I've had a simular fault on a c4800. you couldn't set the pressure to over 900psi. a previous company had replaced the hydraulic pump which cleared the fault temporarilly. on further inspection myself and a colleague found the over pressure valve adjuster on the valve block( nearest the cab} had come loose from the adjuster seating, which is just screwed in. after making a insertion tool , refitted the valve and adjusted up to correct pressure.. all to date is still okay and that repair was 8 months ago