ALT-1 How to Replace a Brake Master Cylinder
There you are, driving along, when your brakes start to feel spongy. In fact, the pedal goes all the way to the floor, and you'll probably see the red brake warning light on your dashboard come on. Best thing to do is to pull over, and arrange for the car to be uplifted. After all, you may feel confident that you can hobble home with no brakes, but if a kid runs out in front of you, how are you going to stop in time.
So how do you know it's your master cylinder? Do some basic checks around the car. Brake fluid seeping out around a wheel brake calliper, points to a brake piston seal having gone. If that's ok, then check for any leaks at brake pipe junctions. Still fine, then it's your master cylinder.
You can't repair the master cylinder, it has to be replaced, but it's not a hard job to do. In fact, it should just take you a few hours to complete. A good investment for your car if you will be doing your own repairs is a repair manual. Every make and model has variation you need to know to do the repair. Although the principles of the braking system are the same for all cars.
First thing to do is to wipe down the reservoir to remove any dirt, and remove the cap. You want to keep everything clean, because you don't want the new fluid to get contaminated before it's even been properly used. You can now drain the master cylinder of brake fluid. Remember, brake fluid is not only poisonous if ingested, it will irritate your skin, so use some of those disposable gloves.
I stole the turkey baster from the kitchen at home, and have found it the best and cheapest way of removing brake fluid. Just remember to buy a replacement for the kitchen, and keep the used one in your toolbox.
A large syringe will do the same job, but if you don't have anything to hand, don't despair. You can use a bleeding kit on one of the callipers. This is a one-way valve on the end of a tube, which allows fluid to drain out, but stops air getting into the system. Once the bleed nipple is opened, just gently depress the pedal until the master cylinder is empty.
Now get some rags and place them around the master cylinder to catch any leftover fluid. Brake fluid can damage paintwork, so you want to contain it as much as possible. If the replacement cylinder comes without the reservoir, remove the old one from the system and put it aside.
You now need to give the brake pipe union bolts a good wipe down. Undo these connections, remembering to make a note of what one goes where, and move them carefully to one side. Don't forget to cover the open ends of the pipes. Again, you don't want dirt to get into the system after all your hard work.
Undo the nuts and washers holding the master cylinder onto the vacuum servo unit. All you have to do now is gently pull the cylinder away from the vacuum servo. The sealing ring should be renewed when you fit the new brake master cylinder. All that is left to do now, is refit the parts in the reverse order of removal. Take the time to check that all the parts are as clean as possible. Once everything is fitted and tightened up, refill the master reservoir with brake fluid.
Gently pump the brake pedal, having checked that your brake bleeding kit is still properly fitted to the bleed nipple on the calliper. Don't let the master reservoir fluid level drop too low, or you will have to start the brake bleeding process from the start.
Once the air is out of the system, tighten up the bleed nipple, and check the pedal travel. I would advise you to bleed all of your braking system, as air may be trapped in any of the four callipers. Once done, if the brake pedal travel is normal, and the fluid level is steady, the job is done.
One other point to note is do you know when your brake fluid was last completely drained. It's recommended that this be done every two to three years, as the brake fluid can be contaminated by moisture over time. So while you're changing the brake master cylinder, you should consider completely flushing the old brake fluid. It is basically the same amount of work, just drain down each calliper until the new fluid is in all the system.
That's it, not only have you successfully replaced the brake master cylinder, but replacing all of the brake fluid will eliminate any possibility of brake fade. Two problems solved at once.
