Why does my brake pedal go to the floor?

Tiny
GFARM1
  • MEMBER
  • 2000 CHEVROLET TRUCK
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 170,000 MILES
When my truck is off can pump up pedal to get firm brake pedal however when engine on pedal sinks to floor with minimal braking power. Replaced rear wheel cylinders, no change, master cylinder no change, replaced ABS unit no change.
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Thursday, August 4th, 2016 AT 4:37 AM

39 Replies

Tiny
HMAC300
  • EXPERT
I would suspect there is air in the lines. These guides can help you fix it.

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.
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Thursday, August 4th, 2016 AT 7:35 AM
Tiny
GFARM1
  • MEMBER
How can I bleed the air out of the ABS unit?
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Thursday, August 4th, 2016 AT 7:43 AM
Tiny
HMAC300
  • EXPERT
You will need a scanner to open the ABS valve to get the air out.
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Thursday, August 4th, 2016 AT 9:28 AM
Tiny
505200
  • MEMBER
  • 1998 CHEVROLET TRUCK
  • V8
  • 4WD
  • AUTOMATIC
  • 124,000 MILES
I have been working on a customers brakes. When he brought it in he just wanted front pads and the rotors turned. I did that and the brakes were still not what I would call acceptable. If you pushed on the pedal it would eventually go clear to the floor. I read in your forum about changing the rear shoes to the origional GM shoes. That helped considerably however they still feel spongy. I have bled them several times and cannot get any air out of them. I am stumped, I have never had the ABS light come on except when I turn on the ignition and it goes throuth the checks.

Thank you
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
JACK42
  • EXPERT
Spongy but has a pedal? What condition are the brake hoses in? Might have one that is swelling giving you a bad pedal feel?
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
LIKETOWIN77
  • MEMBER
Here's how to fix your problem with spongy brake pedal. The master cylinder must have, absolutely has to have, about an 1/8 inch of preload on it in order to work properly. So in other words, when you put it up to the booster to mount it, if the little rod isn't pushing it away from the booster, you need to replace the little push rod or make a new push rod out of a hardened bolt that is the same diameter as the factory rod. You'll have to get one that's about 8 inches long so you'll have enough bolt where there's no thread after cutting it. Buy 2 bolts, you'll mess up the first one. Cut it slightly longer so you can grind it down little by little until it holds the master cylinder away from the booster about an 1/8 inch before tightening it. You will have brakes after that. When I had my shop open years ago, I remember sometimes when replacing master cylinders, they had that little bit of preload on them so I tried it on my truck and behold, I had brakes again.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
KEN
  • ADMIN
Excellent addition to this thread! Please feel free to help out whenever you are on the site :) Cheers, Ken
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
WILEYD
  • MEMBER
  • 1998 CHEVROLET TRUCK
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 90,000 MILES
Brake pedal is soft, it will go almost to the floor, we installed new brake pads and calipers on the front and new brake pads on the rear bleed the system 5 times. There appears to be low brake fluid pressure at the rear wheels
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
HMAC300
  • EXPERT
Try power bleeding the system rental shops carry the tool it may be an abs problem however and you'll need a pro to do that if it is.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
WILEYD
  • MEMBER
The system doesn't have abs brakes, the rear are drum and the front are disc, how much fluid pressure should be at the rear brakes when you bleed them. What I am seeing seems low. Tim
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
HMAC300
  • EXPERT
Is this a pickup? It should have abs if it is. Rear brake pressure won't be a lot as most stopping is done on fronts with discs. We don't have specs on that. That is why I said try a power bleeder.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
FUN4VERN
  • MEMBER
  • 1998 CHEVROLET TRUCK
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 167,000 MILES
Truck was given to me and have been fixing little by little. Your site here has been loads of help but now have a problem that no one else has addressed so far as I see.
No brake pedal and the master cylinder looked like it had leaked some so replaced it. Bench bled and re bled all at all 4 wheels. But then I noticed that when the pedal was pumped brake fluid was coming back into the reservoir? I know that parts are bad off the shelf some time but before I go and remove the master cylinder and do a swap I was wondering if there might be something wrong with the ABS system or a check valve or something to let the fluid go back into the master cylinder resivoir, oh yes it is going into the chamber of the resivoir closest to the firewall. Any assist ya all can give will be greatly appreciated.
Thanks
Vern
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
JACOBANDNICKOLAS
  • EXPERT
Vern:
It sounds like a bad master cylender. When you say there are no front brakes, do they work at all? Are the calipers ok? Finally, do the rear brakes work properly?

