GMC Sierra 1500 / 2500 / 3500:

abs brake system

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#1 5/22/2010 12:15 PM

bonnie's guy
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Registered: 5/22/2010
Posts: 1

abs brake system

:|My 99 sierra brake pedal goes almost to the floor on occasion. I have no leaks and the master cylinder is full. Is this caused by a faulty abs control unit or something I can repair?

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#2 5/23/2010 12:02 AM

TechHelp2
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Registered: 11/23/2009
Posts: 1035

Re: abs brake system

This is something you can repair if you are qualified.

If there aren't any leaks, the fluid is full and you are not adding brake fluid to the master cylinder, the rear drum brakes (if applicable) are properly adjusted then the problem seems to point to a bad master cylinder. The internal seals on the primary or secondary pistons inside the master cylinder are leaking internally causing the pedal to sink to the floor. You can usually give the pedal another pump and it will come up. Either way you look at it the master cylinder should be replaced.

If the ABS system had a problem the trouble light will light up on the dash and the ABS system would be disengaged. You would still be able to stop in a normal and safe manner.

Scanning the ABS system wouldn't be a bad idea just to be sure the system is working properly.


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#3 12/8/2010 10:50 PM

Harold
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Registered: 12/8/2010
Posts: 2

Re: abs brake system

I have a 1997 GMC 1500 that has this same problem. The master cylinder has been replaced, the ABS has been replaced, the brake booster has been replaced,  the brakes have been bled and when the truck is not turned on the brake pedal is 2/3 the way up. As soon as the truck is started and you step on the brake it will almost go to the floor. Even if the pedal is pumped it will not come back up until the engine is turned off. Just wondering if the vacuum to the brake booster is causing the problem. Or is there something else that needs to be looked into.  The back brakes have been replaced as well.

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#4 12/9/2010 6:49 PM

TechHelp2
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Registered: 11/23/2009
Posts: 1035

Re: abs brake system

Did you do the work yourself or was it done in a qualified shop?

The rear drum brakes should have the adjustment checked first. If the drums were cut then the lip should have been ground off and proper brake adjustment can be done. If the rear drums were not cut then the lip would still be there and not let you adjust the brakes properly.

When the lip is left, you can adjust the brakes as far as the width of the lip of the drum. Then when the drum is put on it feels tight till it passes the lip of the drum. When the drum is installed all the way, you will still have a large enough space between the rear brake shoes and the drum to have a low pedal.

If you can't pump the pedal up then you probably have air in the system.

There are two ways you can bleed the system. One is to pressure bleed the brake system. The other way you can do yourself. It involves a small bottle and a rubber hose.

First you go to the furthest wheel from the brake master cylinder which would be the right rear wheel. Open the bleeder valve and place the hose over the bleeder valve. It should be about 5/16 hose. Place the other end of the hose into the bottle.

Start pumping the pedal slowly till you see the fluid going into the bottle. Keep pumping and observe the flow of brake fluid into the bottle. If there is any air in the line you will see it bubble out. After about 10 pumps, check the master cylinder and refill it as needed.

Then go to the left rear wheel and do the same thing. The next wheel would be the right front wheel then the left front wheel.

The reason for the bottle is to make sure air is not sucked into the line again when you take your foot off the pedal. With the hose in the bottle and brake fluid in the bottle, you can take your foot off the pedal and it will suck in brake fluid and not air. Always make sure the brake fluid is over the end of the hose inside the bottle.

After you check all of the wheels this way you should have a better pedal.

Let me know if this information was helpful.


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#5 12/10/2010 4:44 PM

Harold
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Registered: 12/8/2010
Posts: 2

Re: abs brake system

TechHelp2 wrote:

Did you do the work yourself or was it done in a qualified shop?

The rear drum brakes should have the adjustment checked first. If the drums were cut then the lip should have been ground off and proper brake adjustment can be done. If the rear drums were not cut then the lip would still be there and not let you adjust the brakes properly.

