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Swapping Out Brake Fluid

Whenever the hydraulic part of a braking system is invaded, air will be introduced and it will have to be bled out. Though at first the task seems simple, there are some basic mistakes that end up creating other issues.

It is not commonly recognized that even though a hydraulic brake system is considered closed, the fluid in the system, over time and thermal cycles, will absorb moisture, even if it was never open to atmosphere. This happens slowly, but the effects are the same. The formation of corrosion and acids will attack all of the brake parts, ending in a system failure. Every hydraulic brake system should be bled and refilled with fresh fluid to remove moisture. 

A simple method to accomplish this without bleeding is to suck most of the fluid out of the master cylinder with a pump or kitchen meat basting utensil. Make sure you don’t expose the circuit to air when taking the fluid out. Then refill with fresh fluid. The only caveat to this procedure is that you will need to do it a number of times over the course of a few months to exchange all of the brake fluid. The task is so easy that it can be performed when you do another service, such as an engine oil change. After two or three times, most – if not all – of the fluid will end up being exchanged.  

A brake system can also be bled a number of different ways  by using the master cylinder (pumping), introducing a vacuum, through gravity, or with pressure (pressurized brake fluid). The most common way is to have a helper pump the brake pedal (thus using the master cylinder) to evacuate the air. 

A vacuum pump, either handheld or otherwise, will suck fluid through the lines and components, pulling out air and creating a solid fluid mass.

A power bleeder connects to the master cylinder and feeds fresh fluid into the system, pushing out the air and old fluid. 

The best method (though impractical for a farm shop) is the power bleeder. It is an expensive tool, requires many adapters for the different master cylinders, and is awkward to use. 

A vacuum bleeder is the most practical approach since it is inexpensive, simple to use, and doesn’t require two people.

Pumping the brake pedal is effective, but it requires two people and also has the most chance of damaging the system. 

The chemistry of brake fluid makes it very susceptible to wicking in moisture. Buy small cans of fluid, mark them with the date they are first opened, and seal the cap with electrical tape. Buying a  big can of fluid is only a buy if you use it quickly. Fluid on a shelf for more than a year or with the lid not closed tightly should not be used. When moisture is introduced, it lowers the fluid’s boiling point and causes corrosion and pitting in the brake system, which leads to failure.

When using the brake pedal to bleed the system, place either a piece of wood or your other foot under the pedal to limit pedal travel. When the bleeder is opened, the pedal will sink. This allows the piston in the master cylinder to travel past the bore, injuring the plunger cup when it goes back in. Shortly afterward, the master cylinder fails due to the torn piston plunger cup.

Not cleaning the master cylinder cover or bleeder screws before opening introduces dirt into the fluid, which acts as an abrasive and destroys the seals in the master cylinder, wheel cylinders, and calipers.

When bleeding, begin with the wheel farthest from the master cylinder and work your way to the closest wheel.  

Finally, always keep the master cylinder full and the lid tight while bleeding. If it runs dry, your efforts are all in vain. 

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