Wheel cylinder repair and service

The wheel cylinder and piston set up pushes the brake shoes into the drum to slow the wheel down. There is three types of drum brake set ups...

 Duplex

Servo

Simplex

The wheel cylinder that we took apart was from a simplex system. It has two pistons in the cylinder facing away from one another. When the fluid from the master cylinder enters the wheel cylinder the fluid pushes both pistons out with the same force unless one of the pistons are jammed or the brake fluid is leaking past the piston seals. If the seals are leaking they will need to be replaced. The wheel cylinder will need to be honed if it is corroded inside (surface rust) if the wheel cylinder is badly damaged it will need to be replaced. NB If the cylinder is made of aluminium it is not possible to hone it. If the cylinder is made from cast iron use kerosene or methylated spirits as a lubricant when honing it. We measured the inside of the cylinder and the diameter of the piston to make sure that the seals will seal properly. Once the cylinder has been honed the we cleaned it with brake cleaner and replaced the pistons after putting rubber grease on the seals to stop them from being damaged. We then replaced the dust seals.

Brake roller test

The brake roller tester is used to measure the braking force of a vehicle. The  machine we used also tested the shock absorbers. First we drove the vehicle onto the shock absorber tester and the machine tests them then we moved the vehicle's front wheels onto the brake roller tester and applied the brakes according to how much the machine specified and once the test are done we applied the brakes hard. Then we tested the back  shock absorbers, the back brakes and the hand brake using the same procedures.

We discovered that the left wheel failed on the park brake test. The vehicle that we tested had inboard brake drums for the park brake so it narrows the problem down to the drums. It could either be the brake shoes, the handbrake cable that is jammed up or the adjustment of the spreader. In our case the shoes still had enough lining on and the cable was not jammed so it is probably the adjustment of the spreader.

If a vehicle with only drums or only rotors was tested and the back brakes are fine but the park brake is not it is more than likely the park brake mechanism that is faulty or the cable is jammed or the cable needs to be adjusted. If there is a problem with brakes it could be caused by air trapped in the brake lines so it will need to be bled or there could be a mechanical fault in the calipers/drums like the piston/s or sliding bolts being jammed and not allowing the fluid pressure to push the brake pads/shoes against the rotors/drums.There may also be a leak in the brake lines or the seals around the pistons, if it there is a leak the fluid will not have enough pressure to push the pistons out which leaves the brakes with a smaller braking force.

 Brake roller tester

 Shock absorber tester

 Inboard drum Park brake

Inboard drum Park brake

Servicing and repairing master cylinders

The master cylinder supplies the brake fluid to the wheel cylinders. The size of the master cylinder is smaller than the wheel cylinder, this causes the master cylinder piston to move more than the wheel cylinder pistons but the wheel cylinder pitons have more force acting on the pads/shoes. Most of the new master cylinders have two pistons, this is mainly for safety reasons. It has two lines that either split front and rear or split diagonally so each piston is connected to two wheels so if one system fails there is another system to slow the car down.

If the master cylinder fails it is probably the seals around the pistons. The pistons need to be removed and the cylinder should be checked for corrosion, because brake fluid draws in water and if the vehicle is parked for too long it can cause the master cylinder to corrode. If it is a steel cylinder it can be honed using methalated spirits or kerosene with a honing tool and then it should be cleaned out with brake cleaner. If the seals have been replaced, the cylinder honed or replaced (if it is aluminuim) and it is assembled it should be bench tested using blocked off fittings to close off the openings for the break lines. The cylinder should hold pressure if it doesn't then there is still a problem with it and it will need to be disassembled again to find the problem. The cylinder should be cleaned with brake cleaner if it will not be installed immediately, otherwise the brake fluid will get contaminated with water and that will cause the cylinder to corrode.

Honing

Secondary piston

Primary piston

Brake fluid testing

Brake fluid is hygroscopy so it may be contaminated by water. To test if the brake fluid in a vehicle has been contaminated we use a Brake fluid tester which has three different coloured LEDs in it. If the green light goes on then the fluid is not contaminated, if the yellow light goes on it means it has been contaminated but it is still fine to use, if the red light goes on it means that the fluid is contaminated and will need to be replaced. Brake fluid does not stay uncontaminated and will need to replace about every two or less depending on the climate of the country the vehicle is in. When the brake fluid is contaminated the water brings the boiling point of it down and it could cause components in the system to corrode.

