Out with old, in with the new: proper brake fluid renewing procedure.March 16, 2020
Whenever the hydraulic part of a braking system is invaded, air will be introduced, and it will have to be bled out (removed). Though at first, the task seems simple, some common mistakes end up creating other issues. The information provided here applies to any hydraulic braking system.
It is not commonly recognized that even though a hydraulic brake system is considered closed, over time and thermal cycles, the fluid will absorb moisture. Even if it was never opened to the atmosphere.
This happens slowly, but the effects are the same; the formation of corrosion and acids that attack all the parts, ending in a system failure.
Every hydraulic brake system should be bled and refilled with fresh fluid to remove moisture. This is akin to changing the engine oil or transmission lubricant.
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 do not expose the circuit to air when taking the fluid out by actuating the brake pedal.
Then refill with fresh fluid.
The only caveat to this procedure is you will need to do it several times over a few months to exchange all 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 the fluid will end up being exchanged.
A brake system can be bled in several different ways: by using the master cylinder (pumping), introducing 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 by pushing it out with brake fluid.
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 brake 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, but is the most efficient.
A vacuum bleeder is the most practical approach since it is inexpensive, simple to use, and does not require two people.
Pumping the brake pedal is valid, but it requires two people and has the most chance of damaging the system.
Where things go wrong
Using old brake fluid: The chemistry of the fluid makes it very susceptible to wicking in moisture. Buy small cans of brake fluid and mark them with the date they are first opened and seal the cap with electrical tape.
The big can is only a buy if you use it quickly. Fluid on the shop 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.
Not using a mechanical stop: 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: This introduces dirt into the fluid, which acts as an abrasive and destroys the seals in the master cylinder, wheel cylinders, and calipers.
Using the wrong sequence: Begin with the wheel furthest from the master cylinder and work your way to the closest wheel.
Not watching the fluid level: Always keep the master cylinder full and the lid tight while bleeding. If it runs dry, your efforts are all in vain.
Not many farmers put renewing hydraulic brake or clutch master cylinder fluid on their PM list. These simple steps can ward off an expensive and inconvenient machinery breakdown.