Caution when painting brake calipers

So me and a buddy of mine who is a mechanic decided we want to paint my brake calipers. Instead of just taping everything we decided we wanted to take them off and paint them. All in all it was pretty easy take them off, clean them, paint, and then let dry. After they were dry we began putting them back on, no problems, until we started to bleed the brakes... the car would not keep a pressure build at all.... it would build pressure and then it would loose it after a few seconds. We bled the brakes numerous times, we even gravity drained the system to be 100% sure that there was no air in the lines. We then took everything back off and made sure the calipers were put back together correctly. In the end we called my dad and he asked me if the calipers were put back on the same side. I was all yeah I'm pretty sure we did cuz they went on with no problems and there was no sign of it being put on incorrectly. Well in the end that was the problem, we ran out of other options and decided to try it. Of course it worked... So in the end of this I learned to do one caliper at a time, or to at least be sure they don't get switched in the process....

I know this is a long story but in the end it may save numerous ppl this hassle. The thing I don't understand is on his civic we swaped the calipers and it worked fine....

 

the only marking i saw on it was a 4 and a M, and on one side its right side up and the other its not... so that didn't make sense either.