Did it shift correctly with this linkage before the rebuild? Same valve body? Why did it need a rebuild, symtoms? If the throttle lever was all the way back at full throttle, pressure should have been sufficient to prevent slip. If it was freewheeling on the gear change it was definately slipping. It's now possible the rear clutch is damaged. Caused by broken input shaft rings, cut piston seal or lack of throttle pressure caused by incorrect throttle linkage adjustment.
Need more info.
Doug