While backing up a trailer or parking the truck, just a couple of pumps and my brake vacuum is gone. Is there a booster that can be replaced or is this common. What is the solution?

 

The brakes on a diesel are hydro boost, not vacuum.

You have a hydraulic control unit that has few moving parts to control it.

First thing though, check the brake fluid and make sure there is some.

Second, is it ONLY doing it backing up and or at low speed?

If yes, it sounds like there is a valve that may be stuck in the HCU.

But, before I go looking at the HCU, I'd do the simple part of checking the fluid and bleeding the brakes.

 

No, the pedal doesn't go to the floor, it just get hard. It feels the same as when the vacuum is used up after a couple of pumps when the vehicle is shut off. If I am backing a trailer and use the brakes near the stopping point, the pedal gets hard.

 

Okay, then that sounds like a stuck / sticking valve in the HCU.

Hate to say it, but it's a trip to the dealer for this one.

There are no service parts for it, it's replaced as a unit.

 

Now don't get me wrong, the brakes work great, it's just