Week 6 of the Basic Auto Mechanics class was another shop only class. There was no classroom time and I honestly think that classroom time has come to an end. We are holding steady at 5 guys left in the class and I think it is much more entertaining to have the teacher teach in the shop, the material is easier to remember when you can have an example right there in front of you. This class was all about brakes and the systems that are in use when using them. Even though this class was all in the shop, I did manage to jot down some notes and learn a thing or two.
We started with the master cylinder. He explained the different parts of the master cylinder to us and showed us the reservoir and actual cylinder on a vehicle that was in the shop. He explained how that the car we were looking at probably had drum brakes on the back originally, and could still have them now, due to the fact that the second reservoir was smaller than the first one.
We went over brake fluid and how you should never use any other type of fluid in the braking system. He stressed this point a lot, he reminded us about the cooling system and how he told us to put whatever fluid in the radiator that we wanted to in a pinch to get the car home, but stressed that you cannot do that with the brake system. Stories were told about how the incorrect fluid can get hot and completely lock up the brakes. He said to look at the rubber seal for the top of the master cylinder, if the wrong type of fluid was put in the reservoir, the seal will be deformed or discolored.
He then went over how to replace the master cylinder and how you should go about it. Unbolting it from the car is the easy part, he told us to check the depth of the plunger when replacing and to make sure that they are the same from the old one to the new one. It was explained that if they are not, the brake system can build up pressure and eventually lock up the brakes because the plunger does not completely come back to the neutral position.
The next thing he told us was that when replacing the master cylinder, you don’t need to bleed the entire system and that you can bench bleed the master cylinder. The way to do this would be to put the adapters onto the new master cylinder and run hoses into the reservoir which should be filled with the proper brake fluid. You would then push in the plunger of the master cylinder with a screwdriver and allow it to return slowly to the neutral position. After one or two pushes, you should see air bubbles in the reservoir from the hoses that are in there. Once the air stops, take the hoses out of the reservoir and put the cover on. This will keep the leaking of the brake fluid to a minimum while installing it in the car. Then take the adapters out and install the master cylinder. Once the master cylinder is installed in the car, you will want to push in the plunger by stepping on the brake pedal. Find a way to keep the brake pedal depressed while you connect the brake lines to the master cylinder, have a friend hold it down for you or use a stick. Once connected, you can let the brake pedal up. This will draw the small amount of air that is in the lines into the reservoir and the system will be air free. At that point, the system will be completely bled.
There were a couple of cars lying around in the shop and we followed the brake lines from the master cylinder to see where they went. We saw a proportioning valve which keeps the brakes functioning in the event of a failure by blocking off the lines that do not have any pressure. This allows you to still have brake functionality and bring the car to a stop in the event of a brake line failure. I asked if there were multiple proportioning valves in the system to isolate individual discs and the answer was no, the proportioning valve only separates the front and rear brakes not side to side. He told me to think about what would happen if only one front brake worked when I the brakes were depressed in a panic stop situation, the wheel would be jerked off to that side and the car would end up out of control.
We also touched on the ABS systems and how they work. We talked about ABS sensors and how they communicate with the computer to tell the solenoids that control the brake pressure when to open and close. The brake lines actually go from the master cylinder to the ABS actuator which controls all of the functionality of the ABS system. The teacher mentioned that the ABS system actually increases stopping distances but you gain control in exchange for it since you should be able to still steer your vehicle in a panic stop situation.
There were two sets of demonstration brake systems brought out and setup on a bench for us to see. We only worked with one and it was a front disc brake for a rear wheel drive car. It was a setup with the hub and disc all rolled up into one unit. It was demonstrated to us how to disassemble the brake caliper and take the entire assembly all the way down to the spindle, bearings and all. I found it very surprising that all that weight actually rides on very small wheel bearings! We were shown how we should go about packing the wheel bearings for re-assembly, either using a tool or doing it by hand, and how to properly reinstall the entire unit.
At this point there was about 20 minutes left in the class and he told us to have at the demo. We took it completely apart and put it back together a couple of times to make sure that we understood it. I probably learned the most information in this class out of all the classes so far. It may be because I have always been interested in engines and making more power reliably, brakes taking a back seat. But it was very interesting to see how the fluid manages to stop a car so easily. Next week will be another week on brakes, probably going over drums and front wheel drive discs.