I'm changing the master cylinder on my 2001 Fat Boy, and the stock requires DOT 5 brake fluid. Is the DOT 5 a requirement of the master cylinder or the brake caliper, or both? Or is it mandatory just because that's what was being used 15 years ago as the standard? I'm wondering if I will need to switch to DOT 4 since it's a newer design master cylinder.

 

If your system calls for DOT5, continue to use it after the change.

The only real cautions are (short form)

1. Don't mix DOT3/4 and DOT5.

2. Don't use DOT5 in an ABS system.

3. Don't switch from DOT3/4 to DOT5 w/o a thorough flush to remove any possible water deposits.

 

My rocker c use dot 4 in the front and rear brake system.
But I changed the handlebars elements in chrome with a chrome set aftermarket. In the instructions of new chrome master cylinder , say "use dot5". When I installed the new master cylinder, I used dot 5.
Master cylinder is who use the requirement of dot4 or dot 5.