This past week my Dad and I replaced both rear brakes on our Envoy, This was part of my Fathers day gift. Replacing the old worn out brakes (really noisy too) was pretty simple if you have the tools (all hand tools) and the know how (the vehicles owner maintenance book. This project took (me anyways) about two-two and a half hours to do one brake. I also used allot of different sockets so I'm going to suggest that you just get a socket set, and I don't know all the sizes because some were in mm and others in inches. Obviously the brakes are a vary important part of your vehicle (especially one of this size) so if you arn't comfortable with that much responsibility then perhaps you can just read the Instructable for "just in case" scenarios. (just guessing from the maintenance book that this also works on trailblazers)
Step 1: Tools
A socket set
giant pipe wrench (optional)
muscle power (or a big metal pipe to use as a leaver)
hammer (for those who aren't really strong)
small piece of abs/pvc pipe to prevent damage to wrenches being hit by hammer (again for those who aren't strong including me)
new brake pads
brake part cleaner
Those blue paper towels
Step 2: Jack Up Car
Refer to the owners manual or use good judgment when picking a place to support the jack. Place the jack so that it is on a structural part of the vehicle so that it can support the weight, otherwise you'll probably cause damage when lifting the vehicle. Once the lifting jack is in place other non-movable jacks are placed on other structural supports to ensure that the car won't crash down on you. The repair manual has a few pages on safe places to support the car when lifting it. As has been pointed out in the comments it is a good idea to loosen the bolts on the wheel before jacking the car up.
Step 3: Remove Wheel
The removal of the wheel consists of using a ratchet and socket to undo all the bolts that hold the wheel on and then lifting the wheel off. Take the bolts off opposite each other, start with top right then take off bottom left then bottom right then top left (not sure this is necessary but thats the way you put them on and practice is good).
Step 4: Remove Caliper Holder and Pads
Undoing the bolts holding the caliper on is a relatively easy (not like that pad holder) and then removing the pads is also easy. To remove the brake pads pull out the small metal clips that keep the pads in place then the pads come out easily. If there is a groove in the pads then they are still god (one large one in the middle not a bunch of small ones all around).
Step 5: Remove Brake Pad Holder
I don't know if this is it's official name but that's what i'm calling it. These bolts were secured by the mechanics using "lock tight" so they are really difficult to get undone. We used a big ass pipe wrench a large ratchet a medium wrench a hammer a big metal lever and the small piece of abs pipe to undo these bolts.I'm sorry that the pictures aren't vary good but it is a small space with a big person and tools in the way. Anyway if your's aren't secured with "lock tight" it's probably allot easier.
Step 6: Remove Old Rotor
As you can probably tell from the rust this rotor has been on the car for a long time. Once everything else is off (pads,pad holder caliper) the only other thing to do is cut or pry the thin metal ring around the bolts and then the rotor slider right off (in theory). We had to add a little lube to get it to come off easily.
Step 7: Put New Rotor On
first step is to clean both the new rotor and the hub with the brake cleaner (you don't need to read the warnings to know this stuff is bad for you). The blue paper towels were used to stop the rotor from getting dirty after it was cleaned. When both parts are clean(ish) then slide the new rotor all the way on and prepare to put everything else back on.
Step 8: Put Pad Holder Back On
This is were you need to look in the vehicle maintenance book to see how much torque to apply to the brake pad holder bolts. I would post the picture of the exact page from the book but I think thats a copyright infringement so I'll just say it's on the first page of the brakes section. So set the torque wrench to the correct torque and tighten the bolts, when you hear the click stop pushing. To set the torque wrench first unscrew the handle until it is below the lowest marking, then tighten until the numbers on the handle (0-9) and the makings on the wrench read to the correct amount of ft/lbs.
Step 9: Install Brake Pads
the brake pads go in the same way they came out, first put the metal clips in then the pads themselves.
Step 10: Undo Brake Fluid Cap
Next open the hood and find the opening for the brake fluid (yellow cap) and undo it then place it back on so that air can still flow in and out (not screwed on just placed ontop). Remember to do this back up after the next step.
Step 11: Re Install Brake Caliper
First take one of the old brake pads and place it on the circular piece of the caliper (lengthwise), then take the C-clamp and place one side touching the old brake pad and the other on the far end of the caliper. Tighten the c clamp until the circular bit is flush with the side of the caliper. Then place the caliper back in position and tighten the bolts to the required torque with the torque wrench.
Step 12: Put Tire Back On
first put the wheel back on then tighten the bolts up medium tight and then slowly drop the car down onto the ground before fully tightening the wheel nuts. Set the torque wrench to the correct ft/lbs (108) and then secure the nuts via the opposite method (step 1). Then redo the gas fluid cap (if you haven't already). Make sure to do a slow speed test run of the vehicle to ensure the brakes are working properly. It will take a few pumps of the brakes to re-fill the fluid in the brake lines. So there you go if your a willing to put in the time you can save yourself about $500.