Shaun, I think you are looking at this wrong. If there is no air in the system, and for this discussion I'm assuming there is not, then the ram does not have to "fill up". Its already full.
Any fluid movement will result in ram movement.
If the ram does not get enough flow, it will lag behind the regular steering parts. But, it will move.
When you turn the steering box against a load and the spool valve moves, the fluid will be sent to the normal actuator and the ram.
If the fluid flow is insufficient, then things will move slowly, but if the fluid is moving, they will move.
In other words, you should be able to turn the wheel until it has pressure on it, hold it there and the wheels will slowly turn.
If they do not, then the fluid is not flowing. There is no "slop" in a full hydraulic system. Hydraulic fluid is non-compressable. When you push down on a bottle jack handle, the ram will move the same amount no matter what. If the load is too much, the bypass valve will open and the fluid will no go to the ram.
I'm pretty sure this is what is happening to you. The pump is not putting out enough pressure for your system to work.
Either the bypass valve is weak or set wrong, or your ram is too small. When you turn the wheel, the pressure gets sent to the ram but before it can move the bypass opens and the fluid pressure stalls at that point.
Which is not enough to move the wheels.
Given you were having this problem before you went assist, I suspect you have a pump problem.
If you have access to a hydraulic shop, they could probably put some gauges on to see what your pump it putting out.
Failing that, maybe you could try a junkyard pump to see if it makes a difference.
Otherwise, I would look into a new pump.
Remember, with any pump you often trade pressure for volume. Make sure any pump you buy will put out at least stock pressure. If you can get more flow too, thats fine.
But pressure is what you want now.
If you have access to a bigger ram, try it if its cheap.
If you want to test my theory, see if you can loosen the attachment of the ram to the steering.
Not enough to break anything, just enough to see some movement.
Watch the ram while someone turns the steering.
I think you will see the ram shoot to the end of the free play almost instantly and then stall.
That means that it is getting plenty of flow, but the pressure is not there.
i could use a larger bore master cylinder (like one from a '99 c3500 w/rear disc) but that will be farther down the road.
