I never put any water in the system for flush or fill unless it is distilled. Tap water from your garden hose can have all sorts of undesireable minerals, chlorine, sediment, etc. depending on your local water supply.

Jeff
96 T100 SR5 4WD Ext Cab, Warn Hubs, HD torsion bars, Bilsteins, Camper Shell, TJM bar, factory 31 x 10.5's, Powertrax No-slip rear, Recaro driver's bucket