“Bleeding brakes is easy. Just a couple of pumps of the pedal and we’re done”…..sometimes it doesn’t work out like that! In this article we go back to basics and rehearse the fundamentals of bleeding brakes, we cover off on different methods and gives some handy tech tips.
How to Bleed Brakes
Written by Darren Caldwell
Does this sound like you?
You’ve just replaced the rear brake shoes & cylinders and 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 & explain that the car won’t be ready until you source & fit another cylinder and you tell them that the job is going to cost twice as much as before.
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 customers waiting for the car but the cylinder won’t bleed up. Is it a dud cylinder or is it a case of air still being in the system? In 99% of cases it is air in the system and the bleeding technique that causes the problem. Here are some handy hints that will prevent this from happening and save you some valuable time. Sound familiar?
Bench Bleeding
In most cases (except for isolated master cylinder types) it is best to bench bleed a cylinder before fitting it to the vehicle. Bench bleeding removes all the air from the master cylinder, which makes it faster and easier to bleed the individual brake lines after installing the unit. Skipping bench bleeding almost always causes trouble because large pockets of air must travel through to the bleeders at the wheels.
The best way to bench bleed a master cylinder, as shown in the diagram above, involves filling the fluid reservoir and attaching return hoses to the outlet ports to recycle the fluid back into the reservoir while stroking the push rod. Alternatively, if you haven’t got access to return pipes, you can stroke the cylinder in the vice, putting your fingers over the outlets 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 unit 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. Tighten the pipes before releasing the pedal. Keep repeating this procedure until no air is present.
The three methods: Pressure – Manual – Vacuum
Pressure Bleeding
This method requires the use of a pressure bleeding tool. The unit pressurizes the fluid in the reservoir, and you open the bleeders in sequence until no air remains 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 on older vehicles that have had little maintenance. You don’t need to stroke the master cylinder 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. Dirt may damage the seals if they pick it up. Pressure bleeding avoids this.
Manual Bleeding
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. Start at the wheel specified by the proper bleeding sequence for the vehicle. One person opens the bleeder screw, the pedal is depressed, and the bleed screw is locked up before the pedal is released. This is the open-shut method. Usually, the person directs the fluid through a piece of clear plastic hose into a jar containing fluid at the bottom. Keeping the end of the hose inserted in the fluid will prevent air getting into the system when the bleeder is open. Once the vehicle has been bled by the open-shut method at each wheel and a firm pedal is achieved, proceed to pump the pedal and hold pressure and then crack the bleed screw one at a time in sequence. This final step will create a high velocity of fluid to dislodge any trapped pockets of air.
Vacuum Bleeding
This method uses a vacuum bleeder which come in various designs to suck the fluid from the bleed screw. One benefit of Vacuum bleeding is one person can perform vacuum bleeding. Vacuum bleeders usually utilize shop air pressure and a venturi principle to suck fluid from the bleeder. This method has drawbacks because it can allow air to be sucked from around the thread of the bleed screw which creates more problems.
In Summary
All methods have their good and bad points. Some methods work better on certain models than others. Sometimes manual bleeding doesn’t remove a stubborn pocket of air, so you’ll need to use a pressure bleeder. Whichever method you choose to bleed the wheels, always bench bleed the master cylinder first, and don’t forget to fit the bleed screw caps on all bleeders when you finish the job. That way the next time the car is in for maintenance the bleed screws will be free and clean.
Bleeding the individual brakes in the correct sequence is crucial to remove all the air from the system. The sequence varies from one vehicle to another, so always refer to the bleeding sequence specified by your vehicle manufacturer.
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