There is a thread here about a squealing 553 with loss of power. You may have the same issue which is there is a thin metal disc bolted to the flywheel which has a cast steel splined pump coupler which itself is bolted or riveted to the drive plate disc. It has rubber antivibration grommets where it fastens to the disc. The splines on either or both the disc and the coupler may have sheared off due to age and vibration. When I had a 555 we caught it just in time before it ruined the pump splines. Had the same issue on a backhoe too.
