After replacing the master cylinder on your 1996 Ford Aerostar the warning lights on the dash stay lit what is the problem?

Your brakes need bleeding. And if you have antilock brakes, you should take it to a shop to bleed them. It takes special tools to bleed antilock brakes. If you have regular brakes you can bleed them yourself. Could go into further detail but would take too long to explain. Refer to a repair manual.

Note: bleeding antilock brakes without first disconnecting the sensor will damage the sensor and the antilock function will no longer work (learned the hard way, unfortunately).