You've got slack/play in the pushrod, that's good.
My first guess is a flexible line collapsing and acting like a check valve, I know you said they are new, but new is not always perfect.
Does this happen to both front brakes, or only one? If both then my line theory is less likely and you're back to the MC as the primary culprit. Disassemble it to see and or exchange it.
Present your troubleshooting method to NAPA, if it seems logical and points to the new MC then ask what they have to offer.
Grigg
