96 Ford 350 Diesel 4x4

Hi automotive advisors,

I have a 1996 Ford 350 4x4 Super Diesel Truck that drives great. When the truck slows down to 30 mph's or less, the emergency brake light turns on from the dash and the brakes lose pressure. As the truck speeds up again on the highway or something above 30 the emergency brake light goes off on the dashboard and the brake pressure is restored. What is causing that to occur?
P.S The emergency brake is not in the on position:smile:

You make it sound like you have air brakes lol. Have you had the ebrake adjusted? Maybe the cable is caught or siezed up. When you say the brakes lose pressure do you mean you have to push the pedal harder?

Yes, the brakes are harder to press down. I hadn't thought of adjusting the emergency brake. Thank you. If there is anything else, please let me know.

This truck has a vacuum brake booster, right?
If yes, the booster is probably leaking vacuum, enough so that the engine-driven vacuum pump only overcomes the leak when the engine is revving faster, as you pick up speed.

You could probably just rev the engine while holding the brake(not in gear). You should feel the pedal sink?