Hi all. I finally joined the diesel community with a purchase of 93 w250 (pics to come). However, it has this issue... When I drive sometimes breaks work really well, but sometimes they don't. Pedal would get super hard and its a pain to stop the truck. also sometime pedal just falls making that hissing sound (the brake light comes on when it gets hard or falls to the floor) . The cruise works well for about 10 min, then it starts getting confused. I will drive with my foot for 5 min, and it works fine again.

So I decided to do a vacuum test. Well at idle connected to booster it holds 30 well. if you get on the brake it drops down a little, more you push on the brake the farther it drops. the thing it barely comes back up if I hold the pedal down, just kinda stays low. now if I get off the pedal it goes back to 30 really fast and stays there till I push the pedal again.

Now I am assuming its my brake booster right? since if no breaks applied it holds steady 30. What do you guys think my problem can be. Sometimes breaks work very well, sometimes I have to use my both feet especially if I sit it traffic the pedal becomes solid but truck keeps rolling. Please help me and thank you. I learned a lot from this forum already just by lerking.

 

Yes, replace the booster. It's best to replace the master at the same time, usually the booster fails because the master was leaking brake fluid into it.