The to the rear brake lines rusted out, draining all the fluid, replaced brake lines and bleed the brakes, pedal is hard and firm, (no air) UNTIL engine is started, then they go mushy??? what did I miss? even did an ABS reset? thanks.

 

Almost certainly you have a vacuum leak somewhere near or at the brake booster. Check, double check those hoses and also make sure the booster itself doesn't have
any punctures or severe rust issues going on. Could also be an ABS control module problem.