My piston for my front brake master cylinder is very sticky I took it apart and the o ring is very dry and manual says to put lubricant on the o ring and in the bore. The manual does not say what type I should use. I am afraid that if I use the wrong lube it could do something to the brake fluid.

 

You can lube it with the same brake fluid that you fill the cylinder with

 

Agree. You could immerse it in some fluid for a few minutes before refitting. But how old is your bike? If you have it apart how about fitting new seals? Will give it a fresh life.

 

Like the others said use brake fluid.You dont want to take a chance contaminating the fluid with something else.

 

I end up buying the rebuilt kit from Harley and it came with some type of lube that was black and oily. When I bleed the brakes I saw that oil mix in with the brake fluid I am sure it is ok it is what came with the kit. At least I hope so!

 

You should be fine! I would bleed fresh fluid through until it comes clean, but that is me.