drill the head away with a 4 or 5 mm drill (mesure one of the other screws)  - remove the cap - drown the piece in wd40 over nigth - and find a good tool to hold around the little tap and turn it out. (If it's one of the screws on the fluid cap  )

easy out

Carefully drill the head off. You should be able to get the cover off and then have a little bit of bolt left to get it out.

Never force anything. just get a bigger hammer.

phelonius

It sounds like a job for the mechanical repair contractor I used to see on the jobs I was working on...BF & I...brute force & ignorance...motto:  No job too small; no hammer too big.

Seriously, this is a tough job.  As said, you might be able to very carefully drill the head off the screw.  Soak the screw threads in a good penetrating oil, maybe Kroil or B'laster PB, and don't waste your time with WD40*.  After that Vicegrips might get the rest of the screw out.  With the brake fluid removed, heating the body of the unit might help...don't use a flame that will melt the aluminum.  Or, heat then cold shock with ice.  Anything to break loose the crud holding the screw in.  

I had an Easy-Out work once...once in 35 years.

In the future, be real picky about the tool used on the screws and bolts.  It might be work buying a set of JIS screwdrivers instead of Phillips, use only 6-point sockets and box end wrenches, and be sure any Allen wrench is in good condition...grind off the end if it's worn.

*WD40 is, as the name implies, the 40th recipe for a water displacing product.  It displaces water well.  It is poor for lubrication, penetration, and rust prevention.