In the haynes manaul, it shows them pulling the whole master cylinder...

Can I just suck out the fluid and take the resevoir off with the master cylinder still there? I need to replace the o-ring at the bottom of it because its leaking

 

Yes. You should be able to. And I would think by keeping the remaining fluid in the cylinder you're probably not allowing any air into the system. Look for a small screw underneath the cylinder that helps hold the reservoir on.

Did you buy the new rings at Mazda? Damned expensive $8 for little rubber o-rings! The parts guy apologized for having to charge so much.

 

The screw is only on the S5 versions, the S4 versions don't have the screw.

When I pull the res (but not the MC), I just syran wrap the filler hole, then re cap (so there is an air tight seal). Then pull it up and drop it into a small bucket or waste basket that put right next to the MC to catch the res.

Then you can lift the res, over the fender in the waste can without worrying about dropping any fluid on the fender.