What is the purpose of "dead-end" or dummy couplers on a tractor when no trailer is attached?

They let the tractor build extra air pressure for climbing grades

They substitute for the trailer's electrical pigtail

They automatically close the tractor protection valve

Air Brakes on Combination Vehicles Test 2/2

✓

Correct!

Correct. Per AAMVA CDL Manual Section 6.2.5 (p. 6-6), dummy couplers cap off the hoses when they're not connected to a trailer, keeping water and dirt out of the coupler and air lines.

✗

Not quite.

Wrong. Dummy couplers don't add air capacity; they simply cap the open lines.

The correct answer is A: They keep water and dirt out of the disconnected glad hands and air lines.

✗

Not quite.

Wrong. Dummy couplers are an air-line accessory and have nothing to do with the separate electrical pigtail.

The correct answer is A: They keep water and dirt out of the disconnected glad hands and air lines.

✗

Not quite.

Wrong. The tractor protection valve responds to air pressure and the cab control, not to whether dummy couplers are capping the hoses.

The correct answer is A: They keep water and dirt out of the disconnected glad hands and air lines.