You should have a overflow bottle. You have a closed or reservoir cooling system. The radiator cap includes a second sealing gasket under the shell, which rests on the upper sealing seat of the filler neck. The normal overflow hose from the filler neck runs into the reservoir tank and extends to the bottem below the normal liquid level.
When the engine is started and begins to warm up, the coolant expands. As it expands, it is forced out through the pressure valve of the cap, through the overflow tub and into the reservoir.
When the engine is turned off and begins to cool down, a partial vacuum is created in the radiator by contracting coolant. The special upper sealing gasket in the pressure cap then allows this vacuum to draw coolant back into the radiator from the reservoir through the overflow tube.
Because of this back and forth movement of the coolant, practically all the air is eliminated from the system. This guarantees that the engine block, heater and radiator are always completely full of coolant, therby increasing the efficiency of the system.
Generally on a closed systems, coolant is added as needed only to the reservoir tank, instead of through the radiator filler neck. Because of this, car manufacturers have recommended that the radiator pressure caps on these systems not be routinely removed.
I hope this helps you understand how it should work.
