Hi,
I am assuming that it isn't meant to do this. I understand that when the water gets hot it expands and goes into the expansion tank via the spring loaded radiator cap. I would assume on cooling that the contraction of the water in the system would some how cause the the water to come back from the expansion tank via the radiator cap.

I am really after advice. After the car has been run and the engine has got hot. Once cooled I notice that the top hose starts to suck in. Now I replaced the radiator cap last year in the hope that this would fix this, and I thought it had. But I had to replace the top radiator hose on my mothers car today as it had started to leak, and when it cooled I noticed the pipe was sucking in slightly. I assume it is not getting a return from the reservoir tank. Could it be a blockage from there or do I need to replace the radiator cap again. I am not sure how the fluid gets back from the reservoir to the radiator as the radiator cap is sprung. Any ideas?