Help needed -- what size tap?

I have managed to slightly damage the threads in my front right brake caliper so that the bolt/pin (whatever you call that thing) will only turn once or twice. I'm CERTAIN it's the thread, as I can see it. The bolt itself is fine, as I have no problem running either bolt into the bottom hole.

What I'd like to do is to run a tap into the hole to get past the damaged spot, but don't know what size tap to use. I bought a generic kit, but even the largest tap doesn't cut it (pun intended).

Thanks in advance if anyone can help.

The top of the bolt says 10.9 SPS, if that's worth anything.

Obviously, I don't know alot about bolts :D

Since you didn't mention the year of your car, I checked your profile which says 2000 Grand Prix.

The threads on the caliper slide bolt are M14 x 1.5.
The threads on the caliper bracket bolts are M14 x 2.