Airlock!? or....?

[Neshco]

This happen before, a year ago. Water in my bath tub just stopped running, like someone cut the cold line.
I found googling that it is maybe airlock problem and what I need to do to "unlock" it. Advise was to open both cold and hot water in the bath tub and with the hand palm press tap to close water exit at it and push the running line to not running line and it worked.
Now, after a year or so, problem is back, and it affected first bath tub cold line and sink tap hot line. Tomorrow, sink lost hot water line as well. Now out of 4 lines I have working just a tub hot one, sink is completely dead with 0 water running.
It never happen with the toilet flush and the shower, because it is in line with the manifolds both hot and cold. But as you can see at the photos, both affected lines are those that are routed below the floor and up making "U" shape. On the sink side, there is a spot that are possibly problematic and guess there is a problem, but on the bath tub side, there is a clear "U" shape without distortion (or I cant see it?!) and there is a problem as well.
I dont know how air came in the system, guess from the supplier, but fact is that previously I have solved the problem with no physically touch the system.
Anyhow, I have tried to push the water through it by running hose from the shower. I have closed the manifold valve for the hot water and tried to push the cold water (which is not closed) through the hot line on the sink in opposite direction without success. During the procedure I have released the end lock on the hot manifold and there I could hear some air running out, but other than that nothing. I have tried to release it more and less, in the same time to open hot water valve at the manifold, but nothing happen. I have tried opposite way with the hot water running through the cold pipe, with no success.
Now, I am stucked and out of ideas. I would greatly appreciate any suggestion and any tip how to fix this problem.

 

[Breplum]

What is your pressure? On a pressurized system cold water side, all air should be pushed out by fluid dynamics...so can't imagine anything but a problem with the tub valve itself.
Unfortunately, you are in a nearly impossible spot with the tub blind-set.

 

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[Neshco]

Hi and thank you!
Here is a pressure, in average 3,5, but can drop to the 2,8 all the way to the 4.
Bathroom is renovated 2 years ago and all taps are new, and very high quality. Ok, maybe some are not good (sink have 2 on the same line) but I am not sure that 2 taps became bad at the same time. Especially because I had this problem before on both sides and it was resolved on both and after a year again on both sides in the same time. Kind a unlikely that tap(s) are the problem? Or..?
Meanwhile, I have tried to do this

at the bath tub and it fixed the problem. But, on the sink side both lines missing, so not applicable there. Any idea about it?

 

[Breplum]

We don't ever have problems with airlock with pressure like you have. In the USA, Water Heaters are fully pressurized and almost 100% of the pros on this site don't have any knowledge of euro systems. Sorry. Good luck.