Vetus Hydraulic Steering

My vessel has a dual station Vetus Hydraulic steering system with manual pumps. For sometime there has been a "chatter" when reversing direction at the helm. The fluid in the helm reservoirs are kept at the proper levels. I had the cylinder rebuilt and rechromed in December. A new bypass loop was installed and I had a professional purge the system again just to be sure that I was not missing something in my many attempts to remove trapped air. I removed one pump and had it rebuilt. The inside station's pump will be a bare to remove so I deferred its rebuilding to see if there was any improvement in the outside station.
There was no noticeable improvement in the feel of the helm.

Any suggestions? Would it be necessary to have both helm pumps serviced if they run independently?

Thanks for your interest in helping.
The "chatter" is felt, not heard. Whenever either helm is turned it does in fact turn the rudder. Once the direction of the helm is reversed, there is a sort of skipping sensation before the rudder follows. The helm then feels normal until the direction changes again. This sensation happens every time at both stations. Steering a sailboat with Hydraulic steering is a lifeless sensation at the best of times but with the current situation it is down right unpleasant.

We had the rudder out last week to pull the shaft and replace VPP components. With everything disassembled the rudder turned relatively freely. I say relatively because its big and heavy.
We are now back afloat.
The chatter happens when you reverse direction regardless of how far off center the rudder is.
There is a pilot. It is tee'd into the system a couple feet from the cylinder.
The plumbing is copper except for a very limited amount of rubber hose near the cylinder.
New packing in the stuffing box, quite loose at present since we are still re-commissioning and haven't got to tightening it up yet.

thanks for the input,

I really appreciate what is obviously a considered opinion. We will be doing seatrials on the VPP first of the week. I'll be listening to the pilot closely then.

I believe the check valves are in the Vetus helm pumps? I was told that the valves needed work in the pump that was serviced. I am wondering if my continued problem comes from not servicing the second pump at the same time?

Did not change the oil. Only added what was lost in removing the cylinder etc. The tech did say that the oil in the pump was "a little dirty".

Only a limited amount of rubber hose. Maybe 24" total but likely twenty years old.

not sure what the budget is for flying people in, but after the cost of re-chroming the cylinder, replacing the seals, hiring someone to do exactly the same things I did to purge the system, installing a bypass loop, replacing the packing with teflon, and removing and rebuilding one of the helm pumps, if I don't see a noticable improvement soon the budget is likely to include flying me out.
keep a watch on the job vacancies postings, I suspect I'm skating on thin ice...

Recap

To recap,

So far I have,
Had the cylinder re-chromed
Had the seals in the cylinder replaced,
Had the Vetus manual hydraulic helm pum serviced including grinding the seats in the check valves
replaced any rubber hoses
Purged the system,
Had the system purged by someone else
Re-purged the system
drained all the old fluid
repurged the system
installed a bypass loop at the cylinder and repurged the system
removed the ruddre and changed the bottom bearing and repacked the stuffing box with teflon,
and,

I get two annoying "skips" in the helm everytime I reverse direction regardless of what angle the rudder is at when I change direction.

Can hear the "skipping" in the lazerette. Can not hear anything unusual over the sound of the autopilot's motor so not sure if it happens then or not.

Absolutely no air at bleeders on cylinder, pump full, autopilot res. full. Fluid clean, new HY-32.

Frustrated to say the least.

thank you "Gary?" for your reply. I'm going to take some more time to review the drawings but wanted to thank you for your interest and help right away.
Unfortunately my German is weak and the drawings do not illustrate the particular Vetus pumps I have. I do have the drawings and model numbers etc but can not share them now as I am only barely computer literate.
I can say that the non return valves are integral and that there are no adjustment points or check valves external to the pump.
I saw the pump in its disassembled state and the only operator service point is the top of the reservoir.

nrv

Thank you Garyohv for your comments. they inspired me to keep searching. buried in the darkness behing the pilothouse icemakers I found a dual station non return valve along with the helm pressure equalization line. Sounds like this is the most likely culprit so its coming out today bound for the repair shop. A messy proposition given that its barely reachable but if its the problem then its worth the effort.