Joe
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
FUN4VERN
  • MEMBER
Thanks for the reply. Got a new master cylinder and yes have back brakes. The puzzle is when ya hit the brakes fluid returns to the back part of the resivoir of the master cylinder. Everything up front is good. This truck has the ABS system.
Thanks again
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
JACOBANDNICKOLAS
  • EXPERT
Vern:
My guess is that you got a bad master cylender. I can't think of anything that would send the fluid back into the resevoir.

I'll ask a couple of my buddies on this web site to take a look, but I'm not sure they will know what to say.

Let me know what they recommend.

Joe
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
JNOVACK
  • EXPERT
Does this master cylender have the abs attached to it or is the abs on its own this is the most common used This will cause the issue you are having

EHCU Valve

See Figure 1

The EHCU valve is mounted near or under the master cylinder and combination valve. The valve is not serviceable and must be replaced if malfunctioning.


https://www.2carpros.com/forum/automotive_pictures/52960_0900c1528008f6dd_1.jpg


Fig. 1: Exploded view of the Electro-Hydraulic Control Unit (EHCU)
Front Wheel Speed Sensors

With one exception, on both 2WD and 4WD vehicles, the front wheel speed sensors are permanently mounted to the brake rotor splash shield. With the exception of the 1992-98 Suburban, if the sensor fails the rotor and splash shield must be removed. On 4WD vehicles, the hub and bearing assembly must also be removed for access.

The front wheel speed sensors on 1992-98 4WD Suburban vehicles are removable without disassembly of the hub or brakes.

Rear Wheel Speed Sensors

Except for 1992-93 Suburban, the rear wheel speed sensors are held by 2 bolts at each rear wheel. The brake drum and primary brake shoe must be removed for access.

The 1992-93 Suburban receives the rear wheel speed signal from the Vehicle Speed Sensor (VSS) buffer. The VSS is located at the left rear of the transmission. The buffer, a unit for interpreting the electrical signal from the sensor, is located behind the instrument cluster. The buffer, formerly known as the digital ratio adapter controller (DRAC), is matched to the final drive and tire size of each vehicle. If the final drive or tire size is changed, the buffer unit must be replaced to maintain accurate speedometer/odometer readings and proper ABS function.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
JNOVACK
  • EXPERT
Here i another system they used
C/K Series

MANUAL BRAKES

See Figure 2

Disconnect the brake lines at the valve. Plug or cap the lines and ports.
Unplug the switch wiring connector.
Remove the valve-to-bracket bolts.
To install:

Install the valve. Tighten the bolts to 20 ft. lbs. (27 Nm).
Bleed the system.


https://www.2carpros.com/forum/automotive_pictures/52960_0900c1528008f731_1.jpg


Fig. 2: Combination valve-C/K Series
POWER BRAKES

Disconnect the brake lines at the switch. Plug or cap the lines and ports.
Unplug the warning switch wiring connector.
Remove the anti-lock brake system control module from the bracket. See the procedure later in this section.
Remove the bolts attaching the ABS isolation/dump valve to the bracket. See the procedure later in this section.
Remove the nuts that attach the master cylinder and bracket to the booster.
Remove the bracket and combination valve assembly.
To install:

Position the bracket/valve assembly and install the master cylinder-to-booster nuts. Tighten the nuts to 20 ft. lbs. (27 Nm).
Install the ABS isolation/dump valve nuts. Tighten them to 21 ft. lbs. (24 Nm).
Install the ABS control module.
Connect the wiring.
Connect the hydraulic lines.
Bleed the system.
SWITCH CENTERING

Whenever work on the brake system is done it is possible that the brake warning light will come on and refuse to go off when the work is finished. In this event, the switch must be centered.

