Test de práctica CDL Air Brakes

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Las 15 preguntas de un vistazo

¿Prefieres leer antes de jugar? Abajo están todas las preguntas de la muestra. Las respuestas y explicaciones quedan plegadas hasta que las abras.

1. 1. Air brakes are really three separate braking systems. Which three?

   • A. Service, hydraulic and parking
   • B. Service, parking and emergency
   • C. Primary, secondary and emergency
   • D. Service, spring and modulating
   Ver respuesta y explicación

   Respuesta: B. Service, parking and emergency

   The service brake system applies and releases the brakes from the pedal during normal driving. The parking brake system holds the vehicle through the parking brake control. The emergency brake system is not a third set of hardware, it borrows parts of the other two so you still have a way to stop if the service system fails. Primary and secondary are the two halves of a dual air system, which is a different split entirely.

2. 2. The governor decides when the air compressor pumps. On a typical vehicle, it stops and restarts the compressor at about which pressures?

   • A. Cut out at about 60 psi, cut in at about 40 psi
   • B. Cut out at about 100 psi, cut in at about 75 psi
   • C. Cut out at about 125 psi, cut in at about 100 psi
   • D. Cut out at about 150 psi, cut in at about 125 psi
   Ver respuesta y explicación

   Respuesta: C. Cut out at about 125 psi, cut in at about 100 psi

   The manual gives roughly 125 psi for cut out and roughly 100 psi for cut in, and the exact figures for your vehicle come from the manufacturer's specification. You test it by running the engine at a fast idle until the gauge stops climbing, then stepping on and off the brake to bleed pressure down until the needle starts rising again. Do not confuse 150 psi here, that is the safety valve, and it should never open in normal operation.

3. 3. Complete the sentence.

   A safety relief valve is fitted to the first tank the compressor pumps into, and it is usually set to open at _____.

   Opciones para el hueco 1: 100 psi, 125 psi, 150 psi, 175 psi

   Ver respuesta y explicación

   Respuesta: 150 psi

   The safety valve protects the tanks and the rest of the system from too much pressure by venting, usually at about 150 psi. If it ever releases air, something upstream has failed, most often the governor, and a mechanic needs to look at it. Do not keep driving just because the brakes still seem to work.

4. 4. The low air pressure warning light and buzzer come on while you are driving. What should you do?

   • A. Stop and safely park the vehicle as soon as you can
   • B. Pump the brake pedal to build the pressure back up
   • C. Keep driving until the spring brakes come on by themselves
   • D. Pull the parking brake knob out and continue at a lower speed
   Ver respuesta y explicación

   Respuesta: A. Stop and safely park the vehicle as soon as you can

   A warning you can see is required to come on before tank pressure falls below 60 psi, and it means air is leaving the system faster than the compressor can replace it. You still have controlled braking while air remains, so use it and get parked. Waiting for the spring brakes to grab is dangerous, because they do not act on every axle, a loaded vehicle takes a long distance to stop on them, and a light vehicle can skid out of control when they come on.

5. 5. Which of the following are true about keeping water and ice out of an air brake system?

   Marca todas las que correspondan.

   • A. Compressed air carries water and some compressor oil, which collect in the bottom of the tanks
   • B. With manual drain valves, you drain the tanks at the end of each day of driving
   • C. An alcohol evaporator puts alcohol into the system to reduce the risk of ice forming in the valves
   • D. Filling the alcohol container replaces the need to drain the tanks
   • E. Water in the tanks is harmless as long as the temperature stays above freezing
   Ver respuesta y explicación

   Respuesta: A. Compressed air carries water and some compressor oil, which collect in the bottom of the tanks, B. With manual drain valves, you drain the tanks at the end of each day of driving, C. An alcohol evaporator puts alcohol into the system to reduce the risk of ice forming in the valves

   Air pulled in by the compressor brings moisture with it, and that water plus a little oil settles in the bottom of the tanks. Water can freeze and shut a valve, which is how a brake system fails on a cold morning, so manual drains get opened at the end of every driving day. Alcohol helps with ice inside the valves but it does not remove the water sitting in the tank, so daily draining still applies unless the tanks drain automatically.

6. 6. You push the pedal on an S-cam drum brake. What actually forces the brake shoes against the drum?

