Brake pedal goes slowlwy to floor

Tiny
MRT1963
  • MEMBER
  • 2008 FORD F-250
  • 5.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 100,700 MILES
After changing my front pads my brake pedal goes slowly to the floor. When the engine is off I have a solid pedal feel. When sitting with my foot on the brakes and engine running the pedal will drop to the floor. When driving the truck stops but the pedal will keep going down. I did not open any bleeder valves.
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Friday, August 14th, 2015 AT 4:00 PM

7 Replies

Tiny
RIVERMIKERAT
  • MEMBER
Check the calipers for leaks. You probably pinched the piston seal when you collapsed the piston, causing it to leak.
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Friday, August 14th, 2015 AT 9:20 PM
Tiny
MRT1963
  • MEMBER
No leaks anywhere. Followed all lines I am not losing any fluid
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Saturday, August 15th, 2015 AT 7:48 AM
Tiny
RIVERMIKERAT
  • MEMBER
Ahhh. Master cylinder bypassing.

You will need:1 new master cylinder.
At least 1 pint of the appropriate brake fluid.
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Saturday, August 15th, 2015 AT 11:31 AM
Tiny
MRT1963
  • MEMBER
I dont think so. Firm pedal when engine is off. Firm when I first hit the brakes. While holding pedal it will drop only when engine is running. It was fine before I changed pads
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Saturday, August 15th, 2015 AT 1:15 PM
Tiny
RIVERMIKERAT
  • MEMBER
Yep. Firm pedal feel when the engine is off is because you have power brakes. Unless you have a gaping hole in a line, you're going to have a decent pedal feel with the engine off.

If the pedal sinks to the floor with your foot basically just resting on the brake pedal with the engine running, and you're not seeing any leaks anywhere, the only possibility really is the master cylinder bypassing. You won't see any fluid loss because none is being lost. It's going from one side of the reservoir straight into the other. You've got a 2008 truck with over 100k miles. If you haven't flushed the brake system at least twice in that time, I basically guarantee it's the master cylinder. A chunk of sludge has worked its way along the master piston and between the seal and the wall, allowing fluid to go from one side to the other instead of down the lines.

When the pedal won't hold pressure, it means it's losing pressure. There are only two ways for that to happen: a leak or bypassing. Check the lines front and rear and underneath. Check the diverter and proportioning valves. Check the rear calipers as well.

If you have no leaks anywhere, and the master isn't bypassing, it will be the first time for me to hear about something like in more than 40 years of working on brakes.
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Saturday, August 15th, 2015 AT 3:38 PM
Tiny
MRT1963
  • MEMBER
Thank you so much for you help. I am going to go over everything tomorrow. I will let you know how it goes.
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Saturday, August 15th, 2015 AT 4:21 PM
Tiny
RIVERMIKERAT
  • MEMBER
Yes. Please do. Enjoy tonight's games.
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Saturday, August 15th, 2015 AT 4:47 PM

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