It could be associated with the vacuum modulator,and this is where I
would probably start.

Be sure that the vacuum lines to the modulator are in good shape, not
cracked or disconnected at some point, and are connected to the appropriate
vacuum sources.

To change the modulator, you just unscrew the old one, screw in the new
one, and reconnect vacuum.

They are adjustable, by the way. If you look into the opening on the
modulator,
you should see a little slot or similar.  You can adjust that in or out with
an
appropriate screwdriver or other tool to set it for your particular car.