Iowa CDL — Air Brakes: Safe Driving Practices Practice Test

Question 1

On your walk-around you find cracks in several brake drums, and each drum has a friction area 6 inches wide. Which crack is longer than the manual allows?

Accepted Answer

A 4-inch crack — A brake drum or disc may not have a crack longer than half the width of its friction area. With a 6-inch friction area the limit is 3 inches, so the 4-inch crack is the only one over it.

Answer

A 1-inch crack

Answer

A 2-inch crack

Answer

A 2.5-inch crack

Question 2

On Iowa's inspection test, you check the brake lines and hoses at the steering axle. According to the manual, what may a loss of air pressure in those lines cause?

Accepted Answer

The wheels locking up — Iowa's guide explains that these lines carry air or hydraulic fluid to each wheel's brake assembly. Lost hydraulic fluid can reduce braking power and response, while lost air pressure can lock up the wheels. Check that the lines don't leak, aren't cracked, worn, or frayed, and that every coupling and fitting is tight.

Answer

The safety relief valve opening

Answer

The governor cutting out early

Answer

Faster pressure buildup when the engine restarts

Question 3

When you inspect the air and electric lines between a tractor and trailer on Iowa's test, which check applies only to air brake systems?

Accepted Answer

Listening for air leaks — Iowa's checklist has you confirm that air hoses and electric lines are free of cuts, cracks, chafing, splices, tape, and wear, and that none are tangled, crimped, pinched, or dragging. Listening for leaking air is the step marked for air brake systems only. Connections must be checked at both the tractor end and the trailer end.

Answer

Making sure the lines are not tangled or dragging

Answer

Making sure the lines are free of cuts, cracks, and chafing

Answer

Making sure the lines are not pinched against vehicle parts

Question 4

While checking the engine area on Iowa's inspection test, you hear air hissing. Why does the manual treat an air leak there as important?

Accepted Answer

It can point to a failing part, such as the air compressor, that is hard to inspect directly — Iowa's inspection guide says an air leak may signal that a component is failing somewhere you can't easily see, naming the air compressor and other air parts as examples. Look over the hoses and air lines for their condition and for leaks.

Answer

It shows the safety relief valve is working normally

Answer

It matters only if a puddle forms under the engine

Answer

It means the alcohol evaporator needs refilling

Question 5

In the final air brake check, your governor stops the compressor at about 130 psi rather than 125. What does the manual say about the exact cut-out point?

Accepted Answer

The 125 psi figure is approximate, so compare the result with the manufacturer's specifications — The manual gives about 100 psi for cut-in and about 125 psi for cut-out as typical values, not fixed ones, and tells you to check the manufacturer's specifications. Its leakage test also describes governor cut-off as anywhere from 120 to 140 psi.

Answer

Any cut-out above 125 psi means the governor has to be replaced

Answer

The safety relief valve should be reset to open at 130 psi to match

Answer

Cut-out pressure only matters on single air systems built before 1975

Question 6

Several parts of the final air brake check are done with the engine shut off. Which of these checks is done with the engine running?

Accepted Answer

Watching for the governor to cut the compressor out and back in — The governor check is run with the engine at a fast idle: pressure should stop rising at the cut-out point, and after you step on and off the brake it should start climbing again at cut-in. The leakage, low air warning, and spring brake checks are done after the engine has been shut off.

Answer

Timing the air leakage rate with the foot brake held down

Answer

Fanning the pressure down to see when the low air warning comes on

Answer

Fanning the pressure down until the parking brake valve pops out

Question 7

In Iowa's air brake check, you have just finished the leakage test. What should you do about the engine before starting the low air warning test?

Accepted Answer

Leave it off and pump the brake pedal with the electrical power still on — Iowa's procedure runs the warning device test without starting the engine again. With electrical power left on, you bleed the pressure down with repeated brake applications and note where the buzzer, light, and flag come on. They should activate before pressure falls under 55 psi.

Answer

Restart it at a fast idle so the compressor keeps the gauges steady

Answer

Restart it, then shut it off again the moment the buzzer sounds

Answer

Turn the key all the way off so the warning circuit can reset

Question 8

Your truck has automatic air tank drains and an alcohol evaporator. During a cold snap, which daily task does the manual still give you?

Accepted Answer

Check the alcohol container and top it up as needed — The evaporator feeds alcohol into the air system to lower the chance of ice forming in valves and other parts, so in cold weather you should check its container and refill it every day. Daily draining by hand is for systems without automatic drain valves. Automatic drains push out the water and oil on their own.

