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post #1 of 6 Old 09-28-2011, 03:51 PM Thread Starter
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Diagnosing Brake Problem

91 F-250 4x4 7.3 IDI - Went to apply brakes and the pedal was harder than a rock. Panic city. Eventually got it stopped. I am thinking that the problem is probably with the power brake booster. Brakes would work off and on properly on the way back home. Luckily I was only about 3 miles away and all country roads. My question is how to properly diagnose this. To rule out the vacum pump on the engine or the brake booster. Any advice will be greatly appreciated. Thanks
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post #2 of 6 Old 10-05-2011, 08:00 AM Thread Starter
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bump

Come on guys. still need help with this. Thanks.
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post #3 of 6 Old 10-05-2011, 10:38 AM
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I am not too familiar with your truck but if it has a vacuum booster they pretty much all check out the same......If you have a vacuum gauge you can disconnect the vacuum line at the booster and check the source vacuum with the engine running coming from the hose...on a car with a booster its around 18-22 in. hg.....You can start the truck with everything connected with your foot on the brake pedal and the vacuum should pull your pedal down an inch or so.....or you can start the truck with your foot off the brake pedal and shut it off after a few seconds...then go to your booster and pull the plastic check valve (that the hose is attached too) out of the booster and it should make a hissing noise as the vacuum is let out of the booster...if it doesn't hiss the diaphragm in your booster is ruptured....hope some of this helps.
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post #4 of 6 Old 10-06-2011, 07:05 AM
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It's either the vacuum booster, a hose coming off of the booster to the intake, or frozen calipers somewhere . . . Fyi, had a vacuum hose blow off of my mustang while at the drag strip and well let me tell you that was interesting . . . pedal went completely stiff!

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post #5 of 6 Old 10-09-2011, 08:02 AM Thread Starter
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Thanks for the advice. Will be back home soon and try your ideas. Thanks again. Robert
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post #6 of 6 Old 10-09-2011, 09:30 AM
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It sounds to me like you went from power to manual brakes. Look for a missing, loose or cracked vac line. It's cheap enough to just replace them to know they are good as well.

Pull the vac source off and use a gauge to see what you are drawing.



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