Why does my brake pedal go to the floor?

Tiny
OXCARTMAN AND WIFE
  • MEMBER
  • 1999 CHEVROLET SUBURBAN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 170,000 MILES
We replaced the master cylinder due to internal leak on old one. The brakes were very soft and no fluid in reservoir. After we checked lines and wheels and saw that they were not wet, so put on new master cylinder. Did bench bleed before installed and bled at wheels. Now when you push the brake pedal it goes to the floor and stays there will not come back up. You cannot pull it up. It will only come up if you turn off the vehicle or unhook the vacuum line on the brake booster. The brake booster has strong vacuum. What could be causing the brake to stay down?
Do you
have the same problem?
Yes
No
Monday, April 2nd, 2018 AT 6:44 PM

30 Replies

Tiny
STEVE W.
  • EXPERT
Well the description sounds like the master cylinder you have is the wrong one. There are a couple different ones out there and they use different length apply rods from the booster. Using the wrong one lets the booster apply valving stay on and it applies vacuum regardless of the pedal position. It also sounds like you have the typical GM brake issue where you need to bleed the brakes two to three times to actually get all of the air out of the system, especially the ABS unit. 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.
Was this
answer
helpful?
Yes
No
Tuesday, April 3rd, 2018 AT 12:57 PM
Tiny
ACE58
  • MEMBER
  • 1999 CHEVROLET SUBURBAN
I had a rear wheel cylinder seal failure. The pedal dropped down, the dashboard brake light came on, but I still had front brakes. I decided to convert to rear disc brakes instead of repairing existing drum set up, with a complete kit from EGR. I continued to drive the vehicle, on a limited basis, until the parts arrived, adding fluid as necessary. I installed the complete kit and bled all 4 calipers. I still have a low pedal, and the brake light is still on. How can I remedy situation? Thanks
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
BLACKOP555
  • EXPERT
If the fluid got too low it could be that youve gotten air in the master cylinder and it will have to be bench bled before anything else can commence

will pedal get hard with key off even a slight bit harder.
Was this
answer
helpful?
Yes
No
+5
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
ACE58
  • MEMBER
Yes, it is marginally better with key off.

I bled lines at master cylinder and at each wheel, left rear-left front-right rear-right front, with no improvement in pedal or elimination of light on dash
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
BLACKOP555
  • EXPERT
Pump it up the pedal till its hard. From there push about 200 pounds of force on the pedal and hold it, does pedal stay or does it continue to sink lower, if it goes down master cylinder may be leaking internally.
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
ACE58
  • MEMBER
With the hydra-boost, the pedal doesn't pump up but, it does maintain a constant level, although quite low. While running, the same level is maintained and when "200 pounds pressure" is applied the front wheels lock up. The rear discs stay cool indicating they are not working.

A friend said an air bubble in the ABS could cause a problem but, wouldn't air be eliminated through bleeding? Thanks for your help.
Was this
answer
helpful?
Yes
No
+2
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
BLACKOP555
  • EXPERT
I was saying put on the pressure so if it goes down then your loosing brake fluid through a leak causing the pedal to be low.

Air can cause a low pedal, but abs has to be bleed with a scan tool not the conventional techniques.
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
ACE58
  • MEMBER
Thanks for reply. Been away. What type of scan tool is used for ABS brake bleeding? Is this a dealer only tool?
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
BLACKOP555
  • EXPERT
For the price it might as well be
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:46 AM (Merged)
Tiny
GMARCOS74
  • MEMBER
  • 1996 CHEVROLET SUBURBAN
  • 301,597 MILES
Ok, When I reverse and then put my truck in drive my brake light goes on and my brake pedal goes down to the floor. It is fine after I pump the pedal and the light goes off. The pedal has a great feel after that. But as soon as I reverse it happens again. I already had a mechanic check it and he had me replace the Master cylinder and rear brake shoes. But it still does the same, I think my mechanic ( he is old school, used to work on pre 1972 trucks) might not know that much about bleeding the abs system. Would this be causing it. Please let me know
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
GMARCOS74
  • MEMBER
Also, when driving forward there is a slight rubbing noise from the right rear that goes away when I press on the brake pedal.
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
HMAC300
  • EXPERT
Have your brakes checked it sounds like you may have either air in system or a bad master cylinder. Also there is a valve in the abs that you need a scanner to activate it and it can take up to 3qts of fluid to bleed the system and that may be the problem the valve isn't working right. If the abs light is on then you have a code set and a scanner can see what the problem is.
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
HMAC300
  • EXPERT
The rubbing noise may be rust or rotor hitting backing plate
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
GMARCOS74
  • MEMBER
Awesome Thanks
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
CZR1760
  • MEMBER
  • 1993 CHEVROLET SUBURBAN
  • 2WD
  • AUTOMATIC
I have the SUV listed above it is a 1500 with over three hundred thousand miles. I recently started having problems with the brake pedal sinking to the floor. This problem seems to be more noticeable when I am stopped (at a traffic light) and holding. I first tried to bleed the brakes, but this had no effect. I changed out the master cylinder (re-manufactured), bled the brakes and still have the same problem. I also had to install a new front caliper, because the bleeder bolt would not come out of the old one. The truck seems to stop okay, but has a soft pedal. If I press on the brake pedal quickly, I can hear a "woosh" noise. I do not know if I got a bad master unit or if I have a bad vacuum booster as well. Any help would be appreciated. Thanks.
Was this
answer
helpful?
Yes
No
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
DOCFIXIT
  • EXPERT
Rebuilt masters are no good I would get a new one.

Here is a guide to help you:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Please let us know what you find.

Cheers
Was this
answer
helpful?
Yes
No
+1
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
CZR1760
  • MEMBER
I bought it from O'Rieley's so I can just take it back. I did a good bench bleed on it before installing it. I read that I could clamp off the brake lines and then see if the pedal got hard, just to make sure that it was the master. It is really easy to pull off, so I will probably just swap them out. Should I suspect anything with the vacuum or just leave it alone? Thanks.
Was this
answer
helpful?
Yes
No
-1
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
DOCFIXIT
  • EXPERT
If rear wheel cylinders are not leaking or sucking air. Thing to do is with small screwdriver pull boot back if fluid comes out then replace wheel cylinders first. And bench bleed is way to go.
Was this
answer
helpful?
Yes
No
-1
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
CZR1760
  • MEMBER
I swapped out the re-manufactured master for a "new" one. Bench bled, bled out the lines and still have the same problem. While the truck is running, the pedal does not seem to fully engage until about half way. If I apply pressure very quickly the pedal seems like it has correct feel (although you will forward if going too fast). If I apply slowly the pedal is down about half stroke and continues almost to the floor when stopped. The pedal travels so far down, that if I push slightly further than required to stay stopped, the "brake" light comes on the dashboard. At that point the pedal is completely stroked out. Where to go from here? Thanks.
Was this
answer
helpful?
Yes
No
-1
Monday, March 15th, 2021 AT 11:47 AM (Merged)
Tiny
DOCFIXIT
  • EXPERT
Did you check rears?
Was this
answer
helpful?
Yes
No
-1
Monday, March 15th, 2021 AT 11:47 AM (Merged)

Please login or register to post a reply.

Sponsored links