TOPIC:

Can Dot 3 brake fluid be used in a modern master cylinder that uses Dot 4?
I'm looking to replace the mc, and the Nissin master cylinders on some late model Hondas specify Dot 4.
Dot 4 has a higher boiling point, but other than that, is there a reason why I shouldn't use Dot 3? Thanks.

.... note to self ... check the msds

Simple answer is YES.
Dot 3, 4 and 5.1 are all Glycol based and can be mixed or changed without flushing the system as the only difference is their boiling points.
It is DOT 5 you cannot use or mix in dot 3, 4 or 5.1 systems unless the entire system has been flushed of the old stuff as it is silicone based and does not mix with the Glycol based fluids.

Thanks. I was thinking that maybe dot 4 master cylinders used different rubber compounds in the piston kits.

I read that the mfgs say you can top off dot 4 with dot 3 in an emergency type situation, but you were then supposed to flush the system with dot 4 after fixing the problem. Other than the boiling point & the amount of water absorption, I couldn't find any other reason for dot 4.

DOT 3 and 4 are interchangeable. Mix and match no problem. Since 4 has a higher boiling point there is no reason to use 3 anymore. That's most likely why Honda specs the 4.

Something I didn't notice before is that some Honda mc caps say "use Dot 3 or Dot 4" (ie, use either type in the same mc). Most of them I've seen say to use one or the other, but not both. .... Honda Rebel. dot 3 or 4

just some sample oem Fr Br master cylinders from various models. Single disk Fr Br. 1/2. Nissin. mirror orifice.
not really sure how appropriate any of them would be for a KZ650.

Suzuki DR200 .... .... Ninja 250 .... .... Honda CBR500 .... .... Honda Shadow

Honda CBR500 ... Honda oem brake lever new