Hello. I am having a hard time figuring out what is wrong with my brakes. I have an F250 1986 Lariat 4x4. Under normal conditions, ie driving just the truck with no trailer on an average road or highway, I have no problems other than a sort of spongy feeling in the brake pedal. I was told this is normal for a diesel. However, when towing my trailer in heavy traffic or descending a steep hill without the trailer the pedal goes lower and lower to the floor until the truck looses the ability to stop in a safe distance and my only solution is to shift down to a lower gear and pull over as soon as possible. If I let everything cool off for a half hour I get braking back. I talked to a neighbor who is also a mechanic about it. He installed a new master cylinder, bled and refiled the system and checked all the hoses and connections. He said it should be good to go. On my way back to the city I took the truck up a steep mountain road, about 5 miles, and then turned around and drove back down to test the brakes. The same thing happened. The more I used the brakes the worse they got until the pedal went to the floor again. I shifted down and when I got to a level place near the highway I stopped, cooled down and braking came back. I turned around and went back to my cabin. He checked his work and couldn't see any leaks etc.

Has anyone an idea as to what is going on and what else he should check. He is not a diesel mechanic. Are there things about repairing the diesel that are different? He has checked both the front and the rear brakes and says he sees no problems.

 

Sounds like the vacuum pump is going out.

Put your heater on Defrost, and select between floor and dash, floor and dash several times, it should go from one to the next right away. If it takes abit of time to go from floor to dash, dash back to floor etc. The vacuum pump is going out.