Brake Fluid

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Why Flush your Brake and Clutch Fluid?

Brake fluid is hygroscopic (absorbs water) by design in order to minimize corrosion of the metallic parts within the hydraulic system.  Small amounts of water enter through the reservoir cap air vent.  Changing the fluid every two years ensures that the brake fluid does not become overly saturated with water, and that any water present does not cause brake fade by pushing fluid out of the system when boiling occurs in the presence of high temperatures.  Flushing the system is a fairly involved job and may best be deferred to your local dealer or independent mechanic if you are unwilling to spend the time, or acquire the tools to do the job right.  The cost to have the job done is typically around $50, about the same as buying or making consumer-level equipment yourself.

How To Do The Job

In essence, the brake and clutch hydraulic systems are flushed by applying air or clean brake fluid under pressure to the top of the brake (and clutch) fluid reservoirs.  3 to 10 psi is sufficient to push fluid slowly through the system and out the wheel cylinder bleed valves, during which time the reservoir must not be allowed to run dry.  (A vacuum-type method would be effective but you would need to have someone keep the reservoir full for you.  Otherwise it's no easier to use than the pressure method.)  At no time should you press the brake or clutch pedal during this procedure.

It is sensible to drain out the reservoirs and refilling with clean fluid before starting, therefore minimizing the volume of old fluid that must be pushed through the system.

You can buy a kit with all the parts you need except a regulated air pressure source - I've believe the "Ultimate Garage" has such a kit.  There are also numerous descriptions on the internet on how to make your own.

  Brake fluid reservoir with level sensor integrated into cap

  Clutch fluid reservoir

Force at least 400 ml through each rear brake and at least 200 ml through each front brake.  There would normally be no need to actually bleed the brakes after this operation, as there should be no air introduced.   Be sure to use the fluid specified in your owner's manual, DOT 4 for the 530i.   Castrol make a premium fluid of this specification.

There does not seem to be a practical way to specifically flush the ABS portion of the system, nor is there any indication that it is needed.  Try to get the ABS to activate once or twice a year to move a bit of fluid through it.

Making a Pressure Bleed Kit

Find a 500 to 2000 ml container that you can pressurize safely to 10 psi, clean easily, and incorporate two copper tubes into the lid or side.  I used a stainless-steel and plastic container readily available at Target, but it doesn't hold more than 3 psi.  A more suitable container may be found at a sports store for containing fuels for hikers. My latest "find" is a spray gun tank (Binks style) from the swap meet, the most suitable container yet. (photos will be here sometime in Dec)

  Fit and seal two 3/16" dia by 2" long copper tubes into the lid or upper side with epoxy.  On one of the tubes (call it Tube "B") attach a vinyl tube to the lower end, long enough to touch the bottom of the container.

  Cut a 1.665" diameter disk from .020-.025" thick brass shim stock and solder one or two 3/16" diameter copper tubes into it.  Attach one via a 4 ft. length of 1/8" ID vinyl tube to the upper end of Tube "B" above.  The second copper tube is optional and needs to be closed-off if installed.  I used a tire valve as shown.  This disk replaces the fluid level sensor within the original reservoir cap.  Be sure to use the rubber gasket also.

  You will need a source of pressure-regulated air.  I used a small air pump purchased from an industrial surplus shop.  Connect this to Tube "A" on the tank above.

Fill the container with 500 ml of DOT 4 brake fluid and close the lid.  When the air supply is connected, the fluid is forced up Tube "B" towards and into the brake reservoir.  Be sure to avoid running the brake reservoir dry during flushing.

When you are finished flushing, de-pressurize the tank before removing the reservoir cap.