Swing doesn't work sumitomo LS-2800 excavator

I have a problem with our excavator, a sumitomo LS-2800 F7 1988
Swing doesn't work, it looks like it get pressure, and sometime it working but very slowly. What could be wrong?
Anyone who have a hydraulic shematic for this excavator?

Swing doesn't work sumitomo LS-2800 excavator