Master cylinder

[montmil]

Dot 4 is all I'll use in my three Airheads.

[Cliff A.]

Great explanation.
DOT 4 sounds good.

Thanks to everyone

[Bob_Roller]

I tried the original DOT 5 silicone brake fluid in the mid '80's on my '79 Yamaha XS400, the seals started leaking within a month .

So, I've stayed away from it, never have seen DOT 5.1 before .

I've used Castrol LMA GT4 fluid for decades, until  three years ago, can't find it locally anymore, so I use Valvoline semi-synthetic DOT 4 fluid in all the bikes now .

[Barry]

.....never have seen DOT 5.1 before .

It's common and stocked just about everywhere in the UK which is why I tried it.

.....so I use Valvoline semi-synthetic DOT 4 fluid .

More confusing labelling from Valvoline.  I've no doubt that there are minor variations in the latest DOT 4 fluids but essentially all brake fluid is synthetic - they only recently started adding it to the label for marketing reasons.  

[Bob_Roller]

I'll have to check the brake fluid at the local BMW dealer the next time I go in there .

[mrclubike]

What about Pentosin
I think it is made in Germany.
http://www.pentosin.net/f_brakefluid.asp  

[Cliff A.]

The owner of the local BMW dealership was the one who recommended ether 4 or 5.1 no mention of brand.