Now that this old thread has already been reinstated, so I take a chance to reply to such specific problems:
Before replacing the radiator cap, the connection (pipes) of side bottle cap must also be checked. If the side bottle cap pipes are switched, it will surely leave the bottle full and radiator empty.
Radiator cap functions as a valve so when there is high pressure inside the radiator, the cap releases some vapors into the side bottle, and later on when pressure goes down inside, it sucks back that water into the radiator so that it is filled again.
The side bottle cap has 2 pipe connections:
1) IN: which lets the water coming from radiator into the bottle. This should be connect to radiator and MUST HAVE A SMALL PIECE OF PIPE on the inner side which goes down into the bottle. THIS IS VERY IMPORTANT!!
2) OUT: This is overflow from the bottle. The over-flow pipe should be connected here.
Now, you can imagine if the pipes are switched on side bottle cap (IN and OUT wrongly connected): when its high pressure condition, the water coming from radiator will fill the side bottle and then evaporate excess vapors. And, when pressure will go down and radiator will try to suck back the water, it will not be able to suck water back because in overflow port there is no pipe going into water inside bottle.
Perhaps too complicated to explain - but its a very simple reason.