   • A. Air pressure pushes directly on the back of each brake shoe
   • B. A return spring releases and pulls the shoes outward
   • C. Air pressure clamps a caliper around a rotor
   • D. Air fills the brake chamber, the push rod moves the slack adjuster, and that twists the S-cam
   Ver respuesta y explicación

   Respuesta: D. Air fills the brake chamber, the push rod moves the slack adjuster, and that twists the S-cam

   Air only reaches the brake chamber. Everything after that is mechanical: the push rod extends, the slack adjuster turns the brake camshaft, and the S shaped cam spreads the shoes into the drum. That chain is why slack adjusters matter so much. If one moves more than about an inch by hand where the push rod attaches, that brake is not doing its share, and out of adjustment brakes are the most common defect found in roadside inspections.

7. 7. Which statements about a dual air brake system are correct?

   Marca todas las que correspondan.

   • A. It is two separate air brake systems, each with its own tanks and lines, worked by one set of controls
   • B. Before driving you let the compressor build at least 100 psi in both the primary and secondary systems
   • C. The warning light and buzzer must come on before pressure drops below 60 psi in either system
   • D. If one system fails, the other stops the vehicle in the same distance as normal
   • E. Both halves draw from a single shared air tank to save weight
   Ver respuesta y explicación

   Respuesta: A. It is two separate air brake systems, each with its own tanks and lines, worked by one set of controls, B. Before driving you let the compressor build at least 100 psi in both the primary and secondary systems, C. The warning light and buzzer must come on before pressure drops below 60 psi in either system

   One system typically serves the rear axles and the other the front, and they meet only at the controls, so a failure on one side does not take away all braking. Wait until both gauges read at least 100 psi before you move. Treat a warning in either half as a reason to stop, because a vehicle running on one half has lost part of its braking and will take noticeably longer to stop.

8. 8. Complete the sentence.

   If air keeps leaking away, the springs stop being held back and the spring brakes come fully on when tank pressure drops into the _____ range.

   Opciones para el hueco 1: 5 to 15 psi, 20 to 45 psi, 50 to 60 psi, 80 to 100 psi

   Ver respuesta y explicación

   Respuesta: 20 to 45 psi

   Powerful springs are held off by air pressure while you drive. Take the air away and those springs apply the brakes mechanically, which is what lets one set of hardware serve as both the parking brake and the emergency brake. The manual gives 20 to 45 psi as the range, and your own vehicle applies at whatever figure the manufacturer set inside it. This is a last resort after a warning was ignored, never a plan for stopping.

9. 9. On a newer air brake vehicle, the parking brake control is:

   • A. A round red knob you turn clockwise
   • B. A yellow, diamond shaped push pull knob
   • C. A red, eight sided push pull knob
   • D. A foot pedal to the left of the service brake
   Ver respuesta y explicación

   Respuesta: B. A yellow, diamond shaped push pull knob

   Pull the yellow diamond out to set the parking brakes and push it in to release them. On a tractor the red eight sided knob beside it is the trailer air supply, so the shape and color are how you tell them apart in a hurry. Never use the parking brake control to slow a moving vehicle, since spring brakes come on hard and do not act on every axle. Also leave it alone when the brakes are very hot from a grade, or wet in freezing weather, and chock the wheels instead.

10. 10. Air brakes add a delay that hydraulic brakes do not have. At 55 mph on dry pavement, about how much distance does brake lag add?

    • A. About 5 feet
    • B. About 15 feet
    • C. About 32 feet
    • D. About 100 feet
    Ver respuesta y explicación

    Respuesta: C. About 32 feet

    It takes half a second or more for air to travel down the lines and apply the brakes, and at 55 mph the vehicle covers roughly 32 feet in that time before braking even begins. That is why total stopping distance for an air brake vehicle is four pieces added together: perception distance, reaction distance, brake lag distance and effective braking distance. For an average driver in good conditions the total at 55 mph runs over 300 feet, which is longer than a football field.

11. 11. Which of these belong in the final air brake check of a pre-trip inspection?

    Marca todas las que correspondan.

