided lines.
Most GM (non-diesel) trucks have vacuum assist (but not all). That crappy, spongy pedal will let you know how well that vacuum assist works. Or for a little over $200 you can fix that - convert to a Hydroboost setup. You need to buy a Hydroboost unit, about $125 for a rebuilt or junkyard unit. Get the junkyard as the rebuilts don't come with the master cylinder pushrod/spring/retainer and those can't be purchased separately. You need the 3 hoses (power steering pump to hydroboost, hydroboost to power steering, and return line), a 3/8" T for the return line, 4 hose clamps, vacuum cap, and a new brake pedal (GM Part 15906961).
Hydroboost is all bolt-on for the most part other than needing to spend about 2 minutes drilling a hole in the firewall (which is already marked by a dimple on it). Getting some of the lines to thread on and routed can be a bit tedious (make sure you have 16mm and 18mm wrenches).
At this point I have a braking system that will out-brake a brand new GM truck. I have about $1500 or so wrapped up into it, but I also replaced everything except the ABS and Master Cylinder. For someone with a brake system in decent condition, a simple pad swap in the rear, front GMT900 front calipers/pads/rotors and a Hydroboost conversion will get the same results for $500-600. Well worth the money.