The vehicle that I tested showed a green light which meant that the fluid was fine and it did not need to be replaced. If the brake fluid needs replacing then drain all the the fluid out of the reservoir and put new fluid in then start bleeding the brakes on the wheel furthest away from  the master cylinder. Put a piece of wood under the brake pedal and get someone to pump it and then hold it down once they hold it down open the bleeder nipple and allow the brake fluid to escape into an allocated container then close it again. Repeat this until you can see that the new fluid is coming through then do it on the next wheel furthest away from the master cylinder. Do it until all of the fluid is out of the system and make sure that there is always enough fluid in the reservoir.

Servicing brakes

Disc brakes

If the brake pads need replacing, loosen the sliding bolts and remove the bottom one. Then lift the caliper up  and the pads will be able to slide out. The brake pads can be measured to see if they can be used again. Use a DTI gauge to check if the rotor is warped or not. If the rotor is worn then the inside and outside of it should be measured with a micrometer, the measurements should be compared to specifications. If it is worn and with in the specified thickness then it can be machined if it is not or if it is warped then it will need to be replaced. Once the rotors have been replaced the new brake pads can be replaced. The new brake pads obviously have more lining than the old ones so the slave/caliper pistons need to pushed back this can be done with a G clamp or by clamping the brake hose and opening the nippple so that it can be pushed back by hand. Before the pads are fitted make sure they are correct and remember to replace any shims and anti squeal plates. An anti squeal spray may also be sprayed onto the back of the pads. Once every thing has been put back into place the caliper and sliding bolts should be torqued to the correct setting.

If one of the rotors in the front is warped the driver will feel it in the steering  wheel  if one in the back is warped the whole car will vibrate.

Drum brakes


To remove drum brakes make sure that the cars handbrake is off. Then either hit it with a brass hammer or put two bolts in and turn them until the drum comes off. Once it has been opened check under the dust seals to see if the pistons are leaking then clean it with brake cleaner or water to get rid of the brake dust. Then using a screw driver  to move the pads from side to side to make sure that the pistons that activate the brakes are not jammed. Then measure the thickness of the brake shoes and check the inside of the drum. If the drum is worn inside it will need to be machined. Before getting it machine measure the inside diameter of the drum and check it with the specifications. Once the all the checks have been done and it has been repaired if necessary the brakes need to be adjusted by turning the nut on the spreader until the brakes are in the right position. the last thing to do is to adjust the handbrake cable.

The breaking system

When the break pedal is depressed it pushes the piston in the master cylinder forward. The master cylinder pushes the break fluid through the break lines and into the slave cylinders that push the break shoes against the rotors.

The force applied by driver alone is not enough to stop the vehicle therefor the car uses mechanical systems to multiply the force. The first multiplication is mechanical advantage (leverage) by the pedal. The next multiplying step is the break booster which uses vacuum from the engine to increase the force applied. Then the hydraulic system also multiplies the the force by having the master cylinder smaller than the slave cylinders. If the master cylinder is smaller than the slave the master cylinder moves more than the slave but the slave will have more force as it has a larger surface area.

The friction between the break shoes and the rotors convert kinetic energy into heat energy. The wheels lose kinetic energy therefor the vehicle loses kinetic energy too (slows down).

Jacking up a car (safely)

When jacking up a car it is important to make sure that the area on the ground is level and smooth. If the area on the ground is not level the car may roll back and slip off the jack. If the ground is not smooth the jack's small wheels could get stuck and as the jack goes up the jack will pull away from the jacking point, it could cause the vehicle will slip off.

It is important to put the car in park and put the handbrake up so that the car will not roll backwards or forwards off the jack. For extra safety something could be placed behind the wheels. Before jacking up the car make sure that the jack is in the right position (90 degrees to the wheels) and that it is under a suitable  jacking point where it will not damage the car or be likely to slip of. Once the car has been jacked up put axle stands in as far away from the center as possible to increase the vehicle's stability. Lower the jack so that the vehicle rests on the axle stands. The jack may be left in place as an extra precaution but the handle should be lifted up as far as possible so that other people do not trip over it. Once the car has been jacked up give the vehicle a "shake" to make sure that it is secured before going under it.