Brake light is on
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Senior User
Joined: Apr 2011
Posts: 227
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From: Plymouth, MI
Brake light is on
When ever i start my truck the brake light is off, but when i start to drive it the light will start to flicker on and off and sometimes stay on.. What would cause this and how do I fix it?
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Joined: Nov 2007
Posts: 22,415
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From: Denver Metro Area, CO
This could be a wire errantly being shorted to ground, or it could be telling you of a potentially serious problem with the brake system.
Just behind the left-front wheel, down on the frame rail, is the brake system proportioning valve... THIS POST from Mikeo0o0o0 has pictures.
Two hydraulic lines from the brake master cylinder lead to it, and two hydraulic lines to the front wheels and one to the rear lead from it.
There is an electrical plug at the top, the switch in this valve is what is turning on your dash light.
If the wires leading up to that switch get grounded anywhere along their length, the dash light will come one so ensure the wiring isn't being grounded anywhere.
The purpose of that switch in the valve is to warn you of a pressure imbalance between the front & rear hydraulic brake circuits.
Remove the top of your master cylinder and check the fluid level; if one of those reservoirs is empty, that is what is causing the imbalance in pressure, you will need to figure out where all that fluid went and fix/replace the leaking components.
If one of the reservoirs is significantly lower than the other (but not empty) this indicates most likely a leaking caliper or wheel cylinder in that circuit (could also be a rusted/busted hydraulic line) into which air will be leaked and cause your problem.
Could also mean the master cylinder is leaking internally and is incapable of putting out the same amount of pressure in both circuits, in which case you'd rebuild or replace the master cylinder and then bleed the brakes.
Investigate your system, maybe bleed the brakes, go from there.
Just behind the left-front wheel, down on the frame rail, is the brake system proportioning valve... THIS POST from Mikeo0o0o0 has pictures.
Two hydraulic lines from the brake master cylinder lead to it, and two hydraulic lines to the front wheels and one to the rear lead from it.
There is an electrical plug at the top, the switch in this valve is what is turning on your dash light.
If the wires leading up to that switch get grounded anywhere along their length, the dash light will come one so ensure the wiring isn't being grounded anywhere.
The purpose of that switch in the valve is to warn you of a pressure imbalance between the front & rear hydraulic brake circuits.
Remove the top of your master cylinder and check the fluid level; if one of those reservoirs is empty, that is what is causing the imbalance in pressure, you will need to figure out where all that fluid went and fix/replace the leaking components.
If one of the reservoirs is significantly lower than the other (but not empty) this indicates most likely a leaking caliper or wheel cylinder in that circuit (could also be a rusted/busted hydraulic line) into which air will be leaked and cause your problem.
Could also mean the master cylinder is leaking internally and is incapable of putting out the same amount of pressure in both circuits, in which case you'd rebuild or replace the master cylinder and then bleed the brakes.
Investigate your system, maybe bleed the brakes, go from there.
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 785
From: Simi Valley, CA
The plastic "Low Brake Fluid Warning Lamp Switch" threads into the proportioning valve.
This switch is NOTORIOUS for developing cracks in its threads. When this occurres, brake fluid seeps out.
The Low Brake Fluid Warning Lamp on the dash comes on...and stays on!
C8AZ-2B264-A .. Low Brake Warning Lamp Switch / Available from Ford and autoparts stores.
MSRP: $29.73 // FTE Ford Dealer Parts Dept sponsor PARTSGUYED.COM price: $19.62.
Applications: 1968/79 F100/350 & Bronco / 1969/79 Econoline // 1968/69 FoMoCo Passenger Cars.
This switch is NOTORIOUS for developing cracks in its threads. When this occurres, brake fluid seeps out.
The Low Brake Fluid Warning Lamp on the dash comes on...and stays on!
C8AZ-2B264-A .. Low Brake Warning Lamp Switch / Available from Ford and autoparts stores.
MSRP: $29.73 // FTE Ford Dealer Parts Dept sponsor PARTSGUYED.COM price: $19.62.
Applications: 1968/79 F100/350 & Bronco / 1969/79 Econoline // 1968/69 FoMoCo Passenger Cars.
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