I'm wondering if the master cylinder is plumbed correctly.

Does this have a modern, dual-circuit master cylinder or the original single outlet??

If it's a dual circuit, it almost sounds like you have the rear brakes plumbed to the front circuit, and the front brakes plumbed to the rear circuit.

This would cause the issues you're having as the front circuit has a residual pressure valve built-in that not only holds 3lbs of pressure in the line, but activates a split second AFTER the rear.

If you have the rears hooked up to the front circuit, this will cause them to both drag, but activate at the wrong time as well.