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Brake System Problem Diagnosis&

ABS Service Tips

R. Bortignon

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Brake Squeal• is usually the result of high frequency metal-to-metal vibration• on drum brakes…

ensure the 6 brake shoe support pads have a thin coat of high temp lube such as anti-seize compound

• on disc brakes… ensure brake shims are used on the brake pad backings check abutment clips – replace if needed clean & lubricate the caliper slides alternate brake pad composition may help

organic semi-metallic ceramic

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Brake Pedal Pulsation or Steering Wheel Shimmy

• front brake rotors need refinishing or replacement wheel installation with an impact gun can cause this hard, panic stops when the brakes are hot can lead to these conditions

• remember to measure the disc thickness to determine if there is enough material left to machine

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Low Brake Pedal

• this is usually caused by either…

1) brake shoes are out of adjustment (shoes are too far away from drum)

2) air in the hydraulic lines if this is the case YOU HAVE A BRAKE FLUID LEAK!!! air in the system is not a normal condition! brake fluid level should never get below the MIN mark

brake warning lamp may be illuminated

pump the brake pedal rapidly 3 or 4 times if the pedal came up higher: shoe adjustment issue if the pedal is still low: air in the line(s)

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Brakes Pull to One Side

• ensure it is not the steering system creating the pull if it pulls with the brakes off – it is most likely in the steering system & not

the brakes

• one of the front calipers isn’t working properly rebuild or replace calipers

• restricted front brake flex hose prevents brake fluid from becoming pressurizing

• pulling is not the result of metering valve, proportioning valve or ABS problems

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Dragging Brake(s)

• can be associated with sluggish vehicle performance• brakes will smell when overheated• can be a caliper piston is not retracting• e-brake cable is not releasing completely• drum brake shoes adjusted too close to the drum• can be caused by a collapsed flex hose

this allows brake fluid to pressurize but when pedal is released fluid is “trapped” under pressure and piston

can’t retract loosen the bleeder to determine if this is the cause

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Brakes Grab / Lock-up

• if both rear brakes grab, front brakes should be inspected carefully proportioning valve limits pressure to rear wheels if front brakes aren’t functioning correctly then rear brakes will have to work

harder to stop the vehicle with the transfer of weight from back to front on hard braking, this can lead

to rear wheel lock-up• this is usually the result of oil, grease or brake fluid on the brake linings

replace defective grease seal, wheel cylinder or axle seal

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Brake Pedal Sinks• Imagine this…• you are stopped at a red light & the pedal is at its normal height• slowly, the pedal starts to sink to the floor• you quickly pump up the pedal & it returns to normal height• again it starts to sink• this is caused by…

an internal leak in the master cylinder fluid is leaking past the primary seal(s)

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ABS System Service…

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ABS System Diagram

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Servicing Precautions

• bleeding (removing air from hydraulic lines) & flushing brake fluid may be done exactly the same as a non-ABS equipped vehicle or may have specific bleeding procedures which

• when disconnecting brake lines, accumulator pressure may have to be relieved – can be up to 2800 PSI! pumping the brake pedal 30 to 40 times with engine off may be necessary

• read manufacturer's instructions carefully before beginning repairs!

• ABS problem: always begin with a thorough inspection of the base brake system (non-ABS portion) first

• brake warning light indicates a problem in the base braking system – not the ABS

• ABS warning lamp indicates a problem within the ABS

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Warning Lights• both brake warning lights should come on for about 5 seconds when the

ignition is initially turned on this confirms that the warning circuit is operational

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Service Tips• check wheel speed sensors for debris build-up (permanent

magnet)• check sensor wiring & connectors

frayed insulation incorrect routing can chafe wiring loose, corroded connectors

Service Tips• check tone rings for damage/debris build-up

• mismatched tire sizes will confuse the EBCM

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• follow manufacturer’s repair procedures carefully…

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Service Tips

• when replacing brake pads, do not push caliper pistons in without loosening bleeder screw

this prevents dirt from being pushed out of the caliper & into the brake modulator & master cylinder

• regular brake fluid flushes (every 2 years) will minimize modulator & solenoid problems & extend their life span