Brakes on a Hummer - The Job

Trouble-Shooting Article

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Take your truck to a self-serve carwash and spray clean the underside to get all the mud and dirt out of the area you're going to work in. Bolts loaded with mud don't turn to well. It's no fun working under a truck with mud and dirt falling on your face, not to mention contaminating the brake fluid. I would also clean under the hood around the brake master cylinder since it will be open.

Make sure the parking brakes are off. Loosen the parking brake by turning the top part of the brake lever inside the truck counter clockwise until there is no tension.

Open the hood and open the top of the master cylinder. I use a big grease syringe to remove some of the fluid. When the pistons are compressed the fluid will be pushed back into the reservoir causing it to over flow. Put some rags around the master cylinder to catch any fluid that may spill. You will see 2 reservoirs. The one towards the front of the truck is for the rear brakes.

Make sure that the adapter is bolted tight to the differential. These bolts should be coated with red thread locker and torqued to 125 to 150 ft/lbs. There have been instances where these bolts have loosened and backed out. If this occurs the heads of the bolts will contact the back of the rotating rotor and destroy the whole brake.

These bolts are difficult to get to.

Here's what I did when I fixed the front left bracket.

Jack up the truck and block a rear wheel and place a jack stand under the frame for safety.

Remove the front left tire

Shift the tcase into N which allows you to rotate the diff and easily get to each halfshaft bolt.

Remove the halfshaft bolts at the rotor. Let the halfshaft drop down.

Remove the caliper mounting bolts and move the caliper out of the way.

Remove the rotor.

Remove the bolt holding the diff output flange using a 1-1/8" socket.

Now you can get to the mounting bolts. Undo them with a 15/16" wrench.

Clean them off and put Locktite 272 Red on them and reinstall at 150 ft lbs.

You have to remove the rotor to get to these bolts. The photo shows how the brackets are bolted to the differential.

AMG Service Bulletin

A Halfshaft bolt backing out will destroy the brakes

Front Brakes

I usually start with the front's because they're easier to do because they don't have the parking brake. The front right is the easiest and the left can be a pain because the top caliper bolt is next to the cross member which makes it hard to get a wrench on.

I start by prying the pads back with a big screw driver so they are loose around the rotor Don't pry against the rotor.

Next remove the two (14mm) 9/16" cap screws that hold the caliper to the adapter. These screws are secured with thread locker and sometimes it takes heat too loosen. You can get the bottom bolts off easy enough with any wrench. It's the upper bolts that are a pain. Sometimes I use a propane torch and carefully heat the top of the bolt. Be careful not to burn or heat anything else. Keep the flame on the bolt for about 30 seconds. Then use a curved (14mm) 9/16" wrench to break the bolt free. Be careful that you are turning the bolt in the counter clockwise direction. There's plenty of room above the caliper to get a standard (14mm) 9/16 wrench or better yet a ratcheting box end or gear wrench on to take it the rest of the way off. Snap-on RTBM14 (14mm) or a RTB18 (9/16"). The metric wrench is 42.00 and the 9/16" is 32.00.

Push the caliper up and back and slide the old pads out. Slide the caliper yoke out and clean it up as shown in the section below. You can use an inexpensive disk brake piston compressor, a C clamp or a big set of channel lock pliers to retract the piston on the front brakes.

If you are going to replace or resurface the rotor now is the time. Remove the halfshaft bolts and pull the halfshaft away from the rotor. Be careful! The rotor will fall right off the differential hub. Put the new or resurfaced rotor back on the hub. Replace the halfshaft bolts using locktite blue and use new Nord lock washers if available (nord-loc's amg part number 06008029). Tighten to 57 ft lbs.

Check and lubricate the guide pins per the procedure below. Then place the new pads next to the rotor and slide the caliper over the whole thing. I use Locktite Blue on the 2 caliper mounting bolts. Tighten the 2 caliper mounting bolts to 40 ft/lbs.

Rear Brakes

Remove the undercarriage protection in the rear by removing the six 9/16" bolts. Give each bolt a shot of WD40 and they will come off a lot easier. Remove the 2 outside undercarriage mounting angle brackets on the rear rail. Each bracket has 2 bolts. These brackets stick in toward the caliper and will get in the way. Put all the nuts, bolts and washers to the side.

