Yes and here is why. You can see in the diagram the reservoirs have two holes. fluid is all around the piston. the second hole sends back into the master reservoirs when the primary push is moving along.
Normally you would have a separate piston as in the first is pushing the second springs. to allow for the second to keep pushing when the first is hard pressure.
Here is a video showing how it works
How It Works...and here is a cutaway diagram
so yes totally normal and expected.
So if you block the fronts and can push the rears to the floor either you have air, a leak or not enough fluid going the the wheel cylinders (a to small of a master bore).