When the lip is left, you can adjust the brakes as far as the width of the lip of the drum. Then when the drum is put on it feels tight till it passes the lip of the drum. When the drum is installed all the way, you will still have a large enough space between the rear brake shoes and the drum to have a low pedal.

If you can't pump the pedal up then you probably have air in the system.

There are two ways you can bleed the system. One is to pressure bleed the brake system. The other way you can do yourself. It involves a small bottle and a rubber hose.

First you go to the furthest wheel from the brake master cylinder which would be the right rear wheel. Open the bleeder valve and place the hose over the bleeder valve. It should be about 5/16 hose. Place the other end of the hose into the bottle.

Start pumping the pedal slowly till you see the fluid going into the bottle. Keep pumping and observe the flow of brake fluid into the bottle. If there is any air in the line you will see it bubble out. After about 10 pumps, check the master cylinder and refill it as needed.

Then go to the left rear wheel and do the same thing. The next wheel would be the right front wheel then the left front wheel.

The reason for the bottle is to make sure air is not sucked into the line again when you take your foot off the pedal. With the hose in the bottle and brake fluid in the bottle, you can take your foot off the pedal and it will suck in brake fluid and not air. Always make sure the brake fluid is over the end of the hose inside the bottle.

After you check all of the wheels this way you should have a better pedal.

Let me know if this information was helpful.

I am a qualified heavy equipment mechanic so I did the work myself. I have bled the brakes about four times. After bleeding the brakes and with the engine off I have 3/4 of a peddle. Once the truck is started and running when I apply the brake it drops almost to the floor. This is why I believe that the vacuum is the problem. I can disconnect the vacuum hose and I get the 3/4 peddle back.

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#6 12/10/2010 7:22 PM

TechHelp2
Administrator
Registered: 11/23/2009
Posts: 1035

Re: abs brake system

Are the rear brakes drum or disc? You didn't mention it in your question. If they are drum brakes, try these suggestions.

Apply the brake at least five times with the engine off to remove vacuum from the booster before making the check.

Measure the distance from the bottom of the steering wheel to the brake pedal.

Take the first measurement with the brake pedal released.

Take the second measurement after applying the brake pedal with about 445 Nm (100 lbs.) of force using a brake pedal effort gage or estimate if you don't have the gauge.

Subtract measurement from the brake pedal applied from the measurement from the brake pedal released.

Compare the measurement of travel with the specifications below:

Vacuum Booster 80 mm (3.1 inches)

Hydraulic Booster 110 mm. (4.3 inches)

Four-Wheel Disc Brakes 102 mm (4 inches)

Also check the diameter of the rear brake drums if you have rear drum brakes.

The diameter from the factory is 10.00 in. 11.15 or 13.00.

Maximum: 10.05 in. 11.21 or 13.06.

Replace: 10.09. 11.24 or 13.09.

The above measurements will put you in the ballpark of how much travel you are allowed.

Inside Diameter Check:

Measure the inside diameter of the brake drum at two or more places around the circumference of the braking surface. The measurements must be made at the same distance in from the edge of the drum. Compare the results with the wear specifications.

Taper Check:

Measuring a drum for taper involves taking measurements at the inner and outer edges of the machined surface at two or more places around the drum. These measurements should be equal.

You can also check the parking brake. Make sure the cables are not frozen and move freely. An out of adjusted or malfunctioning parking brake can interfere with achieving the proper rear brake adjustment.

If the Brake Pressure Modulator Valve (BPMV) of the Four Wheel Anti-lock Brake (4WAL) system is replaced or suspected to have air trapped inside, it must be bled next.

Try bleeding from all the valves that were replaced. Air can still be trapped in those areas where the hydraulic lines were open for part replacement even though you are not getting air from bleeder valves

When bleeding the brakes try pushing the brake pedal down slowly and holding it there for about 15 seconds. Then lock up the bleeder valve and let the pedal up slowly and wait for another 15 seconds. When you do this procedure slowly it will give the air time to move through the system if it is stuck.

If you can scan your ABS system for trouble codes it would be helpful.

Try these new suggestions and let me know how you make out.


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