I had this same issue with them but i was trying their power brake set up. (which uses corvette master bylinder and booster) No matter how much i tried I could not get them working well. Are you using the ssbc master that comes with it? I ran their 4 wheel disc manual set for a while and their master is a 1 1/32 bore.

They worked well but i had to really really really stand on the brakes to get them to stop. I am currently running that same master with a dual diaphram booster and happy with it, but i am thinking of going to the 15/16 bore like dr diff indicated.