secondary plunger slips in - master cylinder

[mini64]

rebuilt my tandem master cylinder and find the secondary plunger with new seal just slips in the bore, no resistance, loose even. Annoying thing is with the decades old, old seal is just slips in too. Went ahead and put together and on bench bleeding sure enough the back (front?) doesnt pump fluid. its a 3/4" bore 646765976(or C?). rebuild kit is an old stock SP2378/3. Dont know what the /3 means. Any ideas?