The light could be on for several reasons:
* Low brake fluid: This is the most common cause. Check your brake fluid reservoir (usually a clear or translucent plastic container near the master cylinder) and add fluid if it's low. If you have to add a significant amount of fluid, you have a leak and need to have your brakes checked immediately by a mechanic. Driving with low brake fluid is extremely dangerous.
* Parking brake engaged: Make absolutely sure your parking brake is fully released.
* Faulty brake light switch: This switch is located near the brake pedal and tells the light when to turn on and off. It's relatively inexpensive to replace, but requires some mechanical skill.
* Brake system failure: This could be a number of things, including:
* Worn brake pads or shoes: These need replacing.
* Leaking brake lines or wheel cylinders: These require repair by a mechanic.
* Master cylinder failure: This is a serious issue and requires immediate professional attention.
* Problems within the ABS system (if equipped): While less likely on a '78, some later models in the year may have had early ABS. This needs professional diagnosis.
How to troubleshoot:
1. Check the brake fluid level: This is the first and most important step.
2. Check the parking brake: Make sure it's fully disengaged.
3. Inspect brake lights: Have someone check that all your brake lights are working correctly. A burned-out bulb in one of your brake lights can trigger the warning light.
4. If none of the above work, you MUST take your truck to a qualified mechanic. Trying to fix a brake problem yourself without the proper knowledge and tools could be extremely dangerous and potentially lead to an accident.
Do not attempt to simply disable the warning light. That would be masking a serious problem and endangering yourself and others. Fix the underlying cause.
