ABS

Anti-lock braking system. Abs prevents the wheels from locking so that the vehicle slows down faster and so that the driver can control the vehicle while braking hard. ABS uses wheel speed sensors to calculate how fast the wheels are moving and whether they are locked up. If a wheel does lock up the abs system reduces the pressure going to it until it moves the same speed as the other wheels then the pressure is supplied to it again.

The system can be checked with a scanning tool to see if the wheel speed sensors, the motor and the valves  are all working correctly. If there is a problem with one of the sensors a multimeter or an oscilloscope can be used to check the sensor and the wires connecting it to the ABS controller.

Brake booster

The brake booster uses vacuum from the intake manifold to increase the braking force supplied by the driver. The booster consists of a metal canister with a diaphragm, a valve and a rod that goes through it that connects the brake pedal and the master cylinder. When the engine is started the manifold vacuums creates a vacuum inside the booster on one side of the diaphragm. When the pedal is pressed the valve is opened allowing air at atmospheric pressure to enter the booster that pushes on the diaphragm that is connected to the push rod. The push rod pushes the piston in the master cylinder with the force from the driver and the force from the atmospheric pressure/ vacuum from the engine.

The length of the push rod can be adjusted to reduce the clearance between the piston and the push rod. To find out how much the push rod had to be adjusted we measured the length of it from the surface of the booster to the end of the rod. We measured the inside of the master cylinder from the outside edge to the piston and subtracted the distance from the surface that gets bolted to the booster to the outside edge that goes into the booster. We then took the measurement of the length inside the cylinder and subtracted the length of the rod to fined the clearance and how much adjustment the push rod needed. If the clearance is correct the brakes will be more responsive and it could reduce a vehicles stopping distance. 

Brake pipe replacements

If a brake pipe fails (cracks, thread gets strips, damaged) a new one can be made. To make a brake you will need the pipe itself, a cutter, nuts that have the correct thread and a brake pipe flaring tool. Measure the old pipe and cut a piece of the pipe off that is the same length. Check the old pipe to see what type of flaring you need and flare one side of the new pipe then put the nuts on and flare the other side with the same type of flare as the old pipe. Once the pipe has been flared on both sides with the right flare and the bolts are on you can bend it to the shape of the old pipe.

Single flared are generally used when connecting the brake pipes to the wheel cylinders as they have brass inserts that look like olives. Double flares are used on the other end of the brake lines. The brake pipes are flared as another measure to ensure that the connections don't leak.

Bench testing a master cylinder

Once the master cylinder has been serviced/repaired it needs to be tested. We clamped the master cylinder in the bench vice and connected brake lines that have been blocked off. To check if the cylinder was working and if it was holding pressure we filled the reservoir with brake fluid and bled the cylinder to get all the air out. We bled it by pushing the piston in with the brake connectors loosened then closed them and released the piston. We repeated it until all the air was out of the system. Then we pushed the piston in with the cylinder completely blocked off to see if it will hold pressure. If the piston keeps going in it means that the fluid is leaking past the piston seals which means that the cylinder needs to be dismantled to find the problem. If the piston stays fixed it means that the cylinder is fine, the cylinder should be cleaned with brake cleaner if it is not going to be installed in a vehicle immediately.

Caliper repair and servicing

The brake fluid from the master cylinder pushes the caliper piston into the brake pads that push against the rotor to slow the wheel down. The problems that can cause the piston to malfunction is caused by the pistons  seals that are leaking or the piston can rust onto the cylinder. This is why it is important to replace brake fluid regularly and to use the correct brake fluid.

 We had difficulty removing the piston as it was corroded. When it was removed we discovered that the person/s that removed the piston previously had damaged it (damaged caused by hard metal tools). The marks on the piston made it unusable. We cleaned out the caliper cylinder and found that the cylinder had a hole in it. In this case the caliper cylinder and piston will need to be replaced as they are both unrepairable. If they were not damaged the cylinders inside diameter and the pistons outside diameter should be measured to make sure it fits correctly. The pistons seals and the dust seal will also need to be replaced.

 Measuring the pistons outside diameter

Measuring the caliper cylinder's inside diameter

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.