Squeaky clutch pedal and how to avoid it

Hi

My car's clutch has developed a squeak about a month ago, 6 months after changing brake fluid from Castrol Response Super DOT4 to Motul 5.1 (DOT5.1). I used Super DOT4 on both cars for the last 7 years and never had an issue, silent and smooth clutches, firm brake response, the fluid never boiled. Note that I use kevlar pads and managed to boil stock DOT4 fluid by repeated braking on autobahn (lorries unexpectedly pulling out and blocking my lane).

The squeak was faint initially and only happening on a cold engine, then started getting worse, to the point that it could be heard in the cabin rather than just felt, and the clutch pedal squeaked on both cold and hot engine.

Squeaky clutch pedal is a very well known Audi / VW issue that happens especially after using ATE200 or SuperBlue DOT5.1 brake fluid. Basically, after a few months clutch master cylinder seals starts squeaking when the clutch is operated. To avoid it, just do not use ATE200 fluid. Simple, isn't it? ... well, not quite.

I first tried flushing with more Motul 5.1 as I had a spare bottle, the squeak stayed the same. So today, I flushed the whole brake/clutch system with 1.6l total of Castrol React Performance DOT4 (formerly Response Super DOT4). The sound immediately got quieter and then went away as soon as engine warmed up. Hopefully it stays quiet from now on.

Afterwards, I looked a bit into the specs of each fluid, and it turns out that most "performance" and DOT5.1 fluids have much lower viscosity, about 30%-50% lower than Super DOT4 and up to 3 times lower than DOT4 max. allowed viscosity. Of course, VW did not help by putting in plastic piston in clutch master cylinder :wonder: .

So, if like myself you need to use brake fluid with higher boiling point, use Castrol React Performance DOT4, and do not use any DOT5.1 fluids. Even with stock DOT4 fluid, if your clutch pedal develops a squeak, change brake fluid first and use one with high viscosity (at least 1200mm²/s).

Funnily enough, looking at Castrol website, that means that React Performance DOT4 is OK, React SRF is OK (if you plan to sell your car to afford it :p ), so is ordinary Castrol Brake Fluid DOT4 and probably most other DOT4 fluids that do not advertise high boiling point or other "special" properties. But React Low Temp DOT4 is very likely to squeak as it has half viscosity of React Performance.

I also checked Motul specs, and RBF600 and RBF660 both look squeak-proof (very thick, on verge of DOT4 limit, not sure if it is good for ABS unit). But avoid Motul 5.1 as it does cause clutch squeak.

I hope this helps someone before they end up with 2 fluid changes in 1 year, like myself.

Edited 8 October, 201114 yr by dieselV6