if you have had any brake work done lately more than likely their is air in the lines and you need to bleed the air out again if no brake work has been done then their is a couple of other possibilities either the calipers are sticking or the splitter blocks mounted just under the master cylinder are going bad!
You have drum brakes on the rear.
The 1990 Ford Aerostar Maintenance Information manual says 21 gallons.
Brakes are adjusted automatically
In a 1990 Ford Aerostar : You either have the 3.0 liter V6 engine or the 4.0 liter V6 engine from the factory ( each size of engine has 6 engine cylinders )
mine only has rear anti lock brakes!
In the common workshop manual by Haynes.
The distributor for a 3.0 liter in a 1990 Ford Aerostar rotates CLOCKWISE ( looking from the top ) The spark plug firing order is ( 1 - 4 - 2 - 5 - 3 - 6 ) The engine cylinder locations are numbered : 3 - 6 2 - 5 1 - 4 front of Ford Aerostar
Ford Aerostar was created in 1986.
Check the fuses first!
21 gal. probably, full it up and see.
Under tyhe dash driver side
1 - 4 - 2 - 5 - 3 - 6