Simple and Easy to Check:
* Low Brake Fluid: This is the most common cause. Check the brake fluid reservoir (usually a clear or translucent container near the master cylinder) and add fluid if needed. *However, adding fluid without finding and fixing the leak is only a temporary fix and could mask a serious problem.*
* Burned Out Bulb(s): Although it seems counterintuitive, a burned-out brake light bulb can sometimes cause the other brake light(s) or the brake light indicator on the dash to stay on. Check all three brake lights (two on the back and one on the center high-mounted stop lamp - CHMSL, also called the third brake light).
* Bad Brake Light Switch: This switch is located near the brake pedal and activates the brake lights when the pedal is pressed. If it's faulty, it might stay "on" even when the brake isn't applied. This is a relatively inexpensive and easy part to replace.
* Loose or Corroded Connections: Check the wiring connections to the brake lights, the brake light switch, and the brake fluid level sensor. Corrosion can interrupt the circuit, causing strange behavior. Clean any corrosion you find.
More Complex Issues:
* Brake System Leak: If you're low on brake fluid, there's a leak somewhere in your braking system. This is a *serious* issue and requires immediate attention. Don't drive the truck until this is fixed. A mechanic should inspect the system for leaks.
* Faulty Brake Light Indicator Switch: This switch is different from the brake light switch. It's part of the instrument cluster and signals the dash light. A failure can cause the light to stay on even when the brakes are not applied.
* Wiring Problems: A short circuit or broken wire in the braking system wiring harness can also cause the brake light to stay on. This can be more difficult to diagnose and requires tracing the wiring.
* Faulty Master Cylinder: While less common, a failing master cylinder could contribute to the issue. This is a critical component of the braking system.
* ABS Issues (if equipped): If your truck has anti-lock brakes (ABS), a problem with the ABS system could cause the brake light to illuminate. This would usually be accompanied by an ABS warning light as well.
Troubleshooting Steps:
1. Check the brake fluid level.
2. Check all brake light bulbs.
3. Inspect the wiring and connections for corrosion or damage.
4. Test the brake light switch (often requires a multimeter).
Important Note: A brake light staying on indicates a problem with your braking system. Driving with a faulty braking system is extremely dangerous. If you can't easily identify and fix the problem, take your 1991 Silverado to a qualified mechanic immediately. Don't delay!
