81. The air supply pressure gauge shows how much pressure:
Detailed Explanation
The supply pressure gauge reports the air pressure stored in the vehicle's air tanks. A dual air-brake system has a gauge for each half or one gauge with two needles. Drivers watch these readings during pressure buildup and operation so they can recognize a system that is not building pressure or is approaching a low-pressure warning.
Official reference: FMCSA Commercial Driver License Manual, Section 5.1.9: Supply Pressure GaugesCorrect Answer: Is in the air tanks.
Why This Matters on the CDL Test
Do not confuse the supply pressure gauge with an application pressure gauge. Supply pressure is the air available in the tanks; application pressure, when that gauge is installed, is the pressure being sent during a brake application.
How to Remember It
Supply equals stored: the supply gauge tells you how much air is available in the tanks.
Answer Review
"Is in the air tanks" is correct because the supply gauge is connected to the reservoirs. It shows available stored pressure, not the amount used during a trip or the momentary pressure being applied to the brake chambers.
Why the other answers are weaker
- You have used in this trip.The gauge is not a trip totalizer. Its reading rises as the compressor builds tank pressure and falls as air is used or lost.
- Is going to the brake chambers.That describes application pressure, which is different from the reservoir pressure shown by the supply gauge.
Test-Day Checklist
- Read the question once for the topic, then read it again for the exact CDL action being tested in Air Brakes.
- Eliminate answers that sound convenient but skip an inspection, warning sign, or safety step.
- Say the correct rule out loud before moving on so you can recognize it when the exam changes the wording.
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8. The application pressure gauge shows how much pressure you have:
10. The safety valve is set to automatically reduce pressure at:
100. Your bus or truck has a dual air brake system. If a low air pressure warning comes on for only one system, what should you do?
105. How should you check that the service brakes come on when air pressure in the system drops below a certain level?
106. If your truck's air compressor has its own oil supply when should you first check the oil level?
Keep Studying Air Brakes
Question pages help you review one rule at a time. To build test-day speed, move back into the full Air Brakes practice test and answer a mixed set without looking at the explanation first.
State CDL Practice Paths
After you understand this answer, use a state practice path to connect the federal CDL rule with the DMV test pages drivers search for most.