Press the brake then my rear tires brakes keep locking

Some tires are noisy.
Some rear axle HD differentials make noise.
Differential bearings may be noisy.
When the differential was drained, the oil should have been inspected for metal residuals.

Jack the rear of the truck up until the tires are off the ground. Put jackstands under the body on both sides. You should be able to pull the rearend back down to the correct angle.

The reason is, you have a rear anti lock brake system. The anti lock valve is located on the frame just underneath the brake master. It is called a RABS valve. Almost impossible to bleed. But it can be done. A dealer has a tool that plugs into the RABS valve which activates the valve constantly moving the valve back and forth. This will then push all fluid and air out of your open bleeder screws in the rear.
I did'nt go to the dealer. i was too stubborn. My solution was to lift the rear of the truck and place on jack stands. Pinch off the rubber brake hoses on the 2 front brakes with a C-clamp or non marring vise grips, this will keep all fluid from going to front brakes. Next adjust the rear brake shoes by tightening the adjuster on the rear brake drums. Tighten the brakes on the rear until the brakes start to drag and the tires no longer spin freely by hand. Attach a bleeder hose to the bleeder screw on the RABS valve, place open end of hose into a jar or canister of brake fluid, so as not to draw in any air. Open the bleeder screw. Start the vehicle up, put it in gear, and let the rear tires spin. (I got up to 25mph on my speedometer) Step on the brake while it is in gear and the tires are spinning. This will activate the RABS valve and cause the air in the RABS valve to bleed. Do this several times. Put vehicle in park, shut off engine, and close the bleeder on the RABS valve. Attach bleeder hose to right rear of the truck, place open end of hose in a canister or jar of brake fluid so no air gets in and open the bleeder screw. Start the vehicle up, put it in gear, and let the rear wheels spin freely. As the wheels spin, you press your foot on the brake pedal, this will activate the RABS valve, and effectively bleed the brakes. Do this a dozen or so times, tighten the bleeder screw and do the other side. Remove clamps from front brake hoses and you will be go to go. All this may seem a bit extreme, but it is the only effective way to bleed a Rear Anti Lock Brake System on an early Ford. All the steps I laid out are necessary. You cannot skip one. The reason why the rear has to be in the air, is that there is a sensor mounted to the rear differential that senses wheel spin(or axle spin), this goes directly to the RABS valve and tells it that the tires are moving at a certain speed, and to activate the Rear Anti Lock Brakes when the brake pedal is depressed. It will not activate if it does not sense movement. The reason for pinching off the front brake lines, is that there are needle valves in the master cylinder and they will send too much fluid to the front calipers, and stifle or hinder the amount of fluid needed to go to the rabs valve and ultimately the rear wheel cylinders. It seems like a lot, and it is, but I did it and it works. Good luck

sounds like the brake is locking up. reach under car and pull down on emergency brake cable or just wiggle it to see if it works. if not remove tire and check back brake. it could be the springs are broken or the nails that hold on the brake shoes have errode to the point where they broke. just check it out!! have a good day !!

You're going to need to increase the tooth count on the driven gear on the speed sensor. probably with 3 additional teeth in the count.

Your rear emergency brakes could be hung up. Especially if a parking valet or someone else parked it recently.

You need to jack up both rear wheels and then put your car into neutral with the motor OFF. If the rear wheels can not be manually turned it is either the emergency brakes, the differential or the transmission.

The emergency brakes require removing the back tires and rotors. If it is still locked, then it is either the differential or the trans. The driveshaft connects the trans to the differential so remove the driveshaft. If the differential turns without the driveshaft, then the trans is locked up. If the differential does not turn the problem is inside the differential.

handbrake jammed on somewhere ,could be front cable ,because the rear cable is pulled by the compensator so if it was the rear one then i would expect only one cable siezed and that is normally the kerbside one.

