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post #1 of 5 (permalink) Old 05-23-2017, 01:39 PM Thread Starter
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Intermittent brake warning light

I have an intermittent brake warning light issue for a few months now. Disconnected the emergency brake switch and tested; switch is good and light still comes on with it disconnected. I should add the light is very random and sometimes fades on and off. I replaced the brake master cylinder because it's 20 years old, still comes on. The worst of it is, it disables the cruise control when on. Harness?

97 F350 XL std cab, zf5, srw (converted), 4wd, 3.55s, stock wheels/tires & rebuilt 9.18
2 Donaldsons, 3" dp, ACs, T500, HV LPOP, Hydra w/PHP & Jeli, DIY 60 gl tank w/6 gl sump, eFuel/Cat filters, Aeromotive RR, Auber instruments, 140v IDM, KC 300X 63/70/84, RiffRaff bellowed up pipes, 6.0 IC, 3" plenums w/RiffRaff inserts & SB 425HD
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post #2 of 5 (permalink) Old 05-23-2017, 01:42 PM
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Re: Intermittent brake warning light

there is a switch in the brake master cyl for low fluid level.
First check fluid level, second test that switch like you did the one for the foot pedal
I've read the vacuum switch can also set that light, since we have vacuum power brake booster it makes sense.

1997 CCFB 7.3, ZF5, 4x4, 4.10, baby swamps, TS6, E99 spyder, RiffRaff pipes, 6.0 FMIC, RSK, 315 toyo at
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2001 Excursion 7.3, auto, 4x4, 3.73, 6" Suspension lift, 37" Dick Cepeks
2000 E350 7.3, auto 2wd, 3.73, planning a TTB 4x4 conversion
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post #3 of 5 (permalink) Old 05-23-2017, 01:45 PM Thread Starter
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Re: Intermittent brake warning light

I did test the switches on the NEW master cylinder; light is still coming on. I have heard of the float sticking, I was hoping it was that.

What about the vacuum pump or switch?

97 F350 XL std cab, zf5, srw (converted), 4wd, 3.55s, stock wheels/tires & rebuilt 9.18
2 Donaldsons, 3" dp, ACs, T500, HV LPOP, Hydra w/PHP & Jeli, DIY 60 gl tank w/6 gl sump, eFuel/Cat filters, Aeromotive RR, Auber instruments, 140v IDM, KC 300X 63/70/84, RiffRaff bellowed up pipes, 6.0 IC, 3" plenums w/RiffRaff inserts & SB 425HD
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post #4 of 5 (permalink) Old 05-23-2017, 01:57 PM
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Re: Intermittent brake warning light

vaccuuuuuuuum switch then
perhaps the vacuum switch is doing its job and letting you know the vacuum pump is not making enough for the booster = look for leaks and test switch
next would be a vacuum gauge and compare to spec, might have to watch vacuum levels as you drive since you said the light comes on sometimes

1997 CCFB 7.3, ZF5, 4x4, 4.10, baby swamps, TS6, E99 spyder, RiffRaff pipes, 6.0 FMIC, RSK, 315 toyo at
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2001 Excursion 7.3, auto, 4x4, 3.73, 6" Suspension lift, 37" Dick Cepeks
2000 E350 7.3, auto 2wd, 3.73, planning a TTB 4x4 conversion
21.9 liter family
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post #5 of 5 (permalink) Old 05-24-2017, 04:09 PM Thread Starter
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Re: Intermittent brake warning light

The low vacuum switch is bad. I checked the system with my old vacuum gauge and it's showing ~23 inches plus gauge error. I also checked about 45 minutes later and it still had ~23 inches. I took it for a 18 mile drive and vacuum stays above ~18 inches. I would think the switch is a discontinued item? Has anyone found a replacement or repaired the original switch?

97 F350 XL std cab, zf5, srw (converted), 4wd, 3.55s, stock wheels/tires & rebuilt 9.18
2 Donaldsons, 3" dp, ACs, T500, HV LPOP, Hydra w/PHP & Jeli, DIY 60 gl tank w/6 gl sump, eFuel/Cat filters, Aeromotive RR, Auber instruments, 140v IDM, KC 300X 63/70/84, RiffRaff bellowed up pipes, 6.0 IC, 3" plenums w/RiffRaff inserts & SB 425HD
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