I recently cracked a front master cylinder on a 300ex. I have a couple of older street bikes sitting in my shop. Can i use the master cylinder off the street bike on my 4 wheeler even though the cylinder is bigger and will hold more brake fluid?????

 

Using hydraulic principles, If the piston in the master cylinder is larger, it will take less stroke, and take more force on the brake handle to get the same amount of stopping power. If the cylinder is of a smaller diameter it will take futher movement of the handle, but less force on the handle to get the same stopping power.
I would at the least try the street bike Master cylinder, you might find you like it better.

 

Just make sure that they use the same fluid DOT 3 and DOT 4 are not compatable. If you mix them you're in for headaches.

 

I've done a little research on the subject; DOT 3 and DOT 4, are the most common grades of brake fluid (by the way DOT stands for Department of Transportation). And the difference between them is just the boiling point. DOT 4 has a higher boiling point. The higher boiling point prevents the brake fluid from boiling when the brakes are used excessively.
DOT3 DOT4
Dry boiling point 401F 446F
wet boiling point 284F 311F

 

Dot 3 & 4 are indeed compatiable.

Dot 5 is the one that is incompatiable! It's also very neat stuff for resisting fade at extream temps. But to use it you must bleed and flush out all taces of DOT 3 or 4.