LV32 braking systems (2)

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kap all phase & 6/11/03 11:37 am Page 39 Student Workbook LV32 Braking Systems (2) LV32/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV32 BRAKING SYSTEMS (2) Contents Page Introduction Common Faults: Low or spongy pedal Possible causes Progress check Brakes drag Possible causes Brakes pull Possible causes Progress check Brakes grab Possible causes Hard pedal but inefficient braking Possible causes Squealing or chattering when brakes applied Possible causes Progress check Disc Brake Inspection and Overhaul: Procedure Progress check 4 12 14 14 19 19 22 23 23 27 28 ……… Page Drum Brake Inspection and Overhaul: Procedure Progress check 48 48 62 30 31 33 34 34 47 -1Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Introduction When diagnosing brake problems it is important to consider other systems on the vehicle that may when faulty, give similar symptoms to those of the brake system With this in mind, consideration should be given to the following: - • The first inspection should be of the wheels Look for any damage to the rim that could cause buckling Check the tyre inflation pressures and wear patterns Finally check the adjustment of the wheel bearings • Move onto the inspection of the suspension system, checking all components for signs of wear or damage • Wheel alignment problems can often result in the incorrect diagnosis of the brake system For example if the camber or caster had an out of tolerance problem, then this could cause a vehicle to pull to one side in a similar manner to a brake problem • Once the other vehicle systems can be ruled out of a diagnosis process then we can start to look at the brake system itself -3Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Common Faults • low or spongy pedal • brakes drag • brakes pull • brakes grab • hard pedal but inefficient braking • squealing or chattering when brakes applied Low or spongy pedal When the brakes are applied the pedal travel is excessive It nearly touches the floor and in some cases actually does The pedal may also feel ‘spongy’ and the brake’s efficiency is significantly reduced -4Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Possible causes Brake pedal height adjustment The brake servo push rod if incorrectly adjusted can be a cause of a low brake pedal height It can usually be adjusted by removal of the brake light switch and adjusting the push rod length via a locknut Then refit the brake light switch and ensure correct operation of the brake lights Brake pedal freeplay too large Brake pedal freeplay can also be a common cause of a low brake pedal height Stop the engine and pump the pedal until no vacuum is left in the servo Push in the pedal by hand until a resistance is felt Measure the distance, (usually about 2-6m, but this can vary from car to car) If the freeplay is non-existent or excessive, check the adjustment on the brake light switch -5Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Lining to drum clearance is excessive Remove the brake drum Using a vernier caliper, measure the internal diameter of the brake drum and then measure the external diameter of the brake shoes Take the shoe diameter away from the drum diameter and what is left will be the clearance between the brake shoes and the brake drum The usual value would be 0.6 mm; anything larger and the pistons inside the wheel cylinders would have to travel further to push the linings against the drum This would result in the brake pedal having to travel further to supply the required amount of pressurised brake fluid The easiest way to rectify this problem would be to adjust the brake shoe to brake drum clearance There are various methods but most use some sort of ratchet or adjusting rod Fluid leaking from the hydraulic circuit Modern vehicles use a tandem master cylinder, which means the hydraulic circuit is split into two circuits for safety reasons If one circuit fails, (i.e leaks) then the other circuit can still operate two of the four brakes This does however result in the brake pedals travel increasing significantly -6Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Causes of brake fluid leaks could be: • • • • • damaged/worn wheel cylinder seals damaged flexible hoses loose pipe unions faulty master cylinder faulty load sensing proportioning valve Air in the hydraulic system When air is trapped in the system it will affect the brake pedal ‘feel’ The more air the more spongy the pedal When the brake pedal moves hydraulic pressure increases within the brake system In a normally operating system the brake fluid cannot compress and therefore can only move the brake callipers/wheel cylinders and therefore the brake pads On a system that contains air, the air compresses so that not all the movement is transferred to the brake calipers and wheel cylinders The driver has no other choice than to continue pressing the brake pedal until the brakes are applied sufficiently This results in excessive brake travel -7Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Air in the brakes MUST be removed and this is achieved by bleeding the brakes Usually this job is carried out by two technicians, however there are various devices available to make it possible for a single person to carry out the task The basic principle was to have one technician sitting in the vehicle whilst the other carried out the bleeding operation The technician bleeding the brakes would connect a tube to the furthest bleed nipple from the master cylinder Once it had been loosened off, the person in the car would pump the brake pedal and the displaced fluid would travel down the tube to be collected in a suitable container During this operation it is important that the master cylinder reservoir is not allowed to become empty Once the ejected fluid runs clean, the nipple is tightened up and the technician repeats the procedure on the other three wheels Brake disc runout and/or thickness variation If the brake disc has excessive runout (warped) as it turns its overall thickness will increase and therefore the brake pads will be pushed back into the caliper When the driver pushes the brake pedal the pads have further to travel before they come into contact with the brake disc Therefore the brake pedal travel is excessive If the brake disc has a thickness variation, then the same problem will occur -8Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue The procedure to follow if the above is suspected would be: • check wheel bearings for free play • adjust/replace wheel bearings as necessary • check brake disc for runout and thickness variation After removing the front wheels, refit the wheel nuts to ensure that the brake disc is clamped correctly Use a magnetically mounted DTI, (Dial Test Indicator) Set pointer to run on the brake disc surface approximately 10 mm in from the edge Turn the disc until the highest or lowest point is found and set the DTI to zero Turn the brake disc by hand slowly and observe the runout On an average family car the runout should not exceed 0.05 mm However refer to the manufacturers’ literature where available Once this test has been carried out, regardless of the result, a thickness variation test should also be carried out This will give you a clear picture of what the actual fault is For example the brake