ASE S4 Brakes (School Bus) flashcards
ASE S4 Brakes (School Bus) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Governor cut-out pressure
- The pressure at which the air compressor stops pumping, typically 120-125 psi. Cut-out more than about 25 psi above cut-in indicates excessive governor spread.
- Governor cut-in pressure
- The pressure at which the compressor resumes pumping, typically 85-105 psi. A narrow spread causes rapid cycling and compressor wear.
- Air leak-down test
- With a fully charged system and engine off, pressure loss must not exceed 3 psi per minute (single vehicle) or 4 psi per minute (combination) with brakes released.
- Applied brake leak test
- With full brake application held for one minute, leakage must not exceed 3 psi/min single or 4 psi/min combination. Greater loss indicates a leaking chamber, valve, or line.
- Low-air warning threshold
- The low-air warning device must activate before system pressure drops below 60 psi, alerting the driver before spring brakes apply automatically.
- Spring brake application pressure
- Spring (emergency) brakes apply automatically when system pressure falls to about 20-45 psi, stopping the vehicle if air is lost.
- Compressor build-up time
- Time to build from 85 to 100 psi at governed engine speed should be about 45 seconds or less; longer times indicate a worn compressor or restriction.
- Slack adjuster pushrod limit
- Maximum pushrod travel for a clamp-type brake chamber is about 2 inches at 90 psi. Exceeding the limit requires adjustment or replacement.
- DOT 3 vs DOT 4 fluid
- Both are glycol-based and hygroscopic. DOT 4 has a higher dry and wet boiling point than DOT 3 and is preferred for heavy-duty or high-temperature braking.
- DOT 5 silicone fluid
- Silicone-based brake fluid that is not hygroscopic and must never be mixed with glycol-based DOT 3 or DOT 4 fluid; mixing causes seal damage and poor braking.
- Spongy brake pedal
- A soft, spongy pedal indicates air trapped in the hydraulic circuit. Bleed the system starting at the wheel farthest from the master cylinder.
- Sinking brake pedal
- A pedal that slowly sinks to the floor under steady pressure indicates an internal master cylinder bypass leak or a leaking wheel cylinder/caliper.
- Pedal pulsation cause
- Pulsation during braking usually results from rotor thickness variation or lateral runout exceeding about 0.003-0.005 inch, or from warped drums.
- Brake drag at one wheel
- Heat and drag at a single wheel typically indicate a sticking caliper piston, a collapsed flexible brake hose, or a restricted proportioning valve.
- Bench bleeding master cylinder
- Bleeding the master cylinder on the bench before installation removes trapped air and prevents a low or spongy pedal after installation.
- Residual pressure valve
- A valve that holds slight pressure (about 10 psi drum, 2 psi disc) to keep wheel cylinder cups seated. The wrong valve causes drag or a low pedal.
- Minimum lining thickness
- Drum brake lining is condemned at about 1/32 inch above rivet heads; disc pads are typically condemned at about 2/32 inch.
- Caging a spring brake
- A caging bolt mechanically compresses the spring brake for safe service. Always cage the chamber before removal to prevent sudden release.
- Parking brake control valve
- The yellow diamond valve exhausts air from spring chambers to apply the parking brake. Failure to exhaust leaves the brakes released.
- Inversion (anti-compounding) valve
- A valve that prevents service brake and spring brake forces from combining on the same chamber, avoiding damage to the brake mechanism.
- DD3 safety actuator
- A spring brake with a locking mechanism that must be properly charged and released per procedure; improper release prevents vehicle movement.
- Parking brake lever adjustment
- Mechanical parking brake linkage should engage within the specified number of clicks (commonly 4-7) with cables free of binding or stretch.
- Parking brake roll test
- A vehicle that rolls on a grade with the parking brake set indicates worn or oil-soaked lining or misadjusted linkage, not just cable stretch.
- Spring brake stroke check
- Excessive pushrod stroke on a spring chamber indicates a weak spring or an out-of-adjustment slack adjuster and requires correction.
- Hydro-Boost operation
- A power assist unit that uses power steering pump pressure. A hard pedal with a good pump usually means a failed accumulator or hydro-boost unit.
- Vacuum booster check
- Pedal should drop slightly when the engine starts. No drop indicates a bad check valve, leaking diaphragm, or a disconnected supply hose.
- ABS warning lamp
- A lamp that stays on after bulb check indicates a stored ABS fault; retrieve blink or scan-tool codes before diagnosing the system.
- Wheel speed sensor air gap
- Typical air gap is 0.020-0.050 inch. A metal-contaminated or damaged tone ring causes erratic speed signals and false ABS activation.
- Wheel speed sensor resistance
- Sensor resistance is commonly 800-2000 ohms. An open or shorted sensor sets a wheel-speed diagnostic trouble code.
- ABS base brake function
- ABS does not reduce base hydraulic braking; a failed ABS reverts to normal hydraulic braking with the warning lamp illuminated.
- ABS pump motor circuit
- A lamp that stays on with no wheel-speed codes may indicate a failed pump motor or relay; check the pump motor circuit for a specific DTC.
- Tone ring contamination
- Metal debris or damage on the tone ring distorts the wheel speed signal, causing intermittent ABS activation or a wheel-speed DTC.