self applying brakes!!!
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Lead Driver
Joined: Mar 2008
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From: Near Los Angeles
self applying brakes!!!
1975 f250 with SuperDuty master and hydroboost. Drove great for ... like 5 years, no prob. Now, the brake light switch got loose and backed almost all the out, meaning the pedal got further away from the firewall, and the brakes started self activating. Today I repositioned the switch and locked it in place with the two nuts. But didn't realize the plastic threads were stripped. I was driving on crowded LA street with wife and dog and felt brakes begin to drag. Got over to right and they locked up. I had to put it in granny 1st to get to a driveway to be safe. Turn engine off and the problem vanishes because the pump is not spinning.
Can anyone explain how the brakes self apply without the pedal moving? There is zero electronics connected to the braking system. How is that possible?
Can anyone explain how the brakes self apply without the pedal moving? There is zero electronics connected to the braking system. How is that possible?
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Lead Driver
Joined: Mar 2008
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From: Near Los Angeles
So, I don't how the brake light switch could possibly affect the brakes/hydroboost, but it does! Threads on the switch were stripped out. I replaced the switch, threw out the cheap crap nuts and bought steel jam nuts, ground one down the thickness of the cheapos, and installed. Brakes back to normal.
The only thing I can possibly blame it on is that if the pedal moves too far out, something is triggered in the master and locks up the system????
Thanks for the reply, but is pressure were leaking into the master, then p/s fluid would be leaking out. It is bone dry.
The only thing I can possibly blame it on is that if the pedal moves too far out, something is triggered in the master and locks up the system????
Thanks for the reply, but is pressure were leaking into the master, then p/s fluid would be leaking out. It is bone dry.
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Joined: Apr 2004
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When you apply the brakes, fluid from the hydro boost unit travels into the booster under pressure. What I think is happening is the valve that allows this is leaking. Thus it's all internal.
That valve has to be part of the master cylinder.
That valve has to be part of the master cylinder.
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Lead Driver
Joined: Mar 2008
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From: Near Los Angeles
Thank you. The missing part of your theory, and I appreciate the help, is the brake light switch. Switch fine, brakes fine. Switch almost comes loose, allowing pedal to travel further away from firewall, and brakes lock up. Replace switch and all is good once more.
Tuned
Joined: Jan 2019
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That can happen when the spool valve in the HB gets over centered backwards. Applies pressure to the master as if pushed forward. I would fabricate a mechanical device that limits the upward pedal travel instead of relying on the brake light switch.
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Lead Driver
Joined: Mar 2008
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From: Near Los Angeles
I still have to say that I LOVE my hydroboost. It is one of the best things I have done to the truck.
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devino246
1973 - 1979 F-100 & Larger F-Series Trucks
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Nov 18, 2011 07:48 PM
