Thanks for suggestions, I do have a leak on top of injection pump, where line for first injector is exiting, but, I guess this could not be the reason for air entering the system?
My thinking is that here there is a very high pressure during motor running, so can not see how it could suck the air trough there?


I am now running straight from fuel canister into plastic primary filter (using clear pvc line) and this one is totally bubble free.
Preheater circuit is by-passed, so it goes directly into lift pump.
Return is connected from main filter housing back into the canister (also using clear line) and in here I can see clearly bubbles flowing.
The more RPM the more bubbles.....
Where I fail to find the entry point; wondering if there could be some good "trick" how to do that.
I suppose they can only enter in low-pressure part of the circuit?
Could return lines connecting all injectors be the cause; I do not think as in the past I had one broke and car was still running and starting normally; just sipping fuel there....
Any suggestion would be supper helpful...it is driving me crazy as I feel I isolated most of the circuit...