pumps can corrode and become pitted as they sit. I just went through this with a family-heirloom pressure washer. Old school briggs engine with a niiiice cat pump on it. it sat for years, put away wet, but no pump saver. when I got it, the engine finally came to life but the pump was shot. a family member really wanted this fixed, but some shops turned it down--- the engine was worth $20, the original pump $400, and we'd be replacing the pump -- basically putting a $400 pump on a $20 engine. Pretty much what the shops told me was a big-box store pressure washer has a cheap $100 that's not good for more than a couple of years, or 100 hours. they pit, crack, corrode while sitting, or more likely are stored improperly and just don't have the quality in them to handle any mishaps, and can't be rebuilt. And a washer that is quality enough to include a good pump starts at $1200 and goes up.
but that's what the family member wanted---- so we did. brand new $400 pump on an old, neglected briggs. and what they told me as they handed me the repaired washer--- never run without water pressure, always use pump saver, and don't let it freeze.