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.

 

Some newer diesel trucks use the power steering pump for brake hydraulics but the brake system on your truck should be the same as a gasser. It probably has dual piston calipers up front though. When I had a problem similar to this it turned out to be a hole in one of the hard lines on my rear axle. If you arent losing any fluid though, that may not be your issue.

 

I would say to check the brake booster, but the usual result of a bad brake booster is a hard pedal, not a soft one. Does it seem to pull either left or right when the pedal gets soft? Is there any loss of brake fluid?

Have your replaced any of the braking system in the past or is all of it original (besides the master cylinder)?

 

Just how hard are you pushing those brakes for this to happen? From your description it sounds a bit extreme. If that's the case, you may be dealing with a severe case of brake fade. If you're building up too much heat, you can actually make the brake fluid boil and lose functionality completely. Are you running the right brake fluid? If the fluid is correct, brake fade can be improved by changing your braking material, and installing rotors that will transfer heat more efficiently.

 

Also check the thickness of your current rotors to make sure they are within specs.