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brake pressure builds, and locks brakes

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Old 04-19-2010, 05:34 PM
byron74f100 byron74f100 is offline
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brake pressure builds, and locks brakes

Hi, I have a 74 f100 with manual brakes, rear drum, front disk, and I replaced the rear shoes and wheel cylinders, and the master cylinder, and a couple rear brake lines. Now after bleeding the brakes, after I drive for a bit, maybe 10-20 minutes the brake pressure gradually builds without releasing, and I don't understand why. Can air in the system cause this to happen? I did have a bit of trouble getting the brakes bled sufficiently, so I thought maybe that's all it is, but now I don't know any more.

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Byron
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Old 04-19-2010, 08:41 PM
Robert R Robert R is offline
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A bad rubber brake hose can cause this.
Sometimes when they get old they brake up on the inside trapping fluid in the line causing the brakes to hold pressure after you let off the pedal.
Pump your brakes till they lockup then crack the bleeders one at a time.
The one that bleeds off the pressure will tell which hose to change.
Hope this helps.
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Old 04-20-2010, 08:56 AM
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You indicated it didnt start until after the new parts, possible wrong or bad master cyl. Take the cap off and watch for a little "geyser" to shoot out of the holes in the bottom of the reservoir while someone pushes the pedal. If not the pisons are sticking or the pushrod is too long
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Old 04-20-2010, 10:29 AM
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Sounds like the master cylinder pushrod is too long. If it's not releasing all the way, it's not uncovering the ports in the master cylinder and won't allow the brakes to fully release.
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Old 04-20-2010, 11:02 AM
byron74f100 byron74f100 is offline
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Ok, Thanks for the help, I'll check out those things today.
Byron
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Old 04-20-2010, 03:25 PM
byron74f100 byron74f100 is offline
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problem seems to be fixed

It seems the push rod was a little longer on this master cylinder than the one I pulled out, I backed off the brake light switch so the pedal could travel farther out, and so far it looks like it's working. It's funny how the simple things are the most overlooked!
Thanks again for all the help.
Byron
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