The aircraft assigned to us in Oakland was brand new. The airline had put it in service the day before. The co-pilot and I were excited about the possibility of flying her all the way to Florida. The gate agents loaded her to the gills with passengers, bags and freight. We taxied out on schedule for a runway 29 departure. After we received take off clearance the co-pilot slowly pushed the thrust levers forward and at that time one of the hydraulic systems failed. I took over and rejected the take off at about 50 mph; turned off the runway and started trouble shooting the problem. Long story short- the problem was an electrical relay that controls the hydraulic pump on that system. Did we have one in stock at Oakland? No way! Flight cancelled...hop on another airline to Las Vegas. Arriving in Las Vegas one hour late for our Miami departure, we were told by crew scheduling that we had been re-routed to Chicago and to please have a good flight. We hustled over to the Chicago gate and found our airplane to be one of the oldest in the fleet. I have flown her many times. As usual, we filled every seat to Chicago and blasted off one hour and fifty minutes behind schedule. The ride over to the windy city was uneventful and really quite pleasant. The night sky was gorgeous and the tailwinds were strong. Our arrival into Chicago with the rising sun was one hour and twenty late, so we made up thirty minutes. Not bad, considering.... I am extremely exhausted; must lay down. Tonight we are scheduled to Phoenix. Yeah!! Two days off, I hope.