ASE S4 Brakes (School Bus) cheat sheet

Normal air system governor cut-out is about 120-125 psi; cut-in is about 85-105 psi. Cut-out more than 25 psi above cut-in signals excessive governor spread.

Standard air-brake leak-down limit (engine off, brakes released, fully charged): loss must not exceed 3 psi in one minute for a single vehicle; 4 psi/min for a combination.

Applied-brake leak test: hold full brake application for one minute; leakage must not exceed 3 psi/min (single) or 4 psi/min (combination).

Low-air warning must activate before pressure drops below 60 psi; the spring brakes must apply automatically when pressure falls to 20-45 psi.

Compressor build-up time from 85 to 100 psi should be about 45 seconds or less at governed engine speed; slow build indicates worn compressor, slipping belt, or restricted discharge line.

Alcohol evaporator or air dryer protects against moisture; a failed purge valve causes constant spitting and rapid compressor cycling.

Safety valve (pop-off) opens at about 150 psi; a valve that vents below 150 psi indicates a faulty valve or overpressurized system.

Slack adjuster pushrod travel limits: clamp-type brake chamber 2 in. at 90 psi for Type 30; exceeding the limit means readjust or replace the slack adjuster.

DOT 3 and DOT 4 are glycol-based and hygroscopic; DOT 5 is silicone-based and must not be mixed with glycol fluid.

Brake fluid boiling point drops as moisture is absorbed: wet boiling point of DOT 3 is about 284°F vs. dry about 401°F.

A spongy pedal indicates air in the hydraulic circuit; a pedal that slowly sinks to the floor under steady pressure indicates an internal master cylinder bypass leak.

Pedal pulsation or vibration during braking points to rotor thickness variation or lateral runout, typically exceeding 0.003-0.005 in.

Brake drag and heat at one wheel usually indicate a sticking caliper piston, collapsed flexible hose, or restricted proportioning valve.

Bench-bleed the master cylinder before installation; bleed sequence is typically farthest wheel from the master cylinder first.

Minimum brake lining thickness on drum brakes is usually 1/32 in. above the rivet heads; disc pads are commonly condemned at about 2/32 in.

A residual pressure valve (typically 10 psi drum, 2 psi disc) holds slight pressure to keep cups seated; wrong valve causes drag or a low pedal.

Spring (emergency) brakes apply when air pressure drops below about 20-45 psi; they release only when system pressure exceeds roughly 60-90 psi.

A DD3 safety actuator uses a locking mechanism; it must be properly charged and released per procedure or the vehicle will not move.

Parking brake stroke on spring chambers is measured at the pushrod; excessive stroke means the spring is weak or the slack adjuster is out of adjustment.

Mechanical parking brake (driveline or cable) adjustment: lever should engage within the specified number of clicks (commonly 4-7) with the cable free of binding.

Caging a spring brake requires a caging bolt to mechanically compress the spring for safe service; always cage before removing a chamber.

Parking brake control valve (yellow diamond) must exhaust air from the spring chambers to apply; failure to exhaust leaves brakes released.

Inversion valve (anti-compounding) prevents service and spring brake forces from combining on the same chamber.

A vehicle that rolls on a grade with parking brake set indicates worn lining, oil-soaked shoes, or misadjusted linkage, not just cable stretch.

Hydro-Boost uses power steering pump pressure; hard pedal with a good pump usually means a failed accumulator or hydro-boost unit.

Vacuum boosters: pedal should drop slightly when the engine starts; no drop indicates a bad booster check valve, leaking diaphragm, or supply hose.

ABS warning lamp that stays on after bulb check indicates a stored fault; retrieve blink or scan-tool codes before diagnosis.

Wheel speed sensor air gap is typically 0.020-0.050 in.; a metal-contaminated or damaged tone ring causes erratic speed signals and false ABS activation.

ABS wheel speed sensor resistance is commonly 800-2000 ohms; an open or shorted sensor sets a wheel-speed DTC.

ABS does not reduce base hydraulic braking; a failed ABS should revert to normal hydraulic braking with the lamp illuminated.

Check the ABS pump motor and relay circuit for a lamp that stays on with no wheel-speed codes; pump motor failure sets a specific DTC.

Blade-style ABS sensor connectors and harness routing near moving suspension parts are common causes of intermittent wheel-speed faults.