BRAKES (and clutch):
The brake system is a very important system that requires regular maintenance. For the 928, Porsche recommends using a high-quality DOT4 brake fluid. The reservoir is located just in front of the firewall on the driver-side. You should always keep and eye on the fluid. As brake pads wear, the brake pistons will fill with brake fluid and push out to fill the gap. So as the pads wear, your fluid goes down. Most mechanics will recommend (as does Porsche) you replace your brake fluid every 2-years. This is because brake fluid is hydroscopic.
In a recent conversation with Chris Braden from Munk’s Motors, I learned that as a general rule of thumb, brake fluid generally absorbs about 0.75% of its content in water each year. So in 2 years, your brake fluid can have well over 1.5% water content.
You might ask, “Why is this such a big deal?” I’m glad you asked.
As you probably know, fluids generally do not compress – this is why they make an excellent medium to transfer mechanical work through a hydraulic system. However, in the case of brakes, because there is a tremendous amount of heat built up from the friction between the pads and the rotors, the heat is transferred into the brake fluid.
As the moisture content goes up in the brake fluid, the boiling point decreases. If the fluid reaches its boiling point, it will transform into a gas/vapor. Gasses are compressible, so if the fluid boils the result is your brake pedal falls to the floor, and you have no brakes! Yikes!
Moisture can also cause other problems like rust and corrosion on internal parts like the master cylinder or the caliper pistons. Corrosion can then cause other problems like premature wear on seals and the like.
On my track car, I needed to do a complete brake fluid flush – it’s part of my annual maintenance. I opted to use a “One-man brake bleeder” that you can generally get from any auto-parts store for about $5. This make it easy to do this job all by yourself. Be sure to get a high quality brake fluid. I generally use ATE TYP200, but there are others out there. ATE used to sell a blue-colored fluid, which made flushing easy, but the US DOT outlawed it recently. Luckily I had blue fluid in the track car, so I could easily tell when I had fresh fluid.
I started by hooking up the brake-bleeder to the nipple on the master cylinder. Cracking the nipple allowed me to slowly pump the brake pedal about 7 times to fill the small bottle. I did this process 4 times, and then I started to get air. I refilled the reservoir and bled the master cylinder once more for good measure to make sure there was no trapped air. I then topped up the reservoir again.
I then moved to the driver-side caliper. There are two bleed nipples. I started with the outside bleed nipple. I hooked up the brake-bleeder to it, and just cracked open the nipple – about ¼ turn. I found that on the front brakes, 6-pumps of the brake pedal would fill the little bottle. It took about two full bottles to get clear fluid on the outside bleeder. Then I would transition to the inboard bleeder. This only took one additional bottle.
This worked out great. Each caliper took 3 bottles to fully bleed the caliper. - two on the outside bleeder and one on the inside. The best part was that 3 bottles was safe for the reservoir (i.e. no danger of running out of fluid and pumping air into the system). So after each caliper, I replenished the reservoir.
After the driver-side front caliper, I moved to the passenger-side front caliper. Another three bottles. Then I did the driver-side rear, then the passenger rear. I found that when bleeding the rear calipers, the bottle could take more pumps of the pedal. I think this is because the rear calipers are on a different circuit and the pedal travel is less for this circuit – so less volume per pump. 8 pumps seemed to do the trick.
Once the rears were done, I moved to the clutch. It’s the same process. On the lower bell-housing, you will see the slave cylinder for a 928. Hook up the brake bleeder, crack the nipple, and pump the clutch pedal. The only trick is that the clutch pedal will not return on its own with the bleeder open. I suggest you pump it by hand so you can pull it back up. I filled about 4 bottles when I bled the clutch.
Now that the system is fully bled, it’s time to check for leaks. Pump the brake pedal and the clutch pedal several times to be sure there are no leaks and that the brake pedal is firm. If everything is good, take it out for a test drive around your block and make sure everything is operating properly.
If you’re not able to take this maintenance on yourself, be sure to bring it to a qualified mechanic. Being able to stop is just as important – maybe more important – that the ability to go. During panic stops, stop-and-go traffic, and other heavy braking situations, your brakes are subjected to a tremendous about of heat and abuse. So it’s important to pay attention to them every so often.
In the next installment, I will discuss transmission fluid for both the automatic and 5-speed transmissions on a 928.
The 928 crew has a monthly beer night on the second Wednesday of each month from 7 PM to 10 PM at Sneakers Pub located at 22628 Woodward Ave., Ferndale, MI 48220. Everyone is welcome to join. If you would like to get on the 928 mailing list, drop me a line at firstname.lastname@example.org or call/text me at 734-837-7908.
About the author: Andrew Olson
I'm a long-time 928 enthusiast. I like long walks on the beach and a soft shoulder to cry on... where's Brian?