I have a 1989 Ford F350 7.3 diesel. When I apply my brakes the first time they seem to brake normally, but if I let up and apply them immediately the second time they are real hard and the brake does not seem to be holding.

I can go on down the road for a short distance and apply the brakes and they are fine....but if I let up and have to apply them immediately a second time they are hard again, and again do not seem to be holding.

Can anyone tell me what my problem might be?