Brake problem 86 F250

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Jan 13, 2005 | 12:30 PM

salewit

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Brake problem 86 F250

I was driving to a friends house this morning, and I noticed the brake light flicker on. Didn't think much of it. Moments later it came on solid. I stepped on the brakes, and they were hard.... as if the power booster were not connected. I pumped 2 or 3 times and I *thought* that the pedal went to the floor, but not sure (was in heavy traffic... so many things going on). When I stopped, the light had gone out and never came back on. I managed to drive slowly back home. Brakes were between medium hard and normal on entire trip home. Emergency brake works fine. Quick look at front brakes look good. Master cylinder full (in fact fluid came out when I opened top ???). Power booster was replaced about one year ago.

It doesn't seem like the light would come on for a bad power booster (it didn't when it went bad last time). But is the light in any way connected to the vacuum pump? Is there a low vac sensor? If not then it seems to me that a bad vac pump would not trigger the light either. Just thinking/typing out loud.

Jan 13, 2005 | 01:01 PM

HamRadioDude

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From: Eustis, Florida

It sounds to me like you have got an air bubble trapped in the brake pressure differential block down on the frame I would suggest bleeding them also what color is you fluid??

Jan 13, 2005 | 01:25 PM

salewit

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I just checked all the vacuum lines and the pump pressure. I get about 7 inches of vacuum at idle. Needle steady. When I rev to about 1500-2000 RPM, it will climb as far as 10 inches. Don't know what it's supposed to be at.

Haven't checked the differential valve yet. The system hasn't been opened as long as I've had it (almost 2 years) so it seems odd to me that air would develop in the system. Haven't look at the fluid color yet either.

Last edited by salewit; Jan 13, 2005 at 01:33 PM.

Jan 13, 2005 | 01:35 PM

pnose

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From: newfoundland canada

Dissconnect the vac hose from the brake booster and put your vac guage in it.You should have 21 inches hg of vac while idleing for 30 secs.Yes low vac will turn on the brakelight.

Jan 13, 2005 | 10:28 PM

salewit

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Possibly an intermittent problem? At first I disconnected the vac line that goes into this round plastic ball-like object and put my guage on the line, and got 7" of vac. Reving the engine only gave me 10" at the most. I then re-connected there and tapped off another item (I think the temperature control door line) and got the same exact thing. Then I connected it to the booster line and got 7". I then revved the engine and it almost immediately jumped to about 20" and stayed there.

BTW, when I killed the engine with the valve attached, it stayed steady as a rock, so I'm assuming there are no vac leaks.

I've got a new vacuum pump that I'm gonna put in tomorrow. Since it was such an intermittent problem in the first place, I may not know for awhile whether this is it or not.

Thanks for everyones help.
Sam

Jan 13, 2005 | 10:59 PM

PLC7.3

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From: Manitoba

Go here and snoop around this guy has a vacuum test on one of the pages.....

28385&cat=500&thumb=1

Jan 13, 2005 | 11:48 PM

salewit

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Hey thanks a lot! That's a great resource. I just spent hours at the library, and they just don't have diesel repair books, and the standard books assume engine vacuum.