Hardees Foods

BAe 125

Ten minutes prior to landing heavy rain fell on airport. Aircraft made normal touchdown, spoilers and lift dump activated. Pilot-in-command delayed brake application as usual and did not get thrust levers in idle. After aircraft passed mid-field taxiway pic reported brakes became ineffective. Aircraft hydroplaned on water-covered runway and overran departure end, down and embankment, and hit a ditch. Runway had an off center crown, grass was about two feet tall at runway edge, and a dam had formed from previously cut grass which prevented water drainage. Airport manager said he was not aware water would stand on runway.