I'm interested in your symptoms...
The wheel turns freely(loosely) without moving the cylinder?
The wheel is tight?
The wheel loses motion when changing direction?
I've never been really happy with my steering although it got a lot better with a purge & fill. The last 2 times out though, I feel some lost motion again.
You can get a seastar manual online if you don't have it. Use the troubleshooting steps.
Basically you isolate the problem by disconnecting things logically until you figure out whats not working.
Fluid leak/Air in lines/cylinder stuck in one or both directions/a Motor stuck/helm not pushing fluid
I'm inclined to take the mechanic at his word unless you have reason to doubt him.
If you are contemplating replacing it yourself, the only hard part is the filling and purging. Assuming all the hoses and the cylinder are good.