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brake light on- low vacuum?

 
  #1  
Old 02-02-2017, 03:52 PM
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brake light on- low vacuum?

My dashboard brake light stays on, but sometimes turns off and will also flicker, and be dim. I know that it comes on in three situations: park brake engaged, low vacuum, and proportioning valve "detecting" a leak. i know it isnt the parking brake, and ive unplugged the proportioning valve (which is newer) which leaves vacuum being low.

However, the cruise control still works, even when the light is on, and a few months ago i checked vacuum and if i remember correctly it was a pretty good 24lbs. is there a sensor somewhere i can unplug for vacuum to see if the light goes off? and if it does go off and i have low vacuum somewhere but the pump is fine, does that mean i have a leak?
 
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Old 02-02-2017, 05:30 PM
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It's impossible to have 24 psi of vacuum on a planet where atmospheric pressure is 14.7 psi. Perhaps you have 24 inHg.

What year truck? (And no "it's in my sig" is not an answer....). I'd have to check on my '85 Batmobile, and I'm not familiar with the bricknoses, but I know on an OBS, the main, and most critical from a standpoint of safety, reason the light comes on is when brake fluid is low in the master cylinder reservoir. This can be detected either by low fluid level or a leak. It has nothing to do with any proportioning or RABS valve (the OBS would have a RABS valve). If that's what's turning it on, you need to check for a hydraulic leak.

The sensor that turns it on for weak vacuum is on the vacuum "distributor" aka manifold. It distibutes vacuum from the vacuum pump to the booster and, if applicable, HVAC controls and/or cruise. On the OBS, it's on the driver's side fender well. On the slantnoses, it's on the passenger-side fender well. In any event, if you just follow the hose from the vacuum pump, it'll lead you to it. It'll have an electrical connector on it, which would be the sensor.
 
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Old 02-02-2017, 06:18 PM
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I had forgotten the letters for vacuum (inHg) and figured that labeling the number would make more sense than a random 24 in the post
Truck is a bullnose '85 f-250 super cab 4x4 6.9idi zf-5.
I do have a small brake fluid leak and I am thinking it's the lines themselves that are leaking, because I've replaced literally everything else in the brake system, except for the lines. Although, there are no electrical connections or wires coming from the master cylinder
 
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Old 02-02-2017, 06:46 PM
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In my 88' e250. i had the brake light go on when i depressed the brakes or held them in for a while. I was worried about the pump but then found a few vacuum lines leaking/loose/disconnected. reconnected them and my brake light problem went away.

Not sure if it's on your p/u, but is there a vacuum manifold/distributor block connected to the vacuum pump? It's usually on top of firewall above engine. There may be a single vacuum line that ends at a vacuum switch that senses low pressure on the left/driver fender that if you disconnect the switch and the brake light goes out means you have low vacuum pressure from leaks or the pump. the pump vacuum may also drive the climate control inside which could be leaking.
 
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Old 02-02-2017, 07:10 PM
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I'm at work right now but I'll have to check that out
 
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Old 02-03-2017, 05:31 AM
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you can pull 24 inches vacuum and still have a bad pump.
what happens if you hit the brakes 2-3 times in a row? does the pedal get hard? that would indicate a weak pump that can not recover fast enough.
 
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Old 02-03-2017, 02:09 PM
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Brake response is fine, only gets hard if the engine is off
 
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Old 02-03-2017, 03:30 PM
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ok. sounds like low fluid level then.

one other thing to check. if the spring that holds the park brake pedal up is weak, the pedal will drop just enough to turn the light on. try toeing the pedal up and see if the light goes out.
 
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Old 02-03-2017, 03:32 PM
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Hi,

on my 1985 F250 it is a faulty Vacuumcontrolswitch, sometime the Light comes on, and go out minutes later, and than it is out for week. But i think this Switch is not longer aviable as new.

regards

Torsten
 
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Old 02-03-2017, 04:46 PM
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Originally Posted by tjc transport View Post
ok. sounds like low fluid level then.

one other thing to check. if the spring that holds the park brake pedal up is weak, the pedal will drop just enough to turn the light on. try toeing the pedal up and see if the light goes out.
unplugging the switch doesn't cause the light to turn off, unless i have the p-brake engaged at the time. speaking of the parking brake, mine doesnt work very well despite having new parking brake lines, and new brake hardware. however, i tend to think that the park brake lines themselves might be too long inside the drum or something.

I'll locate the vacuum switch and unplug that
 
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Old 02-03-2017, 09:01 PM
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You may have to adjust the shoes on the rear drums to get the parking brake to work correctly. The lines only provide so much pull regardless of their length.
 
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Old 02-04-2017, 12:54 PM
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On my 88's and my 93, there's this little metal can thing with a 3? wire plug and a vacuum line going into it. On my 88's, it was on the passenger side fender well. On my 93, it's down on the driver's side, just above the steering box.

It is supposed to make the light come on when there is low vacuum. On my 93, I accidentally broke off the nipple(so there is /no/ vacuum going to it), and... the light stayed out for a while. Hit a bump and it came on, randomly on and off after that. I'm guessing it was sticky inside(it's winter afterall).
I finally just unplugged it so that my cruise control would work again. Solved that problem!
 
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Old 02-04-2017, 09:29 PM
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that metal can thingy is the most primitive vacuum chamber canister ever devised. You'd think ford engineers would think of something better than an enclosed coffee can to store the vacuum. They hide in the quarter panel so owners don't know what a cheap, effective design it was.

It's used as a reservoir to hold vacuum reserves when applying brakes or changing vacuum controls. My e250 has a vacuum pressure valve on the drivers side wheel well and the coffee can is in the passenger side quarter panel. I had a dent in this quarter panel from the PO. I was afraid the dent damaged the coffee can, but fortunately did not. I didn't see a switch or sensor on the coffee can.
 
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Old 02-04-2017, 10:33 PM
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Originally Posted by eagleye View Post
that metal can thingy is the most primitive vacuum chamber canister ever devised. You'd think ford engineers would think of something better than an enclosed coffee can to store the vacuum. They hide in the quarter panel so owners don't know what a cheap, effective design it was.
Different piece. That metal can is actually more interesting than it seems - one of the two ports also has a check-valve inside it, so the vacuum won't 'drain back' with the engine off.


The piece I'm talking about is about 3" in diameter, 1.25" long. Metal around the diameter, with a mounting tab. The ends are plastic, IIRC, with the vacuum port in the center of one end.
 
 
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