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Brake pedal knocks

Tiny
MANUEL MILLER
  • MEMBER
  • 1979 FORD F-100
  • 3.2L
  • V8
  • 2WD
  • AUTOMATIC
  • 100,000 MILES
I put in a new brake system including; a master cylinder, brake lines and pads. When I was pumping the brakes to fill up the lines and everything there was a knocking in the pedal when it comes halfway up. I don't know what the problem is.
Thursday, July 2nd, 2020 AT 12:38 PM

7 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Can you feel it in the pedal and I assume it did not do this prior to this repair? If not, then we need to suspect the master cylinder first and foremost because the push rod is the only non-hydraulic component that was replaced. The reason I think it needs to be a non-hydraulic component is because brake fluid moving will not cause a knocking noise. It is just fluid. So that means it needs to be an actual moving part.

Let us know this info and we can go from there. Thanks
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Thursday, July 2nd, 2020 AT 6:42 PM
Tiny
MANUEL MILLER
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  • 21 POSTS
No, it didn't knock before I replaced the master cylinder. I replaced the entire brake system because the truck has been sitting probably the last 15 years and the seals in the master cylinder were leaking. You could see the brake fluid on the brake booster.
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Thursday, July 2nd, 2020 AT 7:01 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. Is there a feeling with the noise or can you only hear it? Can you feel a clunk in the pedal when it does it?
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Friday, July 3rd, 2020 AT 6:39 PM
Tiny
MANUEL MILLER
  • MEMBER
  • 21 POSTS
Yes, I can feel it too.
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Friday, July 3rd, 2020 AT 6:55 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. If you feel the knock then it is a mechanical component which is either the lever point on the pedal (doubtful as you didn't do anything to that) or the push rod in the master cylinder. Clearly we could be missing something but if it were me, I would return the master cylinder and have the warranty exchange it for you as that is the most likely cause. Clearly you need to bench bleed the new one and then bleed the rest of the system again.

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

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+1
Saturday, July 4th, 2020 AT 7:20 PM
Tiny
MANUEL MILLER
  • MEMBER
  • 21 POSTS
Thank you I'll try that. And let you know how that turns out.
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Saturday, July 4th, 2020 AT 7:53 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Sounds great. Let us know. We will wait to hear back. Thanks for the update.
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Sunday, July 5th, 2020 AT 8:11 AM

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