This can also
be caused by a bad hose going to the Wheel Cylinders. They deteriorate inside
and a piece of rubber lining falls down and acts like a 1 way valve. Usually
found locking one side up by blocking the return of fluid, but can prevent fluid
from going to the Brakes on one side also.
No matter
what you do, be sure to replace these hoses. Don't forget the rear one
either.
Ray