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1996 GMC C1500 brake pedal leaking down

Tiny
HOOTERMAN10
  • MEMBER
  • 1996 GMC C1500
  • V8
  • 2WD
  • AUTOMATIC
  • 152,000 MILES
I have replaced the master cyllinder 2 times and ran 2 quatrs of fluid through the lines and still have a problem with the pedal leaking down while being pressed what else can cause this problem?
Saturday, June 5th, 2010 AT 5:49 PM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
If you have a brake pedal that slowly goes to the floor, you either have a bad MC or a leak. Since you replaced the master cylender, there has to be a leak. Have you checked rear wheel cylenders for seapage? Have you checked all rubber brake hoses and steel lines for leaks? Are the rear brakes properly adjusted so as to not over extend the wheel cyenders?

Let me know.
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Saturday, June 5th, 2010 AT 6:45 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Hi hooterman10, Welcome to 2carpros and TY for the donation.

If there's no leakage anywhere could be the ABS unit or the master cylinder again is leaking internally thru seals/cups-investigate
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Saturday, June 5th, 2010 AT 6:47 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
Whaen bleeding brakes should truck be running or off?
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Sunday, June 6th, 2010 AT 10:12 AM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Try both ways if it still sinking and no leak- rear properly adjusted I'll lean towards the MC
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Sunday, June 6th, 2010 AT 2:06 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
Still getting air out of driver rear could the power booster be failing? Bled the line at the ABS no air. When pumping brakes to bleed them many air bubbles popping up in master resevour
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Sunday, June 6th, 2010 AT 8:19 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
To confrim the brake booster is working -pump the brake a few times hold it in and start the vehicle the pedal should move down a lil.

Sounds like the master cylinder is gone if you are properly bleeding the brakes
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Sunday, June 6th, 2010 AT 8:25 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
Ok pumped the brakes and held then started truck pedal goes down pretty far then leaks on down
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Sunday, June 6th, 2010 AT 8:40 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Replace the master cylinder
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Sunday, June 6th, 2010 AT 8:42 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
When I pump the brakes with the engine off it leaks down also. Is it common two get 2 master cyllinders bad in a row?
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Sunday, June 6th, 2010 AT 8:44 PM
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Okay lets try this bleed the MC and then the ABS unit and all the wheels from RR/LR/RF/LF-

No its not common -but could happened
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Sunday, June 6th, 2010 AT 8:47 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
Ok that did not work. Am going to change MC and booster tommorow will see if that works
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Sunday, June 6th, 2010 AT 9:23 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Not common, but it does happen. If there are no leaks, fluid has to be bypassing the seals in the master cylender. You may want to try a different brand.
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Sunday, June 6th, 2010 AT 11:24 PM
Tiny
HOOTERMAN10
  • MEMBER
  • 10 POSTS
Ok new booster and still leaking down going with a brand new MC tommorow. Is there anyway this could be my ABS unit it is a Kelsey Hayes?
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Monday, June 7th, 2010 AT 8:01 PM

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