Cj5 Brake Problems - JeepForum.com

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post #1 of 6 Old 02-22-2014, 06:50 AM Thread Starter
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1978 CJ5 
Join Date: Feb 2014
Location: Bristol
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Cj5 Brake Problems

Hi everyone, I would love some help with my brakes. I have a '78 CJ5 and whenever I slowly depress the brake pedal it will sometimes travel almost completely to the floor until the brakes are activated. When it is depressed at a "normal" speed, however, the brakes feel and act normal. I've bled the brakes and don't think there is any air in them. Could this be a master cylinder problem? This issue can occur anytime I slowly depress them, whether it is during start up, after driving around for a while, or when it is hot or cold out. Also, I don't have a brake booster and I have discs up front with drums in the back.

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post #2 of 6 Old 02-22-2014, 08:13 AM
John Strenk
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1976 CJ5 
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Location: Shalersville Ohio
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post #3 of 6 Old 02-22-2014, 11:42 AM Thread Starter
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1978 CJ5 
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Location: Bristol
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The fluid level is fine and surprisingly no leaks. The brakes in the back have been adjusted as well.
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post #4 of 6 Old 02-22-2014, 07:31 PM
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1971 CJ5 
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Probably an internal master cylinder leak. The cups have probably shrunk. When you push slowly down, the fluid is passing between the bore and the lip of the cup. When you blast down on the pedal, the pressure rises quickly, forcing the cups up against the inside of the bore, sealing the cylinder.
I would think about replacing it.
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post #5 of 6 Old 02-22-2014, 07:47 PM
1979 CJ7 
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X2 Jeepdaddy2000. I had a jeep that did that and the rubber cups would let the fluid leak past. If I hit the brake hard, the pedal would be almost normal. If you hit the brake hard and stop, leave you're foot on the brake with pressure. If the rubber cups or the bore is scored, the petal will slowly go to the floor (as the fluid leaks past the cups).
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post #6 of 6 Old 02-22-2014, 10:47 PM Thread Starter
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Looks like I'll be replacing the master cylinder and hoping that fixes it. Thanks and I'll keep you posted if it does!
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