Answer

Open each drain valve by hand at the end of the day

Answer

Run the engine at a fast idle until the safety valve opens

Answer

Bleed the tanks down to 55 psi to push out moisture

Question 9

Your truck's air compressor is driven by gears off the engine rather than by a belt. How does that change the air brake check in the engine compartment step?

Accepted Answer

The drive belt check doesn't apply, since it is only for belt-driven compressors — An air compressor is connected to the engine either by gears or by a v-belt. The extra engine-compartment check for air brakes is the condition and tightness of the compressor drive belt, and it applies only when the compressor is belt-driven.

Answer

You check the compressor gears for tightness in place of a belt

Answer

You drain the air tanks during that step instead

Answer

Nothing changes; you still check a compressor belt for condition and tightness

Question 10

Which of these air brake devices can the driver NOT control from the cab?

Accepted Answer

An automatic front-wheel limiting valve — Older front brake limiting valves have a cab control marked normal and slippery. A modulating valve is a handle on the dash, and a bus's dual parking controls are valves the driver pushes and pulls. Automatic front-wheel limiting valves work on their own. They cut air to the front brakes except on very hard applications of 55 psi or more, and the driver has no control over them.

Answer

The front brake limiting valve on a truck built before 1975

Answer

A modulating control valve mounted on the dash

Answer

The spring-loaded release valve of a bus's dual parking controls

Question 11

Four different rigs each lose 2.5 psi in one minute during their pre-trip air leakage checks, all with the engine off. Which rig should stay parked until the leak is found and fixed?

Accepted Answer

A straight truck with the parking brake released and no brakes applied — With no brakes applied, a single vehicle has to lose under 2 psi a minute and a combination under 3. With the foot brake held down, the limits rise to 3 psi for a single vehicle and 4 psi for a combination. Only the straight truck in the brakes-off test has gone past its limit. Too much leakage can leave you without brakes on the road.

Answer

A tractor-trailer with the parking brake released and no brakes applied

Answer

A straight truck with the foot brake held fully down

Answer

A tractor-trailer with the foot brake held fully down

Question 12

You drive a tractor-trailer with ABS on both the tractor and the trailer. How much braking force does the manual tell you to use when you slow down?

Accepted Answer

Just enough to stop safely and keep control, easing off if either unit starts to get out of line — With ABS on a combination, brake the way you always have. Use just enough force to stop safely and keep control, keep an eye on both the tractor and the trailer as you slow, and ease off the brakes when you can safely do so. Only a vehicle with working ABS on every axle may get full brake application in an emergency.

Answer

A full, hard application every time, so the ABS can handle all of the wheel control for you

Answer

Light, rapid pumps of the pedal so the ABS computer has a chance to reset between them

Answer

More force than you would use in a rig without ABS, since ABS always shortens the stop

Iowa CDL — Air Brakes: Safe Driving Practices practice test

Study questions with answers

1. On your walk-around you find cracks in several brake drums, and each drum has a friction area 6 inches wide. Which crack is longer than the manual allows?

2. On Iowa's inspection test, you check the brake lines and hoses at the steering axle. According to the manual, what may a loss of air pressure in those lines cause?

3. When you inspect the air and electric lines between a tractor and trailer on Iowa's test, which check applies only to air brake systems?

4. While checking the engine area on Iowa's inspection test, you hear air hissing. Why does the manual treat an air leak there as important?

5. In the final air brake check, your governor stops the compressor at about 130 psi rather than 125. What does the manual say about the exact cut-out point?

6. Several parts of the final air brake check are done with the engine shut off. Which of these checks is done with the engine running?

7. In Iowa's air brake check, you have just finished the leakage test. What should you do about the engine before starting the low air warning test?

8. Your truck has automatic air tank drains and an alcohol evaporator. During a cold snap, which daily task does the manual still give you?

9. Your truck's air compressor is driven by gears off the engine rather than by a belt. How does that change the air brake check in the engine compartment step?

10. Which of these air brake devices can the driver NOT control from the cab?

11. Four different rigs each lose 2.5 psi in one minute during their pre-trip air leakage checks, all with the engine off. Which rig should stay parked until the leak is found and fixed?

12. You drive a tractor-trailer with ABS on both the tractor and the trailer. How much braking force does the manual tell you to use when you slow down?

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