Master Cylinder Autopsy

Different manufacturers make their M/C and kits differently. At the time of the FSM they may have shown a metal cupped washer ahead of the primary rubber cup. This centers the spring and supports the rubber cup on the return stroke. No crud should be in the M/C, this is a lack of maintenance. Your concern should be of the pitting in the bore. If extensive, a hone will not clean it up and a rebuild kit will not seal. It may be wise to replace the M/C.