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What you describe sounds like a volume (flow) not a pressure problem. Volume problems are caused by a restriction on the water piping. You get good initial flow because of the built up static pressure but once that depletes the restriction reduces flow. Very hard water by itself can reduce the inside diameter of the piping from mineral deposits and can sometimes cause a significant restriction at some specific point. Also if your main shut off is not a ball valve (lever handle) that (usually gate) valve can fail causing a restriction. Also sediment build up at the piping near the holding tank is another possible culprit.
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