did you have any leaks at the calipers?? confirm there are no leaks before proceeding. did you bench blled the master before installation? if not, then system is air bound and can be difficult to remove all the air.
Sounds like there is still air in the system. Some newer cars with ABS, are very hard to manually bleed. It may require pressure or vaccum bleeding, some models even need a scan tool to put into a bleed mode.