The grommet is the answer nine times out of ten and it is a genuinely small part.
Diagnostic that takes ten seconds: pump with your thumb sealing the head instead of a valve. If you feel strong resistance and pressure builds, the pump body is fine and the head is the problem. If it pushes through easily, the piston seal has gone, which is usually grease rather than parts.
Between those two you have diagnosed almost every dead floor pump without taking anything apart.