2 things to look at. General stuff, not specific to the Ford diesel.
Diesels don't really create vacuum like gasoline engines, they don't have a
throttle plate, so there is no restriction on how much air can come in so
the brakes are assisted by either a vacuum pump or by power steering
hydraulic pressure. The vacuum assist power booster should have a return
spring inside to counteract the vacuum and there is a check valve arrangment
to release vacuum assist when there is no pressure on the pedal. One thing
that I have heard of is the rear seal on the master cylinder leaking and
fluid filling up the power booster, not likley, but worth a look if the back
of the master cylinder is wet. Also there is usually a check valve where
the vacuum hose goes into the booster.
If you unbolt the master from the booster, and move it forward, see if there
is a difference in how the pedal returns and check for fluid.
If your area is like here with rust, chances are there are lots of 13 year
old trucks in the junkyard, so a used replacment might be cheap enough to
give a try without bothering to diagnose. Could also stick your head under
the dash of the junker and see if there is a return spring on the pedal.
Might be a coil spring around the pedal pivot.
Ford dealer could also show you on the parts illustrated catalog.
Good luck
Dave
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