Mixing Brake Fluid

E

E85

Member
 Ontario, Canada
I want to change the brake fluid in the Z4. I understand that you poor the fluid in the same reservoir for the brakes and the clutch right? But my main questions are: OEM brake fluid is DOT 4 right? I heard OEM fluid is actually Motul DOT 4 fluid, is that true? I want to do a flush, but as you it's hard to get it ALL out. So is it ok to mix OEM brake fluid with Motul RBF 600?

TIA
 
Well, Motul's own data sheets say they only fluids you must not mix it with are silicone/polyglycol/mineral ones.

As Motul Dot 4 fluid is 100% synthetic as is RBF 600 (and they both say that they're Dot4 compatible), then there shouldn't be a problem - although I'd still get as much of the Dot4 out of the system anyway as you'll probably want the benefit of the higher-performing fluid - if you only get half of the new fluid in then the resulting mix will perform somewhere between the two specs.
 
thanks for the advice. that's what i figured and yah, i will do my best to get it all out, but i have heard horror stories about mixing and something about the brake lines corroding and stuff like that

i'm not scientist, so don't know how mixing any 2 brake fluids can be bad as i'm sure there are many possible chemical reactions.

anyone know off hand how much brake fluid is in the car for the 4 brakes and the clutch? i used to be a honda guy and most of those are under 1L or 1 quart for all 4 brakes ... then maybe another 1/2 L or 1/2 quart for the clutch as it's 2 separate reservoirs in a honda.
 
Brake fluid needs to changed periodically for two reasons. The extreme temperatures it can be subjected to can degrade the fluid and reduce it's effectiveness. And, brake fluid is hygroscopic. It absorbs water. Over a long period of time, the absorbed water can cause the fluid to boil at a lower temperature and lead to corrosion of the internals of the brake system. In years gone by, that meant you bought or rebuilt master cylinders and wheel cylinders. No big deal. Today, you have to consider the cost of replacing your ABS system. If you are going to change out your brake fluid, get rid of all of the old fluid. I bet there are web based guides on how to do it properly.
 
TO//M: totally agree and will do. but is there anyway to ensure you got ALL the fluid?

as i said, will do my best to get rid of it all, but just wondering if there was any catestrophic things to worry about if i say, only got rid of 80% of the old fluid and the 2 were to mix.

mmm-five's explanation was what i was assuming in the first place since both are dot4 and probably both synthetic.
 
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