Pennsylvania CDL — Air Brakes Practice Test 10

1.In an s-cam brake, what happens when you press the brake pedal?

A wedge is driven straight between the two brake shoesA power screw clamps a rotor between two padsAir enters the chamber, and the rod turns the s-cam so the shoes press against the drumHydraulic fluid pushes a piston against a disc

2.Total stopping distance for an air brake vehicle is made up of which four parts?

Perception, reaction, gear-selection and braking distancesReaction, brake lag, coasting and skid distancesPerception, brake lag, downgrade and braking distancesThe perception and reaction distances, plus the brake-lag and braking distances

3.When using proper (snub) braking on a long downgrade, the brakes serve as what?

The only means of slowing, with the engine in neutralA full replacement for downshiftingA way to keep the wheels locked the whole way downA supplement to engine braking, after you are in the proper low gear

4.What is the purpose of a modulating control valve?

It drains the air tanks automaticallyIt lets you apply the spring brakes gradually if the service brakes quitIt limits air pressure to the front wheelsIt recharges the secondary system

5.In the snubbing technique, once you have braked to about 5 mph below your safe speed, what do you do next?

Hold steady brake pressure for the rest of the gradeShift into a higher gear and coastSet the parking brake to hold the lower speedLet off the brakes, then reapply once you return to your safe speed

6.How does an air brake system light up the brake lights for the drivers behind you?

An air-pressure-operated electric switch turns them on as you brakeThe governor sends a signal back to the taillightsA contact at each wheel is closed by the slack adjusterThe wig wag completes the lighting circuit

7.Why is it a bad idea to repeatedly press and release the brake pedal when it isn't necessary?

It overheats the alcohol evaporatorIt can let air out quicker than the compressor restores it, risking loss of brakingIt forces the governor to stay in cut-outIt makes the safety valve open at 150 psi

8.How does an air-operated disc brake differ from an s-cam brake?

It uses a power screw rather than an s-cam to squeeze the rotorIt has no slack adjuster at allIt runs on hydraulic fluid instead of airIt works without any brake chamber

9.What is true about front-wheel braking on a commercial vehicle?

Front brakes should be disabled on wet roadsFront-wheel braking helps in all conditions and rarely causes skids, even on iceFront brakes are only useful on dry pavementFront-wheel braking sharply raises the chance of a skid

10.What is the function of a slack adjuster?

It stores extra compressed air for emergenciesIt compensates for wear of the brake shoesIt warns the driver of low air pressureIt drains moisture from the tanks