Brake Warning Light
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Brake Warning Light
The brake warning light on my 1991 F-250 7.3 keeps coming on. It will come on and then go off. It seems to do it more while at idle but it come on while going down the interstate. The fluid level is ok as are the brake pads and shoes. Someone suggested the vacuum pump may be going out. Any suggestions?
Post Fiend
Joined: Nov 2003
Posts: 21,285
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From: Nutter Fort, WV
Not sure if it is the same as my 86, but three things turn my brake light on.
Parking brake - if the pedal is not all the way up due to binding cables the brake light comes on. When I have had bad rear wheel cables the switch is not fully depressed and the light will flash or glow lightly when hitting bumps.
Vacuum - if the vacuum level is low the light comes on. Does your heater control switch from defrost to floor OK? You may have a pump going bad or a leak in the system. Low or no vacuum results in the heater going to the default defrost position for the air flow. Could also be a loose belt on the vacuum pump. WARNING Complete loss of vacuum will result in very hard pedal pressure required to stop the truck.
Brake balance switch - if you blow a brake line the switch shuts of the fluid to which ever end failed. This light is on constant till repaired and rebalanced.
Parking brake - if the pedal is not all the way up due to binding cables the brake light comes on. When I have had bad rear wheel cables the switch is not fully depressed and the light will flash or glow lightly when hitting bumps.
Vacuum - if the vacuum level is low the light comes on. Does your heater control switch from defrost to floor OK? You may have a pump going bad or a leak in the system. Low or no vacuum results in the heater going to the default defrost position for the air flow. Could also be a loose belt on the vacuum pump. WARNING Complete loss of vacuum will result in very hard pedal pressure required to stop the truck.
Brake balance switch - if you blow a brake line the switch shuts of the fluid to which ever end failed. This light is on constant till repaired and rebalanced.
Last edited by Dave Sponaugle; Jul 24, 2005 at 11:45 AM.
Senior User
Joined: May 2003
Posts: 455
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From: Alaska
The brake warning light will come on for two reasons, low brake fluid level and/or emergency brake applied or not fully released
Many Ford products, including yours, do not use a proportioning valve switch.
The theory is, if there is a loss of pressure, there will be a loss of fluid and the light will come on.
However, a bad master cylinder can cause a loss of pressure and the light won't come on because there is no fluid loss.
Many Ford products, including yours, do not use a proportioning valve switch.
The theory is, if there is a loss of pressure, there will be a loss of fluid and the light will come on.
However, a bad master cylinder can cause a loss of pressure and the light won't come on because there is no fluid loss.
Senior User
Joined: Dec 2003
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From: central california
look on the frame rail below the master cylinder you will see a valve with the lines coming of the master cylinder to it then from there the line go's to your back brakes it also has a two wire plug going to it. that is your rear abs valve. you will see a bleed nut on that valve and it also needs to be bled. if that does not take care of soft brake that valve is most likly bad. you can get a reman. from napa for about 150.00 or you can do what I did and get some fittings and bypass it you wont have abs but that has not been a problem with me. leave the valve there with the plug hooked up if those wire are dameaged or not pluged in the break light will stay on.
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Joined: Mar 2005
Posts: 400
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From: Colorado
I have the same problem with my 94 F350. Dealer says its a short. My brakes work fine light on or not. Heater diverter works fine as well. Light only comes on once in a while. I bought the truck about 4 months ago and the ford dealer I bought it from had replaced the master cylinder, front and rear shoes and pads, front calipers. Im inclined to think it may be losing a little vacuume pressure, even though the brakes work fine when the light is on.
phish555
1994 F350 turbo diesel crew cab
phish555
1994 F350 turbo diesel crew cab
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