In order of likelihood: a worn nozzle, an inadequate water supply, a failing unloader valve, then pump seals or valves.

Check the nozzle first. An eroded orifice drops working pressure substantially and costs a few units to replace - it is by far the most common cause and the cheapest fix.

Then the water supply. A restricted inlet filter, a kinked hose or a tap that cannot keep up starves the pump, and the symptoms look exactly like a worn pump.

The unloader is next. If it is stuck partly open, flow diverts to the inlet instead of the nozzle and pressure never builds.

Only then look at the pump. Worn seals and eroded valves show as low pressure with pulsing, water in the oil, or weeping from the head.

Diagnose in that order. Fitting a seal kit to a machine whose only problem was a blocked filter is a common and avoidable waste.

It diverts flow back to the inlet when the trigger is released, and it fails by sticking - giving no pressure, no release, or constant pulsing.

With the trigger closed the pump is still turning and still moving water. The unloader gives that water somewhere to go, recirculating it rather than letting pressure rise until something bursts.

Stuck open, the water keeps recirculating even when the trigger is pulled, so the machine will not build pressure.

Stuck closed, pressure does not release when the trigger is let go, which loads the pump and can cause hose or seal failure.

Partially stuck or badly adjusted, the valve hunts and the machine pulses rhythmically - the classic symptom.

Grit and scale are the usual causes. Some unloaders can be stripped and cleaned; many are replaced as a unit. Adjusting an unloader to raise pressure beyond the machine's rating is a common and destructive shortcut.

Vapour bubbles forming in the pump because the inlet cannot supply enough water, then collapsing violently and eroding the internals. Prevent it by fixing the supply.

When the pump tries to draw more water than the inlet can deliver, pressure at the inlet falls low enough for water to vaporise. Those bubbles collapse as pressure rises through the pump, and the implosion pits metal surfaces.

It sounds like gravel in the pump and it destroys valves and heads far faster than normal wear.

The causes are all upstream: a blocked inlet filter, a kinked or undersized supply hose, a tap that cannot deliver the flow, drawing from too great a lift, or air leaking into the suction line.

The fix is to restore the supply - clean the filter, use a larger supply hose, or feed from a break tank so the pump always has water available.

Fitting new parts without curing the supply simply destroys the new parts.

Crankshaft pumps are designed to be rebuilt; axial pumps on lighter machines usually are not worth it.

Professional crankshaft pumps have replaceable seal kits, valve kits, plungers and sometimes sleeves. Rebuilding is routine, well documented, and costs a fraction of replacement. These pumps are expected to be serviced several times in their life.

Axial pumps on domestic and light commercial machines are often built as sealed units with the head and internals not intended for service. Parts may not be available separately and labour exceeds the value.

Damage type matters too. Seals and valves are consumables; a pitted head from cavitation, a cracked head from freezing, or a scored crankshaft usually condemns the pump.

Where a machine has had one rebuild and is failing again, look for the underlying cause - supply, bypass time, freezing - before authorising a second.

Water trapped in the pump head expands as it freezes and cracks the casting. Avoid it by storing warm or flushing with antifreeze.

A pump always retains water after use, and there is no drain that removes all of it. In sub-zero storage that water expands with enough force to split a brass or aluminium head.

The damage is usually terminal and is not covered by most warranties, because it is preventable.

The two reliable preventions are keeping the machine somewhere that does not freeze, or drawing a pump antifreeze through the machine before cold weather, which displaces the water in the head, valves and hose.

Draining hoses and disconnecting the supply is good practice but does not empty the pump.

For machines that live on vehicles or outdoors, antifreeze protection at the start of the cold season should be a scheduled task, not a decision made when frost is forecast.

At the interval in the manual, plus an early first change after bedding in - and immediately if the oil looks milky.

Crankshaft pumps have a small oil charge working hard, and many manufacturers specify an early first change to remove bedding-in debris, then a regular interval by hours.

Milky or creamy oil means water has passed the seals and is emulsifying with it. That is a seal kit job, and continuing to run will damage bearings and the crankshaft.

Low oil level with no leak usually means it is going somewhere - past the seals into the water, or out of a breather that has been over-filled.

Use the specified oil. Pump oils are not motor oils and the additive packages differ; the wrong oil foams or fails to protect at the operating temperature.

Check the level with the machine level and cold, since readings differ hot.

Nozzles, seal and valve kits for the pumps in the fleet, an unloader, inlet filters, and coupling O-rings.

Nozzles are the cheapest performance restoration available and should be treated as routine consumables rather than spares.

Seal and valve kits for each pump model in use turn a multi-day breakdown into an afternoon. They are specific to the pump, so record pump models rather than machine names.

A spare unloader is worth holding for any machine the operation depends on, because it is a common failure and a total stoppage when it goes.

Inlet filters and screens are trivially cheap and prevent the most expensive failure mode.

Coupling O-rings and seals stop small leaks becoming reasons not to use the machine.

For hot machines, add burner nozzles and fuel filters. For fleets, a complete spare pump is often justified by the cost of downtime alone.