I have constructed some small cylinders to actuate the rudder on an ultralight aircraft. They are 19mm and 12mm (3/4" and 1/2") bore diameters.
Two cylinders are at the pedals and two back at the rudder.
When in operation, all four pistons are moving at once as the opposite side cylinders are being pushed back by one pedal pressure.
The problem is; at very low rates of movement, pedal pressures are very high. They are quite acceptable once the pistons are moving at a reasonable speed.
The seals used are automotive brake mastercylinder seals, the type with a hole in the middle which locates them over a pin or post on the piston, and the fluid is brake fluid.
The distance that they travel in the cylinders is 3inches and 1.4inches. The cylinders are aluminium.
Would a normal cup seal, as in auto wheel cylinders have less friction and would they handle the use
Would it be possible to reduce the friction by using a different fluid and a totally different type of seal?
As this is a closed system the seals need to be pretty good or one may find there is no steering in flight!
I would be grateful for any advice

Why in the world does your ultralight need hydraulic rudder actuators?  That's a heavy solution to the whatever problem it's meant to solve.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

It sounds like your problem is that you are using two different sizes of cylinders in a completely closed system. The fluid volume in the two cylinders does not stay constant as different sizes of piston rods (not pistons) go in and out of the cylinders. You are probably creating a vacuum somewhere in the system when it does work. When it locks-up you are probably trying to compress fluid that has nowhere to go.

With two cylinders on the rudder, do you have a geometry issue?  In other words, if you don't have a 1:1 relationship for distance moved by each cylinder at each rudder angle, you'll "lock up".  You can only get to high-speed pedal movement once you've got some distance up (and are out of the locked condition).

Map the cylinder extensions for both cylinders acting on the rudder, vs. rudder angle.  If they don't coincide--Bingo!  There's a place to look.  (Unless you've got a system with matching master/slave geometries from the pedal to the rudder)