Pennsylvania CDL — Air Brakes Practice Test 4

1.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

2.How are the powerful springs in spring brakes kept from applying while you drive?

Air pressure holds them back, and losing that air lets them applyThey are locked open by the slack adjusterHydraulic pressure keeps them retractedThe ABS computer releases them electrically

3.On some buses, what is the separate air tank tied to the parking-brake system used for?

Supplying extra air to the trailer brakesKeeping the alcohol evaporator topped offReleasing the spring brakes briefly so the vehicle can be movedPowering the ABS computer

4.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

5.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

6.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

7.Where is the safety relief valve placed, and what is its job?

In the last tank in the line, where it warns of low pressureIn the first reservoir the compressor charges, where it guards against too much pressureOn the brake pedal, where it limits how hard you can pressAt each wheel, where it keeps the brakes from locking

8.Why can a driver still make several brake applications even if the air compressor stops working?

The reserve of compressed air held in the tanks is enough for several stopsThe spring brakes quietly recharge the systemThe governor makes the compressor run on battery powerA hydraulic backup takes over automatically

9.A safety relief valve is usually set to release air at what pressure?

150 psi100 psi55 psi120 psi

10.When should you skip setting the parking brakes and use wheel chocks instead?

Any time the vehicle is fully loadedWhenever you park on a downhill gradeOnly while the engine is runningWhen the brakes are very hot, or very wet in freezing weather