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1971 D100

My dash is missing the brake warning light bulb so I'm wondering which bulb number to use. The other bulbs look like a #53 but they all have one wire going to them whereas the brake warning light has two.

Maybe it's the same bulb but just the ground side of the circuit is completed by the parking brake being applied?

That light doesn't come on when the parking brake is engaged. It only comes on when there is a failure in the front or rear brake systems. There is a block that is often mistaken for a proportioning valve but it's real purpose is to detect uneven pressure in the front/rear brake systems. It comes on when you start the engine as a bulb test.

Thanks, but a bulb number?

I only have listing for '69,which calls for a #57 bulb.

FastbackJon wrote:1971 D100

My dash is missing the brake warning light bulb so I'm wondering which bulb number to use. The other bulbs look like a #53 but they all have one wire going to them whereas the brake warning light has two.

Maybe it's the same bulb but just the ground side of the circuit is completed by the parking brake being applied?

You've got it almost right. That switch to detect brake line pressure imbalance grounds the sending wire when there's a fault.