CAT 339DL boom hydraulics?

[cyanide]

Hey guys,

About a month ago I brought a 1/12th CAT 339DL electric/hydraulic excavator. This is at the lower end of the price/quality scale, direct from China and unbranded (nothing on machine or manual). I suspect due to not being a licenced CAT product.
Is about the same as a 4200XL as far as quality goes. I knew what I was getting into (cost/quality) and have been taking my time to find and fix the new machine/new operator issues. Pretty happy with it so far, was expecting to be wrenching on it and have not been disappointed in that respect. Snugged up connections, hardware etc. Pump mix and Endpoints set on TX.

My current issue is my bucket & stick rams seem good, but when I operate the main boom rams I see air bubbles in the clear send line (between pump and valve block). It only bubbles on main boom operation at first then clears up.
When not running the bucket & stick hold fast, but the boom moves freely. I see no obvious fluid leaks but it seems to be allowing fliud to move?
I don't know much about hydraulic systems, but the boom should not move should it? The lack of fluid leaks makes me think ram seal or valve block, would that be a correct assumption?

Odd thing is it seems to operate the boom OK when running, so I am not sure if it is a problem or not?

oh, I am on a budget, replacing with German hydraulics is not really an option at this stage.

Thanks!