Originally Posted by KaRLiToS
-I turn on the pump-Then the water level inside the reservoir is lowering (due to pressure I guess)
That right there shows me the issue; air pockets.
Water is, as far as we are concerned, uncompressible. The tubes and rads will expand very slightly with the pump pressure but I very much doubt you will notice a level drop due to that. That just leaves air pockets in your loop compressing, reducing the volume of fluid in the loop and being replaced by water from the res, causing a level drop.
The reason it may have worked fine for a while then presented problems is either that your pump is drawing bubbles into it from turbulence in the res or that some small bubbles from the initial fill have eventually collected somewhere and formed a large bubble.
If your pump is drawing air due to turbulence in the res you either need to re-position your inlet to make sure it is below the water level and / or get some aquarium sponge to put over the outlet. No need to change res, unless you really want to.
Either way you still need to bleed your loop as others have suggested. The bracket you have holding the rear rad has a quick release, so you can detach the rad from your case and move it round to aid the bleeding. Just be careful not to pop a hose off whilst you do it.
The top rad needs another approach; try rotating, as previously suggested, to allow the air to follow the flow path of the rad. Tap parts of the rad with something like the handle of a screw driver to help detach the bubbles from the walls. Also turning your pumps on and off with the rad in different positions can help.
I sometimes find that heating the water helps with the bleeding process. To do this I run OCCT and Furmark and turn off my fans. Doesn't take too long to get the water up to ~45°C (I tend not to go further) which can sometimes help to move the bubbles along.