Raise and support the truck.
Attach a bleeder hose to the rear brake bleed screw and immerse the other end of the hose in a jar of clean brake fluid.
Be sure that the master cylinder is full.
When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be held in the open position (with the tool described in the brake bleeding section installed under the pin mounting bolt). Be sure to tighten the bolt after removing the tool.
Turn the ignition key ON. Open the bleed screw while an assistant applies heavy pressure on the brake pedal. The warning lamp should light. Close the bleed screw before the helper releases the pedal.
To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.
Repeat Step 5 for the front bleed screw.
Turn the ignition OFF and lower the truck.
If the warning lamp does not light during Step 5, the switch is defective and must be replaced.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
ESDLS
  • MEMBER
  • 1996 CHEVROLET TRUCK
Brakes problem HD K2500, Silverado, ABS brakes, vacuum assisted power brakes.

I had good brakes firm pedal, pedal only moved about 1/3 distance to floor to attain firm solid braking action.

Then inspected front disk brake pads, needed replacement. Removed wheels, depressed brake caliper pistons, removed calipers, replaced disk pads, reinstalled calipers and wheels and started the truck, actuated brake several times to reset disc pads.

Now I have soft pedal, low pedal, pedal would travel all the way to the floor with steady foot pressure.
Replaced master cylinder, bench bled master cylinder, bled front brakes did the thing!

Thought new master cylinder might be bad, took new master cylinder back to parts store, exchanged master cylinder for Another new master cylinder bench bled new master cylinder, put the second new master cylinder on truck, bled front brakes, started truck, tried brakes, same thing, had soft pedal, low pedal, pedal would travel all the way to the floor with steady foot pressure.

While both new master cylinders were installed on the truck, removed cap from master cylinder, stepped on brake pedal, brake fluid squirted like a fountain out of the reservoir neared the windshield on both new master cylinders, even after the master cylinder secondary piston moved past the fluid inlet hole located in the reservoir. What has happened to the master cylinders? I do not know if the truck is even safe to drive with the soft pedal, low pedal, pedal that travels all the way to the floor with steady foot pressure.

Please help! Do you have any ideas what might have happened?

Thanks, Bob Zinter, Aurora, CO.
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
MHPAUTOS
  • EXPERT
Hi,

This is a common problem with these trucks you must get a ABS scanner that will open the ABS valves to let the air out.

Here is a video to show you how to connect.

https://youtu.be/rTtAnsOlZU4

If that does work I would get hold of three vise grips and clamp off all the flexible brake hoses, see how the pedal is then. if it is firm the problem will be in one of the brakes if it sinks it is the master cylinder.

Check out this guide.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Mark (mhpautos)
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)
Tiny
ESDLS
  • MEMBER
Dear Mark (mhpautos)

I have suspected that the ABS might be the problem, as when I first bought the truck I had the same problem, but after I changed the master cylinder the problem went away. The problem that the truck had when I first bought it was as follows;

Soft brake pedal, very weak braking power, excessive pedal travel. One day when I was driving the truck on an icy road and had to stop the front wheels began to slip on the ice and the ABS kicked in, after the truck stopped and the red light turned green, I drove to the next red light and when I stepped on the brake, the brakes worked absolutely perfect very little pedal travel, firm brakes pedal, the truck stopped better than it ever did.

Then for no reason at all the brakes would go back to soft pedal and excessive pedal travel again. So I would find an icy patch of road and intentionally skid the front wheels to activate the ABS and most of the time the brakes would begin to work perfectly again. This went on for about a month, then it stopped and never happened again for about a year, until last week when

I changed the front disk brake pads and it began again. Does this additional information provide you with any additional clues on what might be happening to my truck braking system?

Thanks, Bob Zinter
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Tuesday, June 1st, 2021 AT 10:52 AM (Merged)

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