How do you bleed the abs unit on a 1997 chevrolet truck

try to drive on gravel and hit brake hard to make sure the ABS pump run then try to bleed again

do what is called a gravity blead. open all bleeders slightly and do not let the master cylinder go dry. after this start at passenger rear then drivers rear, passenger front then drivers. if this does not help you must bring the car in to a garage for abs bleeding and/or new abs module.

Does your truck have ABS? If so you may have a bad brake
pressure modulator for the abs. Its the big box by the Master that says Kelsey-Hayes on it. You can pick up the complete unit from a salvage yard at a pretty reasonable price.

I also found this on another site. I know you said that you blead the Master but check this information out. The bleeding of a new Master Cylinder is very important to get it exactly right.
http://www.ehow.com/how_4508156_bench-bleed-master-cylinder-chevy.html

The brake system bleeding procedure differs for ABS and non-ABS vehicles. The following procedure pertains only to non-ABS vehicles. For details on bleeding ABS equipped vehicles, refer to the ABS procedures later in this section.

WARNING Make sure the master cylinder contains clean DOT 3 brake fluid at all times during the procedure.

1. The master cylinder must be bled first if it is suspected of containing air. Bleed the master cylinder as follows:
   1. Position a container under the master cylinder to catch the brake fluid.
   2. Loosen the left front brake line (front upper port) at the master cylinder and allow the fluid to flow from the front port.
   3. Connect the line and tighten to 24 ft. lbs. (32 Nm).
   4. Have an assistant depress the brake pedal slowly one time and hold it down, while you loosen the front line to expel air from the master cylinder. Tighten the line, then release the brake pedal. Repeat until all air is removed from the master cylinder.
   5. Tighten the brake line to 24 ft. lbs. (32 Nm) when finished.
   6. Repeat these steps for the right front brake line (rear upper port) at the master cylinder.
      

WARNING Do not allow brake fluid to spill on or come in contact with the vehicle' finish, as it will remove the paint. In case of a spill, immediately flush the area with water.

1. If a single line or fitting was the only hydraulic line disconnected, then only the caliper(s) or wheel cylinder(s) affected by that line must be bled. If the master cylinder required bleeding, then all calipers and wheel cylinders must be bled in the proper sequence:
   1. Right rear
   2. Left front
   3. Left rear
   4. Right front
2. Bleed the individual calipers or wheel cylinders as follows:
   1. Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end.
   2. Submerge the other end in a transparent container of brake fluid.
   3. Loosen the bleed screw, then have an assistant apply the brake pedal slowly and hold it down. Close the bleed screw, then release the brake pedal. Repeat the sequence until all air is expelled from the caliper or cylinder.
   4. When finished, tighten the bleed screw to 97 inch lbs. (11 Nm) for the front, or 66 inch lbs. (7.5 Nm) for the rear.
3. Check the pedal for a hard feeling with the engine not running. If the pedal is soft, repeat the bleeding procedure until a firm pedal is obtained.

Fig. 1: Loosen the front brake line in order to bleed the master cylinder

Fig. 2: Connect a bleed hose from the bleed valve on the front caliper to a jar of brake fluid

Fig. 3: Always follow the lettered sequence when bleeding the hydraulic brake system

Hope this helps to solve it; remember to rate this answer.

well, check the brake lines for leakage after adding brake fluid to the full mark. If the brake pedal goes all the way to the floor, it's likely there is a leak somewhere after the master cylinder. If the pedal gets hard after adding fluid, and pumping it several times with the car off, then the master cylinder is still ok.

If air has gotten into the master cylinder, it may have to be bled out on a bench, or using a scanner if that's the way your vehicle is.

Bleeding the Brake System (ABS) Auto Bleed Procedure NOTE: Perform a manual bleeding procedure. If the brake pedal height and firmness results are not achieved, perform the auto bleed procedure below.
NOTE: Perform this procedure when replacing the brake pressure modulator valve or electro-hydraulic control unit.

1. Perform the manual brake bleeding procedure. Refer to Bleeding the Brake System (non-ABS).
2. Check the master cylinder fluid level often to prevent running the system dry.
3. Press the brake pedal firmly and run the Scan Tool Automated Bleed Procedure (for Gas engines) once, or the Function Test (for Diesel engines) four times. Make sure to release the brake pedal between each test.
4. Perform the manual bleed procedure again. This should remove the remaining air from the brake system.
5. Bleed the system as many times as needed to obtain the appropriate feel of the brake pedal.

PRe bleed the master cylinder, using a kit that hooks up to the brake lines connection and the other end goes into the fluid reservoir. Bleed until no bubbles. then reinstall brake lines, one at a time, and then continue bleeding the system. DO NOT let the fluid run low in M/C or start all over again. Kit can be purchased at after market parts stores.

I know there is ABS in that vehicle, you might still have air in the ABS unit, you will need to get a manual that shows you the correct procedure to bleed the brakes. Master installed u need to start the bleed on the bench, then move to the ABS unit then to the individual calipers and/or wheel cylinders if equipped. I think there is a certain way to do it to avoid incorrect bleeding.

But i have also seen bad master cylinders brand new in the box.

Good Luck.

you should first bench blead a new master cyl before installing it on the car.

Did you bleed the wheel furthest away from the master cylinder? THen next furthest, to closest last. Did u make sure the master cylinder never got low enough to draw in air? To bleed, Pumo brakes 3x, holding pedal down 1/2 way, while bleeder is opened, let air/fluid flow, shut bleeder as flow diminishes so no air gets in the line. Is the car running when u bleed brakes? It should be. If u are doing all of this u should feel your pedal coming back (getting firm). Let me know how you make out. countrycurt0

loosen the front brake line that goes to the master cylinder fill master cylinder up with fluid and bleed it first by attaching a flexible line from master cylinder front brake side submerge that line into jar of brakefluid keeping air from entering line get someone to pump brake pedal while ur holding line in a jar keep refilling the brake resivoir so no air gets in, once the master is bled front brake side then reattach the origional brake line then remove bleed screw from caliper and let fluid gravity flow down to calliper then bleed the normal way. more than likely your master cylinder has air trapped in the front portion good luck