1940 Chevrolet KD Brake Light Switch: A Complete Guide
Your 1940 Chevrolet KD uses a hydraulic pressure switch for its brake lights, a critical safety part prone to failure with age.
- The 1940 Chevrolet KD uses a hydraulic pressure switch, not a mechanical one at the pedal.
- Failure is a critical safety issue; if your brake lights don't work, do not drive the truck.
- A new aftermarket switch from a brand like Standard Ignition is the most reliable and practical replacement choice.
- You must bleed the brakes after replacing the switch to ensure safe brake operation.
Signs of a Failing Brake Light Switch
A bad brake light switch is a serious safety hazard. If you notice any of these symptoms, you should inspect the switch immediately. On a simple system like the one in your 1940 Chevrolet KD, the switch is the most common point of failure.
Critical Safety Warning
A faulty brake light switch means other drivers won't know you are stopping, which can easily lead to a rear-end collision. Driving without functional brake lights is illegal and extremely dangerous. Do not drive your vehicle if the brake lights are not working.
- Brake Lights Don’t Turn On: The most common and dangerous symptom. The switch has failed internally and no longer completes the electrical circuit when you press the brakes.
- Brake Lights Stay On: The switch can also fail in the 'on' position, causing your brake lights to remain lit constantly. This will drain your battery and confuse other drivers.
- Intermittent Brake Lights: If the lights flicker or only work sometimes, the internal contacts of the switch are likely worn out and final failure is near.
- Brake Fluid Leak: Because the switch is part of the hydraulic system, it can develop a leak. You might see brake fluid seeping from the switch body or its threads.
Choosing the Right Aftermarket Switch
For a vehicle from this era, finding a New Old Stock (NOS) original GM part is practically impossible. While you might find a used original part, it's a gamble, as it could be just as old and unreliable as your current one. The most practical and reliable solution is a new aftermarket switch.
The technology in these hydraulic switches is straightforward, and modern manufacturing produces high-quality, dependable replacements.
- Aftermarket (Tier 1): Brands like Standard Ignition (SMP) are a well-regarded choice for classic vehicle electrical parts. They produce switches designed to meet or exceed original specifications using quality materials like copper alloy conductors for reliability. This is the recommended option for a safe and long-lasting repair.
Cost of Replacing a 1940 Chevrolet KD Brake Light Switch
| Part Type | Part Cost | Notes |
|---|---|---|
| New OEM | Not Available | Original parts have not been produced for decades. |
| New Aftermarket | $38.97 | This is the most common and recommended option. |
| Used OEM | $10 - $25 | Not recommended due to age and unknown condition. |
| Shop Labor | $80 - $150 | Typically involves about an hour of labor, including bleeding the brakes. |
Installation Snapshot
Replacing the brake light switch is a straightforward job for a DIYer with basic tools. The switch simply unscrews from the master cylinder or brake line block. However, because it is part of the hydraulic brake system, proper procedure must be followed to ensure safety.
You MUST bleed the brakes after replacing the switch. Opening the hydraulic system introduces air, which can lead to a spongy pedal or complete brake failure. Do not skip this critical safety step.
Pro Tip
Have the new switch ready to go before you fully remove the old one. You can swap them very quickly to minimize brake fluid loss and make the bleeding process easier. However, you must still bleed the system to ensure all air is removed.
Frequently Asked Questions (FAQ)
Do I really have to bleed the brakes after replacing the switch?
Yes, absolutely. Any time the hydraulic brake system is opened to the atmosphere, air can get in. Air in the brake lines is compressible and will result in a soft, spongy brake pedal or even a total loss of braking power. Bleeding the brakes is a critical safety step you cannot ignore.
How can I test if the switch is the problem?
A simple test is to locate the two wires on the brake light switch. With the ignition on, carefully disconnect the wires from the switch and touch them together. If the brake lights turn on, the problem is your switch. If they do not light up, the problem is elsewhere in the wiring, fuse, or the bulbs themselves.
Where is the brake light switch located on my truck?
It is not on the brake pedal. Look for a small sensor with two wires screwed directly into the master cylinder (where you add brake fluid) or into a metal block where several brake lines meet.
Are there any recalls for the brake light switch on a 1940 Chevrolet?
No. The National Highway Traffic Safety Administration (NHTSA), which manages vehicle recalls, was founded in 1970. There are no official recalls or Technical Service Bulletins (TSBs) for vehicles of this vintage in modern databases.
Technical Specifications
Type: Hydraulic Pressure Switch Thread: 1/8-NPT (Common) Electrical System: 6-Volt Negative Ground
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.