Fluids do not pull very well they need to be pushed
Air compresses in the pump & motors rather than purging out
Some times it can take forever to replace all of the air in the lines & internals with oil
A few things will help
1) run the engine as slow as it will go , this reduces frothing
2) have both wheels clear of the ground , this reduced the resistance to moving as low as it can go which aids oil flowing rather than air compressing
3) open bypass valves , same as 2 above
In some cases I have needed to attach a vacuum pump ( sump pump ) to a breather / expansion tank to suck the air out
The worst I had was a Husqvrna ProRider that used a full tank of fuel before the oil replaced all of the air
So that was a case of purge for about 1/2 hour, leave it sand till cold then purge for about 1/2 hour .
From memory that process was repeated for a full week before I got full power to the wheels .
I have no idea why some purge in 5 minutes & other take 5 days but they do