ASE S4 Brakes (School Bus)

A school bus air system builds pressure slowly. The compressor is engaging normally and the governor cuts out at the correct pressure. Which condition is the MOST likely cause of slow build-up?

• AA restricted or contaminated air dryer or excessive system leakage
• BA governor cut-out setting that is set too low
• CAn unloader that is stuck in the loaded position
• DA relay valve with a defective exhaust port

The air compressor governor on a school bus is responsible for which function?

• AMetering air to the front service brakes during a stop
• BControlling compressor loaded/unloaded operation by cycling the unloader at cut-in and cu…
• CProtecting the secondary reservoir if the primary circuit fails
• DLimiting brake chamber pushrod travel as linings wear

Technician A says the air dryer removes moisture and some oil from compressed air before it reaches the reservoirs. Technician B says a saturated desiccant cartridge can allow water to collect in the reservoirs. Who is correct?

• AA only
• BB only
• CBoth A and B
• DNeither A nor B

During an air dryer service, the technician finds the purge valve is not exhausting at governor cut-out. What is the MOST likely result?

• AThe compressor will fail to unload and overpressurize the system
• BThe trailer supply line will lose pressure during braking
• CThe spring brakes will apply with the parking control released
• DMoisture and oil are not expelled, so contamination builds in the dryer and reservoirs

What is the primary purpose of the one-way check valves located between the wet reservoir and the primary and secondary reservoirs?

• ATo prevent loss of stored air from one reservoir if another reservoir or the supply line…
• BTo regulate the governor cut-in and cut-out pressures
• CTo exhaust moisture automatically from the wet tank
• DTo balance pressure between the front and rear brake circuits during a stop

A school bus reservoir drained at the end of a shift shows a steady stream of oil and water. The air dryer was replaced recently. What should the technician inspect FIRST?

• AThe treadle valve for an internal leak
• BThe air compressor for excessive oil passing (worn rings or overhead) before condemning t…
• CThe quick-release valve diaphragm
• DThe slack adjuster free travel

On a dual air brake system, what does a low air pressure warning device do, and at approximately what pressure must it activate per FMVSS 121?

• AIt releases the spring brakes and activates at 45 psi
• BIt loads the compressor and activates at 85 psi
• CIt warns the driver and must activate at or below 60 psi
• DIt applies the service brakes and activates at 100 psi

The compressor on a school bus runs continuously and never cuts out, but pressure stabilizes at about 95 psi. Governor cut-out is specified at 120 psi. Which is the MOST likely cause?

• AExcessive air leakage or demand exceeding compressor output
• BA governor set above specification
• CA check valve installed backward in the supply line
• DAn air dryer purge valve stuck open continuously

What is the function of the safety (pop-off) valve on the supply (wet) reservoir of a school bus air system?

• ATo relieve excessive reservoir pressure if the governor or compressor unloader fails
• BTo warn the driver of low air pressure
• CTo exhaust air rapidly from the brake chambers on release
• DTo proportion air between the front and rear axles

A technician must test air system leakage on a school bus with the engine off and system fully charged. With the service brakes released, the maximum allowable pressure drop in one minute for a single vehicle (no trailer) is approximately:

• A6 psi
• B2 psi
• C10 psi
• D20 psi

With the service brakes fully applied and the engine off on a charged single school bus, the maximum allowable air pressure loss in one minute is approximately:

• A8 psi
• B12 psi
• C3 psi
• D25 psi

Technician A says the wet (supply) reservoir should be drained regularly to remove accumulated moisture and oil. Technician B says an automatic drain valve can replace manual draining if it is functioning. Who is correct?

• AA only
• BB only
• CNeither A nor B
• DBoth A and B

What component controls air delivery to the front service brake chambers in proportion to brake pedal travel?

• AThe primary (front) section of the dual treadle (foot) valve
• BThe governor
• CThe spring brake control valve
• DThe air dryer purge valve

A relay valve is used on the rear axle of a school bus mainly to:

• AIncrease reservoir pressure above governor cut-out
• BSpeed brake application and release by supplying air to the rear chambers from a nearby r…
• CLimit pushrod stroke as the linings wear
• DWarn the driver of low air pressure

During a brake application on a school bus, the rear brakes apply noticeably slower than the front brakes and release slowly. Which component is the MOST likely cause?

• AA worn front brake lining
• BA clogged air dryer cartridge
• CA defective rear relay valve
• DA loose alternator belt

The quick-release valve on the front axle of a school bus performs which function?

