Main Coolant hose from the overflow reservoir hose/pipe keeps sucking itself in.

The main coolant hose from the overflow reservoir is sucking itself in when the engine/radiator cools down and the car has been off for a while. As soon as the car heats up the hose regains normal shape and the car still drives fine. I have installed a new " Egr valve and thermostat myself. (Yes I am really 16) also replaced the oil filter and drained the coolant from the radiator and flushed it with a chemical flush, for rust / erosion etc. Also burped the radiator and bled it accordingly. More than four times might I add. And the the hot air blows freely out of the cars heater. As it should do. So I am definitely sure there is no airlock in the radiator/ hose or heating core. I also think it may be the overflow cap / as the zafira I own does not have a designed radiator cap. The overflow cap is not spring loaded. So it cannot push the coolant back into where it came from when the pressure builds up in the overflow tank. And the cap is also very hard to remove when cool. Also I believe the cap may be causing a vacuum like action in the pipes/over flow tank and in turn sucking the pipe together. Vehicle was fully serviced by myself with me changing the air filter / oil / and windscreen water. Also new water pump and water. So please can you tell me what is wrong as I am stuck? Also, my CD30 standard issue radio/stereo's control panel is not working. Although controls on the steering wheel are, but I cannot use that to control the settings menu and pay attention to the oil levels/coolant temperature, engine temperature etc. Any help would be most appreciated and thank you.