Flow falls off and discharge pressure becomes unstable while the pump sounds normal. A valve that no longer seats cleanly lets fluid return during the pressure stroke, so volumetric efficiency drops without any dramatic symptom. Comparing flow against the original figures is the reliable test.

Yes. All the valves in a multi-cylinder pump do identical work and wear at similar rates, so fitting one new valve among worn ones loads it disproportionately and it fails early. Replacing the set restores balanced operation and is what the kits are assembled for.

Only to the correct adjustment, never beyond. Over-tightening generates heat, scores the plunger and destroys both packing and plunger, converting a cheap consumable job into an expensive one. Packing that will not seal at correct adjustment has reached the end of its life and should be replaced.

When scored, pitted or worn out of round, since a damaged plunger destroys new packing quickly and the fault repeats. Inspect the plunger whenever packing is changed - fitting new packing against a scored plunger is one of the most common reasons a rebuild fails within weeks.

Far less than the fluid end. It is a conventional oil-lubricated mechanism separated from the pumped liquid, so it typically outlasts several fluid-end rebuilds. Oil condition and level, and the seals between the two ends, are the items to check at service intervals.

Abrasive particles in the fluid, cavitation from poor suction conditions, and running above rated pressure. Cavitation is the most damaging and the most often missed, since it pits the valve and seat while the operator sees only that valves are not lasting - the fix is upstream in the suction, not in the valve.

Kits are usually assembled by function - a valve kit, a packing kit, or a full fluid-end kit - so check what is included against what the job needs. A packing kit generally will not include valves, and neither will include a plunger, which is ordered separately when inspection shows it is needed.