It's one of the return lines from the valve back to tank, about 6mm OD. The other 6mm return line seems to be fine. I made sure to cut all lines squarely with a razor blade. Those yellow 6mm hoses are quite soft rubber & maybe I have some brass tube that will fit correctly in the ID, thanx for the suggestion, makes sense.
I used a heat on the suction line, quite a stiff clear rubber/plastic line about 8mm OD, to put a bit of a S curve in it so neither end of the hose was binding on either push lock fitting. I don't see it leaking. I think that's what my problem may be with the 4206 suction line, I need to heat/form the clear hose more so the clear hose is not pulling at an angle on the fittings.
The 4mm push lock fittings have 20 bar pressure on them & I have never had a leaker. But then the 4/2.5mm hose is very rigid walled.
Looking at various builds on german forums of this Fumotec 490 I noticed a lot guys didn't use the push lock fittings at all.