Does a Mercedes require special brake fluid? - Page 2 - PeachParts Mercedes-Benz Forum

BillGrissom · #16 09-06-2015, 01:59 AM

Re post #15, a few facts anyone can easily check:

DOT 5 is actually a rating and doesn't specifically mean "silicone". Recently some glycol fluids were able to meet the DOT 5 standards, and were required to be labelled "5.1" so as not to confuse with silicone, so now "5" sort of does mean silicone.

Many production motorcycles use DOT 5 (silicone) fluid. Probably because they use so little fluid that the bean counters allow it, and motorcycles are marketed as high-end sporty.

I don't think most racers use DOT 5. One can get slightly higher boiling points from brand-new glycol fluids (before they absorb any moisture) and racers don't mind installing new fluid before every race.

A non-internet fact. I have used DOT 5 in my 65 Newport since ~1998 or whenever I learned of it. I replaced the master cylinder a few years ago. It wasn't leaking, I just changed to a safer "dual system" MC (1967+ cars per feds). The MC inside looked brand new, without a hint of rust (recall changing to DOT 5 when I put that MC on). Indeed, I have seen no rust the few times I have bled DOT 5 fluid in any of my cars, unlike the rust gunk I have seen when I used glycol. It simply changes from blue to yellow over time, which is just the blue dye fading. But, if you are the type of guy that would let water drip into your MC, for sure don't consider silicone fluid.

RunningTooHot · #17 09-06-2015, 01:24 PM

Hi Bill,

We're actually in agreement. I specifically mentioned "DOT 5 / silicone", although I should have worded it more clearly by specifically excluding DOT 5.1. My mistake.

And yes, silicone fluids are (typically) no longer used in race cars for several reasons, one being that the compressibility & viscosity characteristics of DOT 5 silicone fluids leads to a loss of (nuanced) pedal feedback, and the higher (dry) boiling points with newer glycol based fluids is obviously advantageous. The other problem is that if a vapor bubble forms, it does not get reabsorbed, and getting bubbles out of a silicone fluid filled system can be a beeyatch. (Those are somewhat esoteric points that are not typically an issue in street cars however.)

For an enthusiastic & knowledgeable owner (like you) of a vintage car, where the owner wrenches on it himself AND never lets anyone else touch it, then silicone fluid can have advantages as you outlined. If, and that's IF someone properly converts to a silicone fluid (which few people do correctly) AND you diligently keep any water from entering the system, then silicone is a great product for those circumstances.

A problem arises when most people convert to silicone - they don't completely remove all traces of the old glycol fluid; they simply flush from glycol to silicone and think they're done. Doing it that way leaves traces of old glycol fluid in the system, and it (usually) already contains some water contamination... which, depending upon the amount left in the system, can eventually wreak havoc because a lot of people think that they'll never have to change the fluid again.

And the diligence in keeping water out of the system is critical no matter what fluid is used. The air bleed hole in the brake fluid reservoir cap is a good example. It is a necessary evil, and it does allow glycol to (eventually) absorb some atmospheric moisture. Question: How many times have you seen someone seal the reservoir cap when washing or steam cleaning their engine or engine compartment? Just a little saran-wrap and a rubber band prevents that issue, but how many people actually do that? I've NEVER seen the clowns at car washes taking preventative measures like that.

BillGrissom · #18 09-07-2015, 01:33 AM

I totally agree on first purging all glycol fluid. I blew air, ethanol, and air again to get it clean and dry, in all my cars I converted (all except 2 w/ ABS). Folklore says that any left-over glycol and silicone will chemically react or such, but not true no matter how many auto parts guys relate it as fact. You can find a web article by an admittedly anal engineer who tested that and other things in considering switching his classic car to silicone.

Excellent point on the MC reservoir vents. Even worse, most W123's I have seen (in junkyard) have degraded rubber "test caps" above the reservoir level sensors. They usually have splits which can let atmospheric moisture in or liquid water if you don't wrap the MC before spray cleaning the engine. You can buy new ones, but similar rubber that may fail in 10 years. I had some left-over port plugs (come on new hydraulic parts) that were soft and tough and fit. I super-glued them on.