Rear brakes' allen key is stripped!

Went to change my rear brakes yesterday. Removed the 14mm bolt that covers the allen key that controls the piston. Unsrewed the allen key, it was tough, and when the key came out half of the groves were stripped off! This is probably the first time in the car's existence that i had to change the rear brakes so i think the allen key is just old. Anyways, this now means that i cannot rotate the allen key to get the piston back so i can put on my new pads.

My question is:
Can i buy a new allen key for the rear brake piston? (The old allen key is partially stripped so this means there are pieces of it still wedged inside the brake piston so a new key might be pointless)
Because the allen key broke inside of the caliper assembly does this mean it's time to buy a whole new assembly should i not be able to work the piston?

Sadly, I do believe a new (rebuilt) caliper is going to be your only option.

Yeah it seems that way but i read on some forums that people were able to rotate the actual piston via clamps but this means i cant adjust the piston once the new pads are in.

You should be able to pull the caliper off and re-tap the hole and blow in the line to get the fragments out then get a new set screw

I would try to retap the groves but i think they're pretty well lodged in there. oh well i guess i'll try something before buying new calipers