Today I replaced the brake fluid of my 928. It's been 6 years that I have owned this car now, and sure enough the fluid has never been replaced. The interval is 2 years, so I'm way behind on this. Its not difficult to do, takes about one hour. I bought 1Liter ATE DOT4 brake fluid, and it takes almost all of it to do the job. Braking performance did not change at all. Older brake fluid tends to boil earlier due to water absorption, which is not good under heavy braking.

It takes a short tube, a check valve, a 11mm wrench, an oil can with scale, large clean funnel, and 250ml DOT4 brake fluid per wheel.

Procedure is like this:
Jack the car up at the left-front wheel.
Take lid and grid off from the brake fluid reservoir
Open the sealed (!) brake fluid can (or bottle)
Put the funnel in the reservoir and top up with the new fluid
Take the wheel off
Take the rubber cap in the caliper off
Put the hose with check valve at one end on the relief valve
Use a 11mm wrench to turn the valve loose just one quarter turn
Put the check valve side of the hose in the oil-can and catch any fluid
Gently put the brake pedal down about 4-5 times. (no more! add fluid!)
Add fluid to top up the reservoir again. Never drain too much fluid at once.
Monitor the fluid that comes out, the check valve should prevent air returning
Repeat these steps until about 250ml is out.
Then tighten the valve again (one quarter turn approx)
Clean the area around the valve at the caliper thoroughly
Now push the pedal really hard and verify that the valve is closed
Re-mount the wheel again.
Repeat for other wheels.
Take a good note of the brake reservoir fluid level (remember level)
Drive the car, first gently while testing brakes
Then do a few really hard brake runs to verify operation
Now check the fluid level again to verify its not dropped suddenly

Any spoiled brake fluid on the pain needs to be removed immediately. Brake fluid ruins the paint pretty quickly. Clean thoroughly when this has happened.

Be aware that air bubbles re-entering the caliper will cause very spongy and poor brake performance. Also use *new* brake fluid, and not any leftovers. The brake fluid tends to absorb humidity and that lowers the boiling point of the fluid dramatically. In normal behavior this hardly noticeable, but under heavy braking it will make serious difference.

1988 928s4 cherry red
The Netherlands