You have a 1989 Ford Aerostar changed the pads and mastercylinder and the wheel cylinders and still every 5 pumps of the brakes the pedal goes to the floor HELP?

I would suspect you have purchased a defective master cylinder, especially if it was a rebuilt unit. Also make sure you have bled the system properly & completely to remove all air. Bleed in this order. RR-LR-RF-LF The master cylinder must be bench bled prior to installing ; I suspect you have air in the system if you did not bench bleed the master cylinder prior to installing it on your vehicle.Try this: obtain a quart of brake fluid and rebleed the system; you want it to be this problem vs removing parts and starting over. I hope this helps you. Mark