Soft brake pedal
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5th Wheeling
Joined: Feb 2019
Posts: 36
Likes: 5
Soft brake pedal
New master cylinder (bench bled) installed, bled brake lines above and below ABS, bled ABS with scan tool, bled below ABS again. Pedal is firm without having to pump when the truck is off, sinks to floor if pressed slowly when engine on, firm if pressed fast. Tested ABS using testing ports. Seems fine. No ABS light. Thoughts?
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Slow going down is fluid bypassing the master cylinder bore ports.
Can't keep pressure that way.
Be sure the push rod is adjusted to proper length.
Certain ABS fault can also cause this effect.
The power brake booster has vacuum on at all times with the engine running. A booster leak can also put a pull pressure on the diaphragm to pull it down in the presence of the bypass condition.
I would say it's not air in the system by the way you describe it's action but a fault you have yet to find.
Good luck.
Can't keep pressure that way.
Be sure the push rod is adjusted to proper length.
Certain ABS fault can also cause this effect.
The power brake booster has vacuum on at all times with the engine running. A booster leak can also put a pull pressure on the diaphragm to pull it down in the presence of the bypass condition.
I would say it's not air in the system by the way you describe it's action but a fault you have yet to find.
Good luck.
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