Do you get negative pressure inside your watercooling loop?

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Even though my loop was tightly sealed, the water level inside the reservoir was slowly decreasing over 1 year or so. This wasn't caused by leaks but from what i have read due to water evaporating throuht the tubes itself (believe it or not they are water permeable) . What I haven't noticed is that while the water was slowly evaporating away no air was getting into the loop in order to take up it's space and as a result got negative pressure inside the loop. What this has caused is that the tubes 'imploded', they were not round anymore but flattened. As soon as I opened the top of the reservoir, air got inside and they they became more rounded but still not the shape that they should be. It might be be that they got permanently deformed and might need to change them. What I do now is leave the stopper in the reservoir slightly uncrewed so that air can get in to equalize the pressure.

Has this ever happened to anyone else?
 
All the time, although never to the extent that tubes implode. I usually just release the cap on the res once a month or so.
 
Air molecules are much, much smaller than water molecules. The negative pressure you are seeing is not necessarily due to water "evaporating" through the tubes.

I would hazard a guess that as you cycle from hot to cold (on/off) the expansion forces some air out but, on cooling, the negative pressure doesn't allow air back in. Your res top is acting like a 1-way valve.

If practical just leave it slightly loose (as you are now doing)
 
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