• ADelivers air to the spring brakes during parking
• BControls compressor cut-in and cut-out
• CRemoves moisture from the supply reservoir
• DRapidly exhausts air from the front brake chambers when the driver releases the brake ped…

FMVSS 121 sets timing requirements for air brake actuation. The brake application (build) time at the chamber farthest from the treadle valve must generally be:

• ANo greater than 0.45 second to reach 60 psi
• BNo greater than 2 seconds to reach 60 psi
• CNo greater than 5 seconds to reach full pressure
• DUnregulated for school buses

A school bus has a slow brake release at all wheels after the driver lifts off the pedal. Reservoir pressure and build are normal. Which is the MOST likely cause?

• AAn undersized compressor
• BA treadle valve that is not fully exhausting the application air
• CA saturated air dryer desiccant
• DA low governor cut-in setting

Technician A says a dual treadle valve provides two separate air delivery circuits so a failure in one circuit still leaves braking on the other. Technician B says the primary and secondary circuits typically serve different axles. Who is correct?

• AA only
• BB only
• CBoth A and B
• DNeither A nor B

A double check valve in an air brake system is used to:

• ADrain moisture from the supply tank
• BReduce pressure to the front chambers
• CWarn the driver of a circuit failure
• DSelect the higher of two air pressure sources to supply a circuit or device

A bobtail (or front axle limiting) proportioning function on some buses is used to:

• AReduce front brake application pressure under light/empty conditions to limit front-wheel…
• BIncrease rear brake pressure during hard stops
• CBypass the relay valve at low speed
• DExhaust the spring brakes faster

When testing service brake apply timing, a technician notices excessive lag at the rear chambers. The reservoir and treadle valve are good. Which is the MOST likely cause?

• AA weak low-pressure warning switch
• BA kinked or restricted air line or a sluggish relay valve at the rear
• CA loose battery cable
• DAn over-adjusted slack adjuster

The stop lamp switch on most air-braked school buses is operated by:

• AMechanical movement of the brake pedal arm only
• BThe parking brake control valve
• CAir pressure in the service application circuit
• DThe ABS modulator

A school bus exhibits a hissing air leak at a brake chamber only when the service brakes are applied. The leak stops when the brakes are released. The MOST likely cause is:

• AA failed spring brake power spring
• BA worn wheel bearing
• CA leaking governor
• DA ruptured or pinched service brake chamber diaphragm

Technician A says the secondary circuit of a dual air system can be tested by bleeding off the primary reservoir and confirming the secondary still applies its brakes. Technician B says both circuits should hold pressure independently with the engine off. Who is correct?

• ABoth A and B
• BA only
• CB only
• DNeither A nor B

A school bus dash gauge shows primary pressure normal but secondary pressure low and slow to build. Which condition best explains this?

• AA failed governor
• BA leak or restriction isolated to the secondary circuit
• CAn undersized supply reservoir
• DA worn front brake drum

When measuring applied air pressure at a rear brake chamber on a school bus, the technician should connect a calibrated test gauge to:

• AThe supply reservoir drain
• BThe governor unloader port
• CThe brake chamber service port or a tee in the delivery line during a full application
• DThe air dryer purge port

A school bus loses all dash air pressure overnight, but builds normally when started. The MOST efficient first diagnostic step is to:

• AReplace the air compressor
• BReplace both brake chambers
• CAdjust all slack adjusters
• DCharge the system, shut off, and listen/soap-test for the largest leak while isolating ci…

The compressor discharge line on a school bus runs hot and shows carbon buildup at the air dryer inlet. This condition is MOST often caused by:

• AA discharge line that is too short, kinked, or restricted, raising discharge temperature
• BAn oversized supply reservoir
• CA low governor cut-out setting
• DA defective stop lamp switch

A governor is found to have a cut-out pressure of 145 psi. The specification is 120-125 psi. What is the MOST likely consequence if not corrected?

• ABrakes that fail to release
• BOverpressurization that may trip the safety valve and stress lines, fittings, and seals
• CA continuously illuminated low-pressure warning lamp
• DSlow brake apply timing

Technician A says the air compressor on most school buses is lubricated and cooled by the engine's oil and coolant systems. Technician B says the compressor inlet is often supplied with filtered air from the engine air intake. Who is correct?

• AA only
• BB only
• CBoth A and B
• DNeither A nor B

A school bus air dryer with a built-in heater is most important in which condition?