    • A. Build to full pressure, then shut the engine off, release the brakes and time the air loss
    • B. Fan the pedal down and confirm the low air warning comes on before pressure drops below 60 psi
    • C. Keep fanning until the parking brake knob pops out on its own
    • D. Drain the tanks while the engine is running at full governor pressure
    • E. Pump the pedal 20 times first to warm up the compressor
    Ver respuesta y explicación

    Respuesta: A. Build to full pressure, then shut the engine off, release the brakes and time the air loss, B. Fan the pedal down and confirm the low air warning comes on before pressure drops below 60 psi, C. Keep fanning until the parking brake knob pops out on its own

    You build to full pressure first, because every figure in the check is measured from a charged system. Then the order runs downward in pressure: static leakage with the engine off and the brakes released, then applied leakage, then the warning as you fan the pedal down, then the knob popping out lower still. The same check also covers governor cut in and cut out and the buildup rate, which should take pressure from 85 to 100 psi within 45 seconds at operating rpm on a dual system.

12. 12. You are already in the right low gear on a long downgrade and using the snub braking method. When do you release the brakes?

    • A. As soon as you feel the vehicle slow at all
    • B. Only when you reach the bottom of the grade
    • C. Once you smell the brakes getting hot
    • D. When your speed is about five miles per hour below your safe speed
    Ver respuesta y explicación

    Respuesta: D. When your speed is about five miles per hour below your safe speed

    Apply firmly enough to feel a definite slowdown, hold it about three seconds until you are roughly five mph under your safe speed, then release and let the engine hold you until you drift back up to that speed, and repeat to the bottom. Light steady pressure the whole way down is what cooks the linings. Heat changes the lining chemically and expands the drums, so the shoes travel farther and press with less force, and once fade starts it feeds itself. That is why picking the gear before the hill matters more than anything you do with the pedal on it.

13. 13. Complete the sentence.

    Pressing and releasing the brake pedal when you do not need to lets air out of the system _____ than the compressor can replace it, so pumping the pedal to raise your air pressure actually _____ it.

    Opciones para el hueco 1: faster, slower, more evenly

    Opciones para el hueco 2: raises, lowers, has no effect on

    Ver respuesta y explicación

    Respuesta: faster, lowers

    Every release dumps some compressed air out of the system, and the compressor only refills the tanks at the rate the governor allows. Pumping is a habit carried over from hydraulic brakes, where it can help, and here it does the opposite. If pressure is falling on its own you have a leak, and the answer is to park, not to pump. The one time you fan the pedal on purpose is during the pre-trip check, when the whole point is to bleed pressure down.

14. 14. System fully charged, engine off, service brakes released. How much air can a single vehicle lose in one minute and still pass?

    • A. Less than 2 psi
    • B. Less than 5 psi
    • C. Less than 10 psi
    • D. Any amount, as long as the compressor keeps up
    Ver respuesta y explicación

    Respuesta: A. Less than 2 psi

    Static leakage has to stay under 2 psi in a minute for a single vehicle and under 3 psi for a combination. Then you press and hold 90 psi or more, let the initial drop settle, and time it again: applied leakage allows under 3 psi a minute single and under 4 psi combination. The applied figures are higher because air has now moved into the chambers and lines, so there is more of the system able to leak.

15. 15. A roadside inspector finds defective brakes. At what point does the brake condition alone put the vehicle out of service?

    • A. As soon as any single brake is defective
    • B. Only when more than half the brakes are defective
    • C. When 20 percent or more of the brakes are defective
    • D. Only when the vehicle cannot be stopped at all
    Ver respuesta y explicación

    Respuesta: C. When 20 percent or more of the brakes are defective

    The North American out of service criteria that inspectors work from take a vehicle or combination out of service when 20 percent or more of its brakes have a defect that impairs braking, and a fractional count rounds up, so a ten brake combination is out at two. Brakes out of adjustment are the most common defect found, which is exactly why the walkaround includes pulling on the slack adjusters. Other conditions can park a vehicle on their own, including an air loss rate over the limit or a low air warning that does not work.

Qué es el test de air brakes del CDL y por qué importa

No existe una licencia aparte de air brakes. El test de air brakes es un test teórico que haces junto a los demás tests teóricos del CDL, y aprobarlo es lo que mantiene la restricción de frenos de aire fuera de tu licencia. Si te lo saltas, lo suspendes o haces el examen práctico en un vehículo que no lleva frenos de aire completos, la licencia vuelve con una restricción L. Esa restricción te prohíbe conducir cualquier vehículo comercial equipado con frenos de aire, lo que descarta la mayor parte del trabajo de tractora con semirremolque y de camión pesado por el que la gente se saca el CDL. Quitarla después significa volver al DMV, aprobar el test teórico de air brakes y repetir el examen práctico en un vehículo con frenos de aire si así es como te la pusieron.

