Some of these diesels have a vacuum pump for the vacuum booster. If you changed it the rods sometimes need adjusted. I think the previous link was about manual brakes. Lay a straight edge across the front and use a ruler and measure the length. You can turn it in about 1/16 inch and try it. Don't disconnect the master cylinder try to just move it forward enough.
I have had a bad distribution valve in the brake line towards the back this splits up the fluid between front and back brakes.
How tight are the back shoes adjusted this can make a difference.
You can pull the fuse for the anti lock brakes for a few days if this solves the problem at least you isolated the problem. They sell rebuilt units on the web do a google search if you need a unit. They also offer techinical support which you can call.
If you have a vacuum pump make sure it is working.