vodkag

i wouldnt worry about it, test it at 2 year for water content. Honestly is an overkill the replace brake fluids every 2 years unless you car constantly tracking the car and live in a high humidity area. I have not touched my brake fluid in my Audi for 3 years. I test it every year and is always 0% water content

HenryPcar

It is not a closed system. The brake reservoir cap allow air to get in when the fluid level goes down due to wear on the brake pads. Under normal street driving you seldom ever boil the brake fluids even if the water content is high because the brakes never get hot enough to reach that point. Porsche being a sports car are quite often driven on the track or DE sessions that require repeated high speed braking and that's when the fluid could potentially boil due to heat and you will lose your brakes entirely. When the fluid boils, gas not like fluid, is not compressible and your brake pedal will feel spongy and totally ineffective.

Joec500

For piece of mind I always do it every 2-3 years/20k miles.

991carreradriver

I have found that the reservoir reading and fluid collected are identical. Having said that the tester I use on measure in full percentages not micro readings. I also find that once I see H20 on the tester it gets bad real fast.

Bob Z.

Clutch fluid every 2 years as well?! That is overkill.

991carreradriver

I doubt that there is a perceptible difference under normal or spirited driving conditions. I also think that the OEM brake fluid is not very good (lower boiling point) relative to Motul 660 or Castrol SRF. For example, when I first tracked my 2014 C2, I noticed a real difference in sessions as the brakes got really hot using OEM fluid. I don't notice that difference using racing fluids. The most important reason to change brake fluid every two years in a DD is to stop corrosion in the brake system from moisture not brake fade or failure.

991carreradriver

The system is not really closed. Every time you take off the reservoir cap, moisture from the atmosphere is introduced to the system. I suspect this is why newer cars use see through reservoirs to eliminate the need to open the cap to check fluid level.
Below is a chart which shows the average percentage of water in DOT 3 brake fluid as it ages:

titan7

MarkG123

I am the OP.....The local Porsche dealer quoted $260 for a brake flush. After reading everyone's reply I watched a youtube DIY brake flush and it looks simple. So I will make it a DIY. I will spend more time removing centerlock wheels than the brake flush. Not sure about water contamination but the flush is easy enough to remove this as a variable for me.

Bemo

My Indy does a brake flush for $50, that includes the fluid. Why the hell would I give Porsche a dime more of my hard earned money?
Not that I can't afford it but on principle alone, I'd never spend more than I can get away with.

DOUGLAP1

Lot's of interesting opinions.

The bottom line is that we replace brake fluid to maintain a low boiling point. A low boiling point is mostly needed for repeated heavy use of the brakes. Obviously this occurs on a race track, but it can also occur with a spirited drive in the mountains. Many years ago when I had a new 914, I was young and ignorant, and went about 6 years on the same brake fluid. All was fine until I went for a hard drive in the Carolina mountains. The brakes faded out, but I was able to slow it down enough and drive for a few miles without using the brakes to cool them down, and they came right back.

So, as a much older and hopefully wiser fellow, I change my fluid before every track event, and make sure any car I drive in the mountains has reasonably fresh fluid. For a single panic stop event, I don't think it is so critical, but it sure can't hurt.

The brake fluds we use now are not all equal either. I normally use Motul 660, but my race car prep shop keeps saying I should try this Castrol fluid that they say is good for a whole race season. It does have a very high wet boiling point of about 520 F. It is pretty expensive, as Pegasus sells it for about $ 70.00 a bottle, so this may be the closest thing there is to a lifetime fill fluid:

worf928

Reservoir and fluid is shared for the brakes and clutch. If the fluid in the reservoir and brake lines is 5% water after two years and change, then so is the fluid in the long line to and in the clutch slave cylinder.

It certainly is much more of a PITA on the 911s. On a 928 it adds about 4 minutes to a brake fluid flush.

That may be the bottom line for some and if you turn-over your Porsche(s) every few years then that's fine. For you. For the folks down the line not so much.

The water in the brake fluid does decrease the boiling point. But, it also enables rust.

My bottom line is that I want the brake and clutch components to not rust from the inside out. If you go 5-6 years between flushes or never flush your brakes, in about 10 years the probability of needing to replace a master cylinder, caliper pistons, bias valves, or the ABS pump gets way above zero.

It's not my opinion either. It's stuff we see on the 928 forum all the time. And not just with brakes.

928s starting in 1990 (and 911 C4s) had a computer-controlled hydraulically-activated limited-slip differential: PSD. That system used a S-Class Mercedes ABS pump, brake fluid, and a slave cylinder. Of course, since the system used brake fluid the maintenance interval between flushes was two years. But, since this system was very uncommon and required - until we reversed-engineered the process - the Dealer Diagnostic Tool to perform, and because folks thought the dealer was trying to rip them off with unnecessary service, it was almost never done. It was almost certainly never done by Indys. The result is that now, any of those PSD systems that were never flushed or only occasionally flushed are hunks of rust on the inside and don't work at all. It's about $9k in parts now or $2-$5k to rebuild.

In summary: if you are going to keep your Porsche and want the brakes to perform as-new in 10-15 years, if you don't flush your brake fluid when it needs to be flushed, you'll be replacing lots of components in the braking system.

CSK 911 C4S

Motive Power Bleeder

Makes bleeding brakes a one person job.

Ohio Performance

DBH

The brake system is not a totally closed system. It's designed to allow some air in. There is always some degradation/contamination building up in the brake fluid over time. Under normal (not racing/high performance) driving you can probably get away with letting the two year cycle slip a little (6 to 12 months?). However, in an emergency situation I'd want to know that my brakes are working at their very best. To me, it's not worth the trade off of saving a few dollars vs. saving a life...just sayin'.