First build ,cat D11

[Lil Giants]

I use 4s in my Vario hoe with no troubles... virtually the same pump & motor that you're using. What voltage are your drive motors rated at?

I don't remember seeing a hyd filter during your assembly, do you have one? You could change your oil now, but a little discolouring is nothing to worry about.

[9W Monighan]

You may have to put a woodruff key way on your drive shafts. The set screw on the flat will most likely keep loosing from the forward and backward movement. You may consider having a press fit on the sprockets too.