The T-51b power armor is a special type of power armor that doesn't take off points from your agility, charisma, and other special skills.

The armor is, however, extremely expensive to repair.

The only way to repair it is through Traders. I'd recommend Lucky Harrith after you Upgrade his inventory at Canterbury Commons.

His repair skill will be high enough to repair the armor to 81%.

If you've killed Harrith, though, you can still go to anyone else and get it repaired to 50%.