I have an 03 Cigarette Mistress T/S with Marine Machine full hydraulic steering. It does not have an external res, just the stock one for the 500 EFI. Steering works fine at low RPM and up until around 4K and anywhere past that the boat wants to drift starboard and you have to keep turning the wheel port to keep the boat straight? Seems like the pressure is bleeding off somewhere and cannot steer the drives, you can keep steering port without hitting a stop. Im not losing any fluid anywhere either and have the res filled all the way up with it overflowing. Once I come back down in R's the boat steers correctly. Any help/advice would be awesome.

Id guess the problem might be in the helm, fluid bleeding off, what helm is it ?

That's what I was thinking too, assuming the helm is Marine Machine as well but would need to double check. What's involved with rebuilding the helm assembly?

Not really sure about the rebuild, my Cig has a Char Lynn helm by Eaton............is MM still in business ?

I don't think they are. I know the rams and my controls are all MM, would the anodized external helm be MM but the head unit be Char Lynn?

Most likely an Eaton Pump at the helm. There should be a data tag with a part number on the back of the pump if it's an Eaton.

I had a similar issue. It was a bad seal in my ram. Pressure would override the rod end and the boat would drift.

so no lost fluid on yours at it was bleeding back internally then? I already had my port side ram resealed due to an external leak but not the starboard. Any way to test to eliminate the ram over the head unit?

Another thought is air in the system, the drives should not move side to side if you push on them, they should be solid

MM has been out of business for long time