Spray WD40 on the parking brake area to clean and lubricate the moving parts. Remove the cotter pin, washer and clevis pin holding the parking brake cable to the lever. Remove the "C" clip securing the parking brake cable to the caliper bracket. I stick a long screwdriver into one of the 'C' clip voids and turn it 90 degrees. Don't loose or damage the clip or you'll be stuck looking for another.

Pry the pads back with a big screw driver so they are loose around the rotor. Don't pry against the rotor.

Remove the two (14mm) 9/16" cap screws that hold the caliper to the adapter using the same procedure as the front brakes. If you use a torch to loosen up the bolts be VERY careful because the fuel tank is right above you. Be extra careful if you are working on a gas truck. Gasoline is much more volatile then diesel.

You can now move the caliper assembly up and away from the adapter bracket. You may need to use a small pry bar or screwdriver. Once the caliper is out of the way remove the old pads.

If you are going to replace or resurface the rotor now is the time. Remove the halfshaft bolts 15mm and pull the halfshaft away from the rotor. The rotor will pull right off the studs. Put the new or resurfaced rotor back on the hub. Replace the halfshaft bolts using locktite blue and use new Nord lock washers if available (nord-loc's amg part number 06008029). Tighten to 57 ft lbs.


The washers come pre-assembled in pairs, cam face on cam face. When the bolt and/or nut is tightened, the teeth grip and seat the mating surfaces. The washer is locked in place, allowing movement only across the face of the cams. Any attempt from the bolt/nut to rotate loose is blocked by the cam effect. The increase of tension makes the bolt/ nut self-locking.

If you have a special rear caliper compression tool you can retract the piston at this point without taking the caliper off the truck.

Click here to see how to do the job with the brake tool.

This tool will allow you to turn and retract the piston without removing the caliper from the truck. If you don't have the tool continue below.

Carefully position the caliper so you can see the brake line and fitting. Take care not to bend or stress the brake line. Take a 5/8" open end wrench and put it on the brass fitting that is screwed directly into the caliper. Put a 3/8" flare nut wrench over the brake fitting nut. While holding the 5/8's wrench break loose and remove the brake fitting. You can now remove the caliper from the truck. Brake fluid will start dripping. You can either put a pan under it or a couple of rags.

Check the master cylinder and fill if necessary. I don't like to let the master go dry because too much air will get into the system making bleeding the brakes more difficult.

Take the caliper to your work bench and clean it off. Keep dirt away from the exposed brake line fitting. If the caliper needs rebuilding now's the time. I'm not covering this in this paper.

Put your "C" clamp on the caliper so one side is on the piston and one on the caliper. Snug up the clamp. You will see brake fluid squirt out from the fitting. Move the clamp out of the way and take the channel lock pliers and turn the piston clockwise about one half turn. Then snug up the clamp to compress the piston again. Continue this until the piston doesn't want to go in any further. The piston will be sticking out about 3/16" or so. Never force anything. In order to make sure the piston was retracted enough I placed the new pads in the caliper and measured the distance between them for the rotor. The rotor is about 3/4" thick. The distance I measured was about 7/8".

Pull the yoke out of the caliper. Clean up the caliper and more importantly the 2 caliper guide pins on the yoke. If these pins aren't free to move the pads will wear unevenly. I first spray solvent or WD40 on them and wipe them off. I then take a single edge razor blade and scrape off the remainder of the buildup. If there is still some corrosion I use some steel wool or fine 600-800 metal oxide sand paper to clean them off. If the pins are really bad get some new ones. Then lubricate the pins with the green Permatex caliper lube.

Next clean the plastic sleeve and bushing that the guide pins go into. Once again I sprayed the bushing with WD40 and ran a shop rag through the hole until it was clean.

If the rotor is dirty or full of oil wipe it off with some solvent. Place the new pads on the mounting adapter so they sit on the metal steps. If the pads are different sizes put the larger pad on the piston side.

Don't bother using any shims or backing pads in this application. The pads are supposed to be loose and 'float' in the caliper.

Place the caliper assembly up on to the caliper mount sliding it over the pads. There are a lot of ways to reassemble. I rested the caliper on the bracket and temporarily screwed in the lower 9/16" bolt to hold the weight of the caliper. I then lined up the brake line fitting and screwed it into the caliper using only my fingers. Once it was finger tight I put the 5/8" wrench back on the brass caliper fitting and used the flare nut/ split nut wrench to tighten the fitting. This will insure that you will get the threads started correctly. If you cross thread the brake fitting it will always leak and need to be replaced.

