On my D6B the left steering clutch and only the left is much harder to pull than the right. It also does not return forward without pushing it unless you have it pulled back and let go. The oil level is fine. The right clutch functions as it should.
Sounds like a tight linkage problem, also try loosening off the right clutch until its adjustment is correct, then check the left one again.
The booster is pretty much bullet prove but it is sensitive to proper adjustment. Also be sure that the levers/linkage completely return to the stops or one side can affect the operation of the other.