SRaouf

As my system is completely drained Im debating weather I should switch to DOT 5 which is a silicone based brake fluid, as its basically fill and forget and never needs changing. Also doesnt rot seals or damage paintwork and unlike DOT4 is non hygroscopic so doesnt absorb moisture.. Downside is it is a lot more expensive and doesnt mix with DOT 4. Ive had it in my Healey for around 15 years with no problems and was wondering if anyone has tried it in a 928?
Sam 85S2

davek9

It still needs to be changed, else it will turn to a gel if left sitting (I've seen it
Never used it in a Porsche / 928, but did in the past on other stored cars, it's mostly used in museum type stored cars IIRC.

Dave K

davek9

And, yes it was a standard fill in Harley's, however they no longer use it in the new bikes (same reason, it will gel).

Hey_Allen

The other problem with DOT-5 fluid is that any moisture that gets into the system will puddle in the low points, instead of dispersing in the fluid.
This can lead to rust in the lines or calipers, depending on where it gets trapped.

FredR

DOT 5 brake fluid [silicon based] is not compatible with systems designed for DOT3/4.

Mrmerlin

we had a customer that tried DOT 5 on their 1983 928s.
Then after a few months of sitting
the brakes had locked up after he had driven the car 2 miles
the pedal had gotten spongy the MC would not return fully
The rubber flex lines also appeared to have swollen
The calipers appeared to stick or drag
We had to replace every rubber component in the system

I would suggest that you only use the ATE brake fluid in the 928,
its readily available from our vendors or online sources
Change it every 2 years and you should be good

dr bob

There are enough horror stories on cars that were retro-filled with Silicone DOT 5 that it's not worth the risk IMHO. The failure mode is severe, and the recovery method is to replace everything in the system Everything.

As Stan recommends, use the ATE fluid, swap it regularly (2 years max) and you'll be golden (or blue if you can find it).

There are plenty of good common brake fluids that work fine in the 928. I used Castrol LMA DOT4 for decades in everything. Last 5 years I've been using Valvoline synthetic DOT4 due to limited local availability with ATE and Castrol. Any name-brand DOT4, changed at least once every couple years (for street cars), will get the job done well. HDPE & track days might shorten the interval to less than a year, and maybe every event weekend just to be sure.

Petza914

I use Motul RBF600 in all my vehicles, which I believe is a DOT 4. I use it in some like the supercharged 997 to have the extra safety window of the higher boiling point for spirited driving, and in others (even my pickup) because once you're using it in a few cars, it's just easier to stock one fluid in the garage and every spring fill up the Motive Bleeder reservoir and hit all the cars one after the other to renew the fluid. I do use it in my 928 as well. The other benefit is that if you ever get a sticking caliper or an improperly adjusted parking brake assembly, the higher boiling point will keep the brakes from failing when you go to use them and then correct the cause of the issue.

SRaouf

OK thanks all will stick with DOT4 as better safe than sorry. As I said I've had DOT5 in my other car for over 15 years with no problems whatsoever, but the brakes and clutch in that one are far simpler and every component was new when I rebuilt it so there was no potential for cross contamination with DOT4 (which I suspect is the cause of at least some of the horror stories above)

FredR

It seems even small amounts of cross contamination can cause the DOT5 to gel up.

Jim Devine

Don't know if its been corrected (if possible) since but it was next to impossible to get a good pedal with silicone. Way back in the CanAm days, our car owner was given a 5 gallon can of silicone fluid by Dow Corning. We took it out after qualifing & replaced it with Castrol LMA. Driver had no confidence in the pedal. Aparently at that time silicone had microscopic bubbles that were almost impossible to bleed out.

dr bob

Amen! Early attempts to use silicone fluid in my LeGrand sports racer yielded similar results. We didn't have a braking problem by the time we built the second-gen car but tried it anyway because the Big Guys were trying in. This was early 1980's as we dipped our toes into carbon brakes and the extreme temps they generated. Anyway, we vacuumed the fluid in the container for a day to draw the bubbles out, then did a vacuum fill on the system so no air was introduced while pouring the fluid into the reservoir. Fluid is drawn in from the caliper end towards the reservoir, filling each circuit one by one. There was no actual push or pressure bleeding done in the conventional sense. Way different from what we were were used to.

928 Content: The same vacuum-filling technique was successful at eliminating a few very stubborn air pockets in my S4. I somehow managed to introduce air into the system during an annual flush-and-replace exercise. Conventional and pressure bleeding restored most of the brake pedal feel, but it just didn't have that nice firm, confidence-building feel hat the car arrived with. After reading about a similar problem in Nicole's S4, she mentioned that the solution came with vacuum bleeding at a shop in the bay area. Her car was off line for a year or more as she and the NorCal group tried different methods, but none worked well enough for her to feel safe driving it. The whole flush-and-vacuum-fill effort for me was part of a whole system refresh that lasted a few days overall with paint drying times included. The fluid fill part was a few hours all in, doing it single-handed.

Mrmerlin

FWIW a lot of bleeding issues in the S 4 happen because air gets into the ABS unit
If this unit is not bled after the MC is done then you will usually trap air in the unit
I have had no issues with bleeding if I crack all of the B nuts on the ABS unit and push fluid out till no more bubbles show at the B nuts

smokeygt

I too have used silicone in my older cars for +15 years with no problems, but, admittedly, they are simple systems compared to modern systems.

siscogts

I use 5.1 brake fluid, it is not silicone based, it is a simple evolution of dot 4.it Is more heat resistant and it is good for people using car also for random track days...