It’s probably the way your bleeding them. The system is not full or has traped air. If you have the flat tops with no fill plug on the masters then it’s best to bleed backwards. You should take the caps off and open the bleeder. Then force fluid into the bleeder untill clean air free fluid flows out of the master. I use a pressurized canister but something like a turkey baster works also. Anything that can force clean air free fluid thru the system will work.