El número de preguntas, el límite de tiempo y la nota de aprobado los fija tu estado, no un organismo nacional. Es habitual un test corto de opción múltiple con un 80 por ciento de nota de corte, pero el único número del que te debes fiar es el de tu propio manual del CDL estatal, así que búscalo antes del día del examen. El contenido es más predecible. Los estados escriben el test a partir de la sección de air brakes de su manual, que es casi el mismo documento en todas partes porque desciende de un modelo de manual compartido.

Esa sección espera que sepas los tres sistemas de frenado, el compresor y su regulador, los depósitos y por qué se purgan, la válvula de seguridad, el evaporador de alcohol, el pedal del freno, los frenos de fundación con la leva en S y el slack adjuster (el ajustador de holgura), los sistemas de aire duales, el aviso de baja presión de aire, los spring brakes, el retardo del freno y la distancia de frenado, cómo frenar en una bajada larga y los pasos de air brakes de la inspección previa al viaje. Aquí los números importan más que en la mayoría de secciones del CDL. Algunos, como el umbral de aviso de 60 psi, los fija la normativa. Otros, como la presión exacta a la que salta un botón, vienen del fabricante, y el manual da un rango. Cuando tu manual diga que el rango es de 20 a 45 psi, apréndete el rango, no un número de dentro.

Cómo usar esta muestra y cómo estudiar los air brakes

Estas 15 preguntas se quedan dentro del propio sistema de frenos de aire. Preguntan qué está haciendo el regulador, qué pasa mecánicamente cuando pisas el pedal, por qué el testigo de aviso existe a la presión a la que está y qué demuestran de verdad las comprobaciones de la inspección. No tocan el acople del remolque, las glad hands (los acoplamientos rápidos de las mangueras de aire) ni el suministro de aire al remolque, que pertenecen a la sección de combination vehicles y tienen su propia muestra enlazada abajo.

Hazlo de una sentada sin el manual abierto, después lee todas las explicaciones, también las de las preguntas que acertaste. Trata un fallo como un tema, no como un dato. Si fallaste la pregunta del ritmo de fuga y la del regulador, siéntate con la secuencia de inspección entera en vez de memorizar dos números, porque los números solo tienen sentido en el orden en que se comprueban.

La parte que casi todo el mundo prepara de menos es la inspección previa hablada. Muchos estados te obligan a decir la comprobación de air brakes en voz alta durante el examen práctico, en orden, mientras el examinador mira. Leer sobre ella no es lo mismo que decirla, así que practícala de pie junto a un vehículo o, como mínimo, en voz alta y de memoria: sube a presión máxima, apaga el motor, calza las ruedas y suelta los frenos, cronometra la fuga en estático, aplica el freno y vuelve a cronometrar, después bombea el pedal y vigila el aviso y, después, el botón saltando. Di los números según avanzas. El test teórico y el examen práctico preguntan por la misma secuencia desde dos lados.

Cómo StudyPDF convierte tu manual del CDL en tests completos

Quince preguntas te enseñan qué partes del sistema de frenos de aire todavía no sabes explicar. Por sí solas no te preparan, y un banco genérico no sabe de qué manual estatal te examinan ni qué números va a esperar tu examinador.

StudyPDF va por otro camino, parte de tu material. Descarga el manual del CDL de tu estado, o coge el dosier de air brakes que te dio tu autoescuela, súbelo y Bo, el agente de estudio, crea tests de práctica completos con esas páginas exactas. Cada pregunta sale de tu documento y cada explicación señala de dónde viene la respuesta, así que puedes comprobar tú mismo la redacción cuando un número no coincida entre fuentes. Puedes generar tests nuevos tantas veces como quieras, centrar uno en una sola área como la comprobación de air brakes previa al viaje o los spring brakes, y seguir qué partes sigues fallando.

No hace falta ningún archivo para empezar. Si tu manual está en algún camión, di solo un tema, por ejemplo las comprobaciones de la inspección de air brakes o la frenada en una bajada larga, y Bo escribe un test de práctica sobre eso. Empezar es gratis.