Comparison of Brake Testing Methods

PROBLEM	TEST DRIVE	BRAKE TESTER	VISUAL INSPECTION
Brake Noise	Will be heard during a few normal stops.	May or may not be heard during a test stop.	Most causes very difficult to detect.
Pedal Pulsation	Will probably be felt during a few normal stops.	Can be felt but duration will be very short.	Rotor parallelism can be checked but is a time consuming procedure.
Lining Thickness/worn scored rotors or drums.	Impossible to detect unless severe metal on metal condition exists – then a "groaning" sound is usually present.	Difficult to detect unless metal on metal condition exists – then brake balance will be uneven because metal on metal is more aggressive than pad on metal.	May be checked visually with wheels pulled.
Out of Adjustment Brakes	Very difficult, often impossible and potentially dangerous to detect in test drive situation on public roads.	Quickly displays as a wheel or axle imbalance. Wheel with the shortest skid marks is the target.	Difficult to detect - Brake pads will look good because very little wear is taking place.
Leaking Axle Seal, wheel cylinder, caliper piston … causing brake grab. (Small leak that just wets the lining material)	Often difficult and potentially dangerous to detect in test drive situation on public roads.	Quickly displays as a wheel or axle imbalance. Wheel with the longest skid mark is the target.	Small leaks are often overlooked. Axle seals, unless cleaned and rechecked, are impossible to tell if already repaired.
Leaking Axle Seal, wheel cylinder, caliper piston … causing weak brake. (Large leak that soaks the lining material	Very difficult and potentially dangerous to detect in test drive situation on public roads.	Quickly displays as a wheel or axle imbalance. Wheel with the shortest skid marks is the target.	Large leaks are relatively easy to detect visually.
Seized Wheel cylinder or caliper piston.	Very difficult and potentially dangerous to detect in test drive situation on public roads.	Seized wheel cylinder or caliper pistons will show up as greatly reduced braking (short skid mark) on one or more wheels.	Very difficult to detect visually. Requires at least partial disassembly of brake system – not just removal of wheels.
Low Fluid level	Impossible to detect unless injecting air into system which results in soft pedal feel.	Impossible to detect unless injecting air into system which results in soft pedal feel.	Easily checked during visual inspection.
Air in Hydraulic System	Large quantities detectable by pedal feel. Small quantities are very difficult to detect but can affect performance.	Trapped air in system results in loss of brake force and brake balance suffers.	Impossible to detect.
Loose Wheel Cylinder	Very difficult and potentially dangerous to detect in test drive situation on public roads.	Will show up as a very high brake force on wheel(s) with loose cylinder.	Difficult to detect visually. Requires at least partial disassembly of brake system – not just removal of wheels.
Internally Collapsed Hose	Difficult to detect on the road particularly on rear axles.	Will show up as a delay in brake force application on one wheel in relation to other wheel on the same axle.	Impossible to detect.
Dragging Brake	Difficult to detect.	Will display as a brake force before the brakes are applied.	Usually detectable with a thorough inspection.
Damaged, Misadjusted, load sensing proportioning valve. (Due to damage, collapsing springs, and/or misadjustment)	Very difficult, often impossible, and potentially dangerous to detect in test drive situation on public roads.	Will easily display incorrect load sensing proportion valve position as either too strong or too weak on rear axle.	Damage to valve linkage can be detected visually. Misadjusted valves are very difficult and labor intensive to detect visually. Internal valve problems are impossible to detect. Slightly collapsed springs are not detectable without taking ride-height measurement.
Malfunctioning Fixed Proportioning Valve	Very difficult, often impossible, and potentially dangerous to detect in test drive situation on public roads.	Will easily display incorrect proportioning as either too strong or too weak on rear axle.	Impossible to detect.
Metering Valve Not Operating Correctly.	Virtually impossible and potentially dangerous to detect on public roads.	Will display as either front or rear brake force applying before the opposite axle. Fronts early if valve is not working. Rears early if valve is working too well.	Impossible to detect.
Broken or Out of Adjustment Parking Brake	Detectable only if BOTH brakes are not functioning.	Will show effectiveness of each parking brake	Visual inspections typically as labor intensive as actual adjustment.