XRAY - Model racing cars

Hello,
I've a got an issue with shock pistons - I observed very strange behaviuor.

The shocks were moving very hard, sometimes even stopping so I decided to rebuilt them.
First I disassembled one front and one rear shock - the pistons were very hard to be pushed out. The oil inside was very dirty. I cleaned both the shocks and the pistons.
After that I tried to move the piston up and down (with all the o-rings out) and it was still very hard to move, almost blocked.

I left everything disassembled through the night, and the next day the problem was gone - the pison moved without problems.

I disassembled the other two shocks and the behaviour was the same. Through the night the issue dissapered.


I measured the diameter of the piston before and after "resting".
I can confirm, that all the pistons became narrower after "resting" through the night. The difference is less than 0.1mm, but enough to create problems.

Is this normal ?
Do pistons expand, when they are soaked in oil ?

Thanks
Andrej