Decided today was the day to install new brake pads on the front calipers.  I thought getting the cotter pin out would be the hardest part.  Ha ha, wrong.  The 5mm Allen head retaining pin will not budge and worse, it is rounded/stripped out to a point where my Allen head sockets and wrenches can't get a grip and slip too much.
So, I need ideas.  Yes, I soaked the pin head with  WD40 penetrating spray multiple times.  I've considered grinding down a larger Allen head socket and pounding it in the pin head but suspect it will eventually spin and not grip.  Should I try drilling it out?  ???