you'll have to check the pressure at the pump.
it may have a pressure sensor mounted on it, remove the sensor and fit a pressure guage (may need an adapter). the pressure should read 1250psi when steering is turned to the stop and held briefly. if you get any readings higher check the relief valve, if lower then the pump is most likely worn out or faulty.
Also check the bearings and pins in the steer axle, try greasing them if there are grease zerks.