Slow Cranking Truck wont start

So I did a bit of testing today.

I have 2 positive wires going to the starter, 1 for each battery set, one of them cranks a lot better than the other.

The one that cranks better also showed way less voltage drop when cranking at the starter, it drops to about 8-10, whilst the other one plummets to 6, so I am hoping the other set of cables are bad, I haven't had time to check them yet.

I tried to jump it with just 2 batteries and my Ute, but no go.

 

Well I am at a loss, I checked all the cables I could find, cleaned them all up, traced them and found no issues.

I connected a battery directly to the starter with some 1 ft cables, whilst still connected to the other 3 batteries, still no luck

Crank is real slow, no chance of starting and only goes for 1-2 secs.

So I have pulled the starter out, bench tested it and it seems ok, doesn't get hot and engages straight away, but not sure if that relates to how it performs when trying to turn over an engine.

 

So I am thinking it was the starter, I did clean up a few grounds when I had the starter out, but nothing that I think would have made a difference, but I just put a new starter in and she fired up with the same batteries

I havent pulled the old starter apart to see if it's no good, I have never done that before so I will have to do some research, it's not something I am too skilled at.

Cheers all