briknope

Asking for advise on this? I'm about to order rear struts thinking that's it. The car won't budge in the summer but once it gets cold the rear slams to the ground. I can't hear any air leaking anywhere. If I start the car it comes right up in less than a minute. Seeing as how the car is 15 years old it can't hurt to replace them anyway but does anyone have any other ideas ? Valve block ? Water in the system?