disc could and probably will have an element of both problems -9Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Drum Brake Inspection and Overhaul Procedure The following set of photographs/descriptions, will take you through a general method for checking and replacing light vehicle brake shoes Note: For vehicle specific instructions; refer to the vehicle manufacturers’ repair manuals Remove rear wheels Remove rear wheels, being careful not to damage the alloy wheels with the wheel brace etc Ensure the handbrake is off Release the hand brake fully Make sure that it is fully off - 48 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Remove brake drum Note use of bolts Remove the brake drum Use the threaded holes supplied to pull the brake drum off If no threaded holes are supplied, care must be taken not to damage the drum or back plate, when using screwdrivers and/or hammers, to remove the drum Check brake lining thickness Minimum Thickness 1mm Check the brake lining thickness at its thinest point It is normal for the leading shoe to wear significantly faster that the trailing shoe If at any part of the lining is mm or less, replace the entire set of shoes, (i.e both sides) - 49 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Ensure wheel cylinder is not seized Place two screwdrivers as shown and move the handles from left to right, pushing the shoes and wheel cylinders from left to right If the wheel cylinder is seized or sticking you will feel it Be careful not to go too far as the wheel cylinders could pop out or get damaged Check the wheel cylinder seals Good seal, (Dry) Leaking seal, (Wet) Carefully pull the dust covers back on the wheel cylinders to see if the cylinders are leaking If they are you must repair or replace the wheel cylinders otherwise contamination of the brake shoes will take place - 50 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Operate the hand brake on and off to check the linkages are not seized Check linkages move and return back against stop Get a second person to operate the hand brake on and off (There will be no feel as the brake drum is not fitted) Whilst the lever is operated, check that the linkages/hand brake cable move correctly When the lever is fully released, the linkages return fully? (See diagram) If not then the cable is either sticking or the cable adustment needs to be backed off Do this now if necessary Location of the leading brake shoe Rotation Trailing Shoe Leading Shoe When overhauling the brake shoes, it is important to note the location of the leading and trailing brake shoes The steel part of the shoes are often the same with only the linings being different, so be careful As a rule of thumb, the leading shoe is usually located towards the front of the vehicle but not always If in doubt refer to the manufacturers’ literature In our example, this is the left hand rear wheel - 51 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Remove the tension spring Unhook spring Remove the tension spring With a pair of pliers, grip one end of the spring and unhook it from the brake shoe Remove retaining springs Remove the brake shoe retaining springs and clips They may appear to be the same as the ones in the diagram or they may be coil springs On the coil spring variety, simply grip the head of the retainer with a pair of pliers, push in and turn ¼ of a turn and they will come off - 52 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Remove the leading shoe The leading shoe will probably fall off now! This is OK It will probably be hanging by the lower tension spring Simply unhook it Remove the trailing shoe Remove the trailing shoe retainer as before, if not already done - 53 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Remove the hand brake cable Using a pair of pliers, remove the hand brake cable Compare the old against the new It is important to compare the old brake shoes against the new This is an opportunity to first of all check that the shoes are correct and secondly a chance to lay out the shoes in the correct order, i.e leading and trailing - 54 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue De-adjust brake adjuster Back off adjuster Check the brake adjuster is not sticking or seized If it’s OK then back it right off (make it as short as possible) Clean backplate and apply high melting point grease High melting point grease Clean the back plate and apply high melting point grease to the contact points - 55 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Re-fit handbrake cable Re-fitting the hand brake cable is just a reverse of the removal Re-fit trailing shoe and retainer Re-fit the trailing shoe and the retainer Ensure that the ends of the shoe are located correctly in the wheel cylinder end and the stopper end - 56 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Refit lower spring Simply re-fit the spring and locate the bottom end of the leading brake shoe in the front side of the stopper Re-fit the adjuster Ensure ends are properly located After hooking the lower tension spring in place, swing up the leading brake shoe Re-fit the brake adjuster You will have to hold the leading shoe in position as the retainer and top tension spring are not yet fitted - 57 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Re-fit the cross spring Re-fit the cross spring whilst stopping the brake adjuster from dropping out Once the spring is in place the whole mechanism should stay in one piece Check that the adjuster is located correctly as the brake drum will not go back on if incorrect Re-fit retaining spring - 58 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Check the brake drum for wear Adjust the brakes Take up the excesive clearance between the new shoes and the brake drum Have a second person operate the hand brake on and off to ensure the automatic adjustment is taking place - 59 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Re-fit brake drums Repeat on other side and re-fit wheels Always replace the brake shoes as a set - 60 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Carry out a road test Road test to ensure the brakes are properly bedded in Make sure the brakes are not binding Check on a slight hill from a standstill, release the parking brake Check hand brake freeplay when you return (number of clicks) - 61 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Progress check Answer the following questions: Explain how you would check a wheel cylinder for seizure or leaks Why must you always ensure that the parking brake mechanism is fully off, (back against the stop) before overhaul of brake drum systems? What must be done if a wheel cylinder is found to be leaking? By using a drawing, demonstrate how you would recognise the ‘leading’ or ‘trailing’ brake shoe - 62 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue ... Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue -2Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Introduction When diagnosing... vehicle systems can be ruled out of a diagnosis process then we can start to look at the brake system itself -3Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2). .. round - 18 Copyright © Automotive Skills Limited 2003 All Right Reserved LV32: Braking Systems (2) Issue Brakes pull When braking the vehicle should slow down and stop in a straight line The driver