Position the parking brake cable in the caliper bracket and rotate the caliper over the new brake pads. Put some red Locktite 271 on the two 9/16" cap screws that hold the caliper to the adapter and reassemble. I've been using Locktite Blue which doesn't require heat to loosen, but the book says use the Red 271. Tighten these screws to 40 ft/lbs which is good and tight. The brake pads should be free to move around a bit.

Reassemble the parking brake clip and clevis using a new cotter pin. Make sure that the parking brake lever is retracted all the way and resting against the bracket. If it's not you have to adjust the parking brake cable tension by turning the top part of the brake lever inside the truck counter clockwise. If you don't retract the brake it can drag causing friction and heat.

Do the other side and fill up the master cylinder. Clean up the area.

Bleed The Brakes

You now have to bleed the brakes. If your truck has over 60,000 miles and you have never changed out the brake fluid now is a good time. You will need about 3, 11oz bottles of dot 5 silicone brake fluid (non abs truck). Clean off the top of the master cylinder so junk doesn't get in when you remove the top. I spray it down with carburetor cleaner and wipe it with a rag. Remove the top and wipe the top off being careful not to get any dirt and grit on the clean underside of the cover or in the tanks. Take a mechanics syringe or a 'turkey baster' and suck out all the fluid in the 2 reservoirs. Take a clean rag and wipe all the residue from both compartments. Fill both tanks with new fresh DOT 5 fluid (98 and earlier trucks w/o ABS). Note that the small front compartment is for the rear brakes.

When you bleed brakes you always bleed the brake that is farthest from the master cylinder first. Then do the wheel that's next farthest and so on. There are two ways to bleed your brakes. One is where an assistant depresses the brake while you open the bleeder and the other is a passive gravity method. John Ward from AMG told me that the best way to bleed the brakes on the Hummer is the gravity method. I like this method because I don't have to have an assistant.

Flarenut WrenchFlare nut or box wrenches grab the nut all the way around so you don't risk stripping the nut.

Gravity Method

This is the way I do it. Open the master cylinder reservoir. Then open the bleeder on the rear right caliper a couple of turns and let it slowly drain making sure that the reservoir doesn't go empty. I let the rears on my 98 drip for about 90 minutes. Close the bleeder off once an ounce or so has drained out. Then do the rear left caliper. Make sure that you never have more than 1 bleeder open at a time; it could siphon air into the system. Next do the front right and then the front left. The fronts drained much faster than the rears. The front right went for about 30 minutes and the front left for about 20 minutes..

Standard Method

Get an assistant to sit in the drivers seat. Start the truck. Have your helper lightly press down on the brake. Crack open the first bleeder on the caliper farthest from the master cylinder (rear right) as the assistant pushes the pedal to the floor. Using this method it's a good idea to push a plastic tube over the bleeder with the other end in a container. Close the bleeder before the assistant lets up on the brake. In the beginning you won't see anything coming out because you are purging the air in the cylinder. Repeat this process until you get a solid stream of brake fluid streaming out. Remember, don't let the master cylinder go dry. Do the other calipers. Go back and repeat the whole process again. You should have a hard pedal by now. Shut off the engine. Clean up the brake fluid and wipe the rotor if necessary. Fill the master cylinder and close it up.

Too keep vibration from rubbing a hole in your brake line reach up and make sure that the steel line is not contacting the edge of the frame rail.

If you want you can jack up each wheel under the lower control arm, put the transfercase in Neutral and rotate the tire to make sure that the brakes are not dragging. If the brakes are dragging they will generate a lot of heat which can cause a dearth of other problems. If they are dragging you may have to retract the piston some more. Dragging is also a symptom of a warped rotor.

If the wheels aren't binding go for a test ride. Take it easy on the brakes, no panic stops. Don't forget to adjust the parking brake cable tension by turning the top part of the brake lever inside the truck clockwise to tighten the cable so when you pull the lever back it applies the brakes enough to stop the truck from rolling when the truck is in drive with the engine idling.

Check the fluid level in the master cylinder, check for any leaks under the truck and reassemble the undercarriage protection and you're done. For the next week keep an eye out for any leaks. I would also keep an eye on the master cylinder fluid level.

Pictures are by Chuck Kopelson

Drawings are from the 1998 AMG Hummer Shop Manual.

Caliper Rebuild Manual Pages