In a tractor-trailer, blended air is air from
- the compressor and the supply tank.
- the primary and secondary service tanks. ✓
- the tractor and the trailer.
- the foot valve and the quick release valve.
Answer: B. the primary and secondary service tanks.
Blended air in a tractor-trailer is air from the primary and secondary service tanks.
Why this is the answer
The handbook describes a dual air tractor-trailer system where the trailer supply valve gets blended air through a two-way check valve. In the dual system, the two service reservoirs are the primary and secondary reservoirs, each supplying one section of the dual foot valve. So blended air is the combined air available from those two service tanks, which is why the automatic trailer shutoff depends on the service reservoir with the highest pressure. The supply reservoir is separate and is not one of the two service tanks.
Why the other answers are wrong
- A. the compressor and the supply tank. — The compressor and the supply tank are not the two service tanks that feed blended air; the handbook treats the supply reservoir as separate from the primary and secondary service reservoirs.
- C. the tractor and the trailer. — Blended air is not a mix of tractor air and trailer air; it is air combined within the tractor's dual system before it signals the trailer control line.
- D. the foot valve and the quick release valve. — The foot valve has primary and secondary sections, but blended air is supplied through a two-way check valve from the service reservoirs, not from a quick release valve.
Remember it
Blended means the two service tanks pool their air through a two-way check valve: primary plus secondary, not tractor plus trailer.