Block the front tires. Break the rear lugs loose. Jack up the rear of the truck at the center if possible. remove the wheels & brake drums. Remove the brake shoes,return springs & all the other hardware. Remove the differential cover & drain oil. If you have a posi the next step sucks. You need to remove the threaded pin from the housing & remove the axle retaining clips. All the while,being carefull of the spring in the posi unit. If no posi,then same procedure,minus the "pucker factor". After the axle is out,take a large flat blade screw driver or pry bar & pop the seal out. Keep in mind the axle bearing is probably gone too , so that's another procedure. Keep it apart,you'll be glad you did.

SECTION 205-02B: Rear Drive Axle/Differential — Ford 8.8-Inch Ring Gear 2003 Mustang Workshop Manual IN-VEHICLE REPAIR Axle Shaft Removal

1. Raise and support the vehicle. For additional information, refer to Section 100-02 .

1. Remove the wheel and tire assembly. For additional information, refer to Section 204-04 .

1. Remove the rear brake disc (2C026). For additional information, refer to Section 206-04 .

1. Remove the differential housing cover (4033) and drain the lubricant. For additional information, refer to Differential Housing Cover in this section.

1. Remove the differential pinion shaft (4211).
   1. Remove the differential pinion shaft lock bolt (4241).
   1. Remove the differential pinion shaft.

1. CAUTION: Do not damage the rubber O-ring in the axle shaft groove.
   Remove the U-washer (4N237).
   1. Push the axle shafts (4234) inboard.
   1. Remove the U-washer.

1. CAUTION: Damage to the rear brake anti-lock sensor (2C190) may occur if it is not removed before the axle shaft.
   Carefully remove the rear brake anti-lock sensor. For additional information, refer to Section 206-09A .

1. CAUTION: Do not damage the wheel bearing oil seal.
   Remove the axle shaft.

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Rear Wheel Bearing and Axle Shaft Oil Seal Removal

1. Remove the axle shaft (4234). For additional information, refer to Axle Shaft in this section.

1. NOTE: If the wheel bearing oil seal is leaking, the axle housing vent may be plugged.
   Using the special tools, remove the rear wheel bearing and wheel bearing oil seal.

Installation

1. Lubricate the new rear wheel bearing with the specified lubricant.

1. Using the special tools, install the new rear wheel bearing.

1. Lubricate the lip of the new wheel bearing oil seal.
   • Use Premium Long-Life Grease XG-1-C or equivalent meeting Ford specification ESA-M1C75-B.

1. Using the special tools, install the new wheel bearing oil seal.

1. Install the axle shaft. For additional information, refer to Axle Shaft in this section.

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Installation

1. Lubricate the lip of the wheel bearing oil seal
   • Use Premium Long-Life Grease XG-1-C or equivalent meeting Ford specification ESA-M1C75-B.

1. CAUTION: Do not damage the wheel bearing oil seal.
   Install the two axle shafts.

1. CAUTION: Do not damage the rubber O-ring in the axle shaft groove.
   Install the U-washer.
   1. Position the two U-washers on the button end of the axle shaft.
   1. Pull the axle shaft outward.

1. NOTE: If a new pinion shaft lock bolt is unavailable, coat the threads with Threadlock and Sealer EOAZ-19554-AA or equivalent meeting Ford specification WSK-M2G351-A5 prior to installation.
   Install the differential pinion shaft.
   1. Align the hole in the differential pinion shaft with the case lock bolt hole.
   1. Install a new differential pinion shaft lock bolt.

1. Install the differential housing cover and fill the rear axle with the specified lubrication. For additional information, refer to Differential Housing Cover in this section.

1. Install the rear brake anti-lock sensor. For additional information, refer to Section 206-09A .

1. Install the rear brakes. For additional information, refer to Section 206-04 .

1. Install the tire and wheel assembly. For additional information, refer to Section 204-04 .

1. Lower the vehicle.

You are correct, it's not brake fluid. Check the level in your rear axle, it sounds like you have bad seals in the axles allowing the rear differential gear oil to leak inside the brake drums.
This is a heavy weight oil, 80 or 90 weight. This leakage will ruin your brake pads, and if enough leaks out, your rear differential will blow. The seals aren't expensive, but it's a lot of labor to pull everything apart to get at the seals, plus new rear brakes. My Ford ( and I'm sure your GMC) require removal of the rear axles to change the seals. Good Luck! countrycurt0