Originally Posted by worf928

The WSM procedure for the 2-year scheduled brake fluid replacement does not require the PIWIS. As for the ABS pump, certainly some amount of 'old' fluid will remain after a flush, but how much all depends upon the internals of the pump and how it flows when not doing anything.

The WSM section (470107) for flushing the brake fluid specifies that the PIWIS is only required in the context of replacing the ABS pump. Thus, I opine that the PIWIS procedure is used to ensure that all air is flushed from a new pump.

Keep in mind that modern ABS pumps have two purposes: one is to relieve fluid pressure (as per the original purpose of ABS) and the other is to selectively increase pressure to one or more corners for stability control. Under the vast majority of on-public-road circumstances the ABS pump is doing nothing. If you've done nothing that would have activated PSM or the anti-lock brake functionality then I wouldn't worry about non-LV having gotten into any 'cavity' of the pump that won't be flushed with the basic procedure. However, I didn't stay at a Holiday Inn Express last night and I am not factory-trained (this assumes that that 'deep' level of understanding of the system is covered in training, which is not certain....)


There is a specific caution that non-LV DOT4 should not be used to top-up a system that requires LV. If the 991 clutch system didn't 'circulate' fluid - as it appears to do - then I think you'd be right about the mix.


Two flushes is probably a good idea for a reason I will present below.


If you have easy access to the PIWIS then I would definitely use it to completely flush the brake system.

After that, based upon what I think the 991's clutch hydraulics do, if you pump the clutch a bunch (say 20 times) you should circulate all the fluid from the clutch master, slave and lines back to the reservoir.

Then do a second flush of the brakes and reservoir. That should get you to 99-ish percent LV.

After looking at the WSM, I'm mildly convinced that the clutch hydraulics are 'self-flushing' and that actually bleeding the clutch isn't something that needs to be done on a schedule as long as brakes are done on schedule.

However, in your case, doing something to get the non-LV fluid out of the clutch is, I think, a good idea.


More-or-less what I wrote above. Alternatively, do a two-man flush. Make your left leg tired on the clutch and then do a second two-man flush. That way you stand a good chance of getting it all done with only one C/L wheel R&R.



I can't imagine what a dealership would do in a situation like this. Regardless of what they would want to do, or what the Mothership might recommend, I too want the dealer (anyone other than me really) to touch my Porsches as little as possible. This is why I'm slowly, but steadily accumulating WSM sections and 9[98]1-specific tools.

Good luck with this and post back on the outcome.

Originally Posted by AdamSanta85

Interesting speculation in this thread, but curious as to why wouldn't be more widespread, considering 991's have been out for around 8 years now, and numerous brake fluid flushes have been done with Endless, Motul, Castrol SRF etc with no reports ill effects.

Originally Posted by worf928

Exactly. Which is why I’m not yet convinced that the non-LV fluid is the cause of these BMC ’squeakies.’

But, it does seem pretty clear that Porsches after 2004 are supposed to use the LV fluid.

What isn’t clear is 1) the rationale for why a “little bit” of non-LV is verboten in LV systems, 2) the lifecycle effect of a little, or a lot, of non-LV fluid on hydraulic components that are expected to be used with LV fluid and 3) why do we have this LV fluid anyway? What’s it good for?

Originally Posted by maverick1715

Hi! I have the exact same problem and exact car, did brake flush with ATE 200. Took it back to my shop, did flush like 4 times, we did go through almost 5 Liters of ATE 200. Brake feels good, performance is awesome, but squeaks like cleaning the windows! Did you solve the problem without having to replace the master cylinder? Replacing the master cylinder sounds extreme since the problem started immediately after the brake flush.

Originally Posted by maverick1715

UPDATE on the squeak:

Pentosin LV solved the problem. One simple flush with Pentosin LV solved completely the squeak, doesn’t squeak on low speeds or high speeds. The annoying squeak is finally completely gone without the need of replacement of booster or cylinder. ATE fluid also comes in Low Viscosity and it’s called ATE SL.6. However at this point I wouldn’t use anything else than Pentosin DOT4 Low Viscocity and Ladies and Gentlemen I rest my case.

Originally Posted by maverick1715

UPDATE on the squeak:

Pentosin LV solved the problem. One simple flush with Pentosin LV solved completely the squeak, doesn’t squeak on low speeds or high speeds. The annoying squeak is finally completely gone without the need of replacement of booster or cylinder. ATE fluid also comes in Low Viscosity and it’s called ATE SL.6. However at this point I wouldn’t use anything else than Pentosin DOT4 Low Viscocity and Ladies and Gentlemen I rest my case.

Originally Posted by ddolbi

Maverick, thank you for update. I also just finished the flush with Pentosin LV and that faint squeak is completely gone. I saw quite a bit of bubble so I think tech didn't do a great job last time. I used 2 and half bottle to make sure complete flush.

Originally Posted by maverick1715

Nice job! This community is so amazing and always there to help. I’m thankful to everyone. I used exactly 3L using the PIWIS which goes through fluid like crazy, I didn’t flush the clutch but pumped it a lot in between the brake cycles. I’m sure that I got out 99% or more of the “old” fluid, pedal feels very responsive again, without noise. I also realized how very few people and mechanics know about low viscosity dot4. Thankfully passionate Porsche drivers on rennlist do know.