Friday, March 26, 2010 - 07:17 am, by: Adam Lonergan(Alchemistal)
Boris, the standard (FMVSS116) does not actual specify that DOT 3 is to be polyethylene glycol based (don't believe everything you read on wikipedia or hear from a mechanic), although it *mostly* is as this is the most economical method. Even still your statement is incorrect and could mean you a mixing fluids which operate on two different chemistries. The same goes for mixing brands of DOT 3 fluid for that matter.
Another thing to consider is that the larger the DOT number the higher the DRY boiling point, which is not really that important in a daily driven car where the fluid only gets changed every year or less as the fluid will take in moisture. The WET boiling point is the important one as the boiling will drop by up to a third with the moisture that gets passed the seals. Goood/expensive DOT 3 will outperform poor/cheap DOT 4 within a few weeks/months in most if not all applications. The 'drop off' in performance in DOT 4 fluids tends to be worse than DOT 3 I am lead to believe also.
The viscosity of DOT 3 and DOT 4 and DOT 5.1 for that matter are also different and even small changes can lead to things like torn seals in systems that are not designed/capable to accommodate a change in viscosity.
The price of the fluids is the same (or VERY similar), the performance is so similar in street cars that I'd be amazed if even the most skilled driver could tell the difference in a blind test and all are easily available in every town/suburb in Australia so I can't see why you would run anything but what was recommended and designed for.
Personally never had a problem using dot 4 and 5.1 in cars which specified Dot 3. But if what you mention above is true, then the best choice is to use DOT 3.
Friday, March 26, 2010 - 09:57 pm, by: Damian Ware(Frozenpod)
Difference is there and can be noticed between using Castrol DOT 3 and super response DOT 4 (which is a DOT 5.1 fluid). CSR DOT 4 has better pedal feel and IMO is a great brake fluid except...
I have used CSR DOT 4 in my car for nearly 4 years but my master cylinder seals died in my TRC brake unit.
Bench testing alternative fluids indicated no change in seal size at ambient temperature between regular DOT 3 and CSR DOT 4, but apply heat and things change. DOT 3 and heat no problems with CSR DOT 4 and heat the seals died.
So no more CSR DOT 4 in my car.
I am planning to use Valvoline heavey duty DOT 3 next as it has a higher boiling point than regular DOT3 260 deg C and is the same cost as CSR DOT4.
The other alternative I have been looking at is silicon fluids higher temp but compatable. The only down side I can see from my research is the cost, about $300 depending on how much you waste when flushing.
Saturday, March 27, 2010 - 07:42 am, by: Dave Hart(Davyboy)
Is it the additives that they put into the polyethylene glycol that changes it from DOT3 through the other iterations? If so what is the additive(s) that damages the seals? Apart from engine bay heat, only the brake pistons should get hot so are their seals of a different composition?
Saturday, March 27, 2010 - 08:42 am, by: Adam Lonergan(Alchemistal)
Dave, I believe that DOT 4 is made to an American standard... AFAIK it MUST be polyethylene glycol based and contain borates (these are what lifts the DRY boiling point). Again AFAIK the additives are not part of the standard and vary from brand to brand, performing functions like corrosion suppression and possibly viscosity modifiers. Each company tends to keep its additives secret.