• AHot weather, to cool discharge air
• BHigh altitude, to maintain pressure
• CCity driving, to reduce compressor cycling
• DCold weather, to prevent the purge valve from freezing closed

An S-cam foundation brake uses the S-shaped camshaft to:

• ASpread the brake shoes apart against the drum when the slack adjuster rotates the camshaft
• BApply hydraulic pressure to the caliper
• CLimit pushrod travel automatically
• DExhaust air from the chamber on release

On a school bus with S-cam brakes and 30-square-inch (Type 30) clamp-type chambers, the typical maximum allowable applied pushrod stroke is approximately:

• A1 inch
• B2 inches
• C3-1/2 inches
• D5 inches

A slack adjuster on an S-cam brake functions as:

• AA valve that meters air to the chamber
• BA device that exhausts the spring brake
• CA lever that converts chamber pushrod force into camshaft rotation and adjusts for lining…
• DA seal that retains wheel bearing grease

An automatic slack adjuster is found out of adjustment with excessive pushrod stroke. Before replacing it, the technician should FIRST:

• AManually adjust it back into range and return the bus to service
• BReplace the brake chamber
• CReplace the camshaft bushings only
• DInspect for proper installation, lining/drum condition, and worn foundation hardware caus…

Technician A says brake lining must be replaced when worn to the manufacturer's minimum thickness or to the rivet/backing-plate limit. Technician B says cracked, oil-soaked, or glazed linings should also be replaced even if thickness is adequate. Who is correct?

• ABoth A and B
• BA only
• CB only
• DNeither A nor B

When measuring an S-cam brake drum, the technician finds the diameter exceeds the maximum machine-to / discard limit cast on the drum. The correct action is to:

• AMachine it anyway to clean up the surface
• BReplace the drum because machining beyond the discard diameter weakens it and reduces hea…
• CInstall thicker linings to compensate
• DIgnore it if there are no cracks

An air disc brake used on some school buses applies braking force by:

• ASpreading two shoes outward against a drum with an S-cam
• BForcing hydraulic fluid through a master cylinder
• CUsing the chamber pushrod and an internal actuator/caliper to clamp a rotor between frict…
• DEngaging a transmission-mounted parking pawl

During a brake reline, the technician notices the camshaft has excessive radial play in its bushings. The correct repair is to:

• AAdd grease and return to service
• BReplace the slack adjuster only
• CMachine the camshaft journals oversize
• DReplace the camshaft bushings (and seals) and verify camshaft end play within spec

A school bus pulls to the right during braking. Tires, alignment, and air pressure to both chambers are equal. Which is the MOST likely brake-related cause?

• AContaminated or glazed lining, a seized cam/slack, or unequal shoe contact on one side
• BA faulty governor
• CA saturated air dryer
• DA weak spring brake on both rear wheels

Technician A says shoe-to-drum clearance on a manual-slack-adjuster S-cam brake is set by adjusting the slack adjuster until the shoes lightly contact the drum, then backing off to spec. Technician B says automatic slack adjusters should be set the same way at every PM. Who is correct?

• AB only
• BA only
• CBoth A and B
• DNeither A nor B

Blue or heat-discolored brake drums and a strong burning odor on a school bus most likely indicate:

• ANormal brake operation
• BA worn wheel bearing only
• CDragging brakes from out-of-adjustment shoes, a stuck cam/slack, or incomplete air release
• DA low governor cut-in setting

When relining S-cam brakes, anchor pins and rollers should be:

• AWelded to prevent movement
• BPainted to prevent corrosion only
• CLeft unserviced if the linings look good
• DInspected for wear and replaced if worn, then lubricated per the manufacturer where speci…

A long-stroke brake chamber is identified on a school bus by:

• AA marked stroke rating and a square or specially shaped non-pressure housing or identifyi…
• BIts bright red color only
• CThe absence of a pushrod
• DHaving no clamp band

A technician measures applied pushrod stroke with a 90-100 psi application. To check brake adjustment accurately, the measurement should be taken:

• AWith the engine running at idle and brakes lightly tapped
• BFrom the fully released position to the fully applied position with the brakes adjusted/c…
• COnly after driving the bus until the brakes are hot
• DWith the parking brakes applied

During a foundation brake inspection, a return spring on an S-cam brake is found stretched and weak. The likely effect is:

• AFaster brake apply timing
• BIncreased reservoir pressure
• CThe shoes may not fully retract, causing brake drag and premature lining/drum wear
• DImproved fuel economy