Does this sound like you?
You've just replaced the rear brake shoes and cylinders, attempted to bleed the system, and *BANG*, the pedal goes straight to the floor. The master cylinder has just given up the ghost. You ring the customer and explain that the car won't be ready until you source and fit another cylinder, and you tell them the job will be twice as much as you originally expected.
The new cylinder turns up when you're absolutely flat out; you bang the cylinder on and try bleeding at the wheels, but can't get a good pedal. The customer is waiting for the car, but the cylinder just won't bleed up. Is it a dud cylinder, or is air in the system? In 99% of the cases it is air in the system, and it is the bleeding technique that is causing the problem.
Here are some handy hints that will hopefully prevent this from happening and save you some valuable time.
Nobody likes doing it, and everybody says they've been fitting master cylinders for forty years and have never needed to do it, but believe it or not, it is worthwhile bench bleeding a cylinder before fitment. Why? Well, air obviously gets trapped in a master cylinder and the fastest way of getting rid of that air is simply to do it on the bench. It makes it a lot easier to remove the air from the lines if you?re starting with a properly bled master cylinder.
The best way to bench bleed a cylinder is to fill the fluid reservoir with brake fluid, and attach return hoses from the outlet ports to recycle the fluid back into the reservoir as the push rod is stroked. Alternatively, if you have no access to return pipes, you can stroke the cylinder in the vice, putting your fingers over the outlet ports as the piston returns. Keep going until a steady flow of fluid comes out of each port. Then put plugs in the ports and fit the cylinder to the vehicle.
The next stage of bleeding is to bleed the system at the pipes on the master cylinder. This removes air from what is usually the highest point in the system. To do this, have an assistant in the car, crack all outlet pipes then depress the pedal. Do the pipes up before the pedal is released. Keep repeating this procedure until no air is present.
Now it is time to go to the wheels. There are three commonly used bleeding techniques which all have their advantages and disadvantages: pressure bleeding, manual bleeding and vacuum bleeding.
This method requires the use of a pressure bleeding unit, such as the Partswise® Bleedpro system. The unit pressurizes the fluid in the reservoir, and the bleeders are opened in a sequence until no air is left in the system. There are 2 main advantages with pressure bleeding. First and foremost, it is fast, and one person can do the job. Secondly, pressure bleeding has an advantage when used on older vehicles that have had little maintenance. The master cylinder does not have to be stroked to push fluid through the system. On an older car there is sure to be corrosion and dirt inside the master cylinder. Stroking the pedal to the floor moves the piston deep into the master cylinder where it would otherwise never travel. The seals may pick up dirt and be damaged. Pressure bleeding avoids this.
This method requires the use of the brake pedal and master cylinder as a pump to expel air and brake fluid from the system. This method is usually a 2 man operation. Starting at the wheel that is determined by the proper bleeding sequence for the vehicle being worked on (you will probably need a manual or database for this,) one person opens (during the pedal operation) and closes (prior to pedal release) the bleeder screws and observes the air bubbles while the assistant pumps the pedal slowly. Usually the fluid is bled through a piece of clear plastic hose into a jar with fluid at the bottom. Keeping one end of the hose inserted in the fluid will prevent air getting into the system when the bleeder is open. To ensure no air can be sucked back into the system, the bleeder screw must be closed at the end of each stroke, before the pedal is released.
Vacuum bleeding can be done by one person. Vacuum bleeders usually utilize shop air pressure and a venture principle to suck fluid from the bleeder. Vacuum bleeding does have drawbacks in that it can allow air to be drawn in past the seals, thus creating more problems.
To sum up this article on bleeding, all methods have their good and bad points. Some methods work better on certain models than others. Sometimes manual bleeding won?t be enough to remove a stubborn pocket of air and pressure bleeding will be required. Whichever method is used to bleed at the wheels, always remember it is best to bench bleed the cylinder first. Also remember to fit bleed screw caps when finished. These are available in packs of 20 from NBS, and mean next time the car is due for maintenance the bleed screws will be fresh and clean.