I don't know the Capitalino manual hydraulic pump. I have a Wagner pump and what you describe appears to be the check valve. It has a small steel ball and spring at each end and installs some friction and feel to the wheel
, holding a bit of pressure for the current helm
setting. The balls or their seats can become corroded and even a bit will allow fluid to bypass the ball held off it's seat by such crude.
I've had a this problem, and before finding a bit larger ball for one end, steering would drift 5 degrees in 10 seconds without correction. You'll need a brave (ok, even tho' I don't have a pump blow up and can't get parts) hydraulic man to clean, scour, and match balls for the check valve I think, but that is just my current
understanding of my own problem. My problem, after putting a 11mm ball in a 10mm seat, is that there is no more drift but a bit of wheel
bind and resistance still. Good luck.