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So two common bad things can happen that will allow you to pull out the shaft, or excessive endpla (more than 1/8"). 1) the shaft is broken. 2)the circlip that holds the rotor on has broken or come off. If you can pull the shaft out get another pump.
If not check out the pic above. You should be able to pull off the high pressure hose, then remove the fitting on the pump to see the parts in the pic above. Part number 2 is the regulator piston. The nut on the right has a screen that should be clean. There should be no washers between the nut and regulator body. The shorter the assembly the higher the pressure, the longer the lower. 2nd thing is to check the size of the hole in part #1. The hole should be 5/32". The Chevy's with hydroboost brakes and power steering used the "hot rod" version of the saganaw pump with the highest flow and pressure possible
If you can't make any headway go to the parts store and get Chevy saganaw pump. Take the regulator out of it and make sure it has no washers and a 5/32" hole.