I just experienced thi9s for the first time. I did a few laps at the track then trailer the car. 2 days later I go open the hood and see that both my upper and lower hoses are sucked shut....Its never done this before. I opened the radiator cap and the vacuum went away and the hoses opened. I did make 2 recent changes. I took out the thermostat and changed the radiator cap to a new Stant 16lb one. There is no heater core or thermostat bypass hose on this car. I have a weak electric fan and just wanted to see if no thermostat would help the car run cooler, and it did. Its just odd on the suction thing. Like the others, I'm for sure going to add in the spring. I have other issues but the spring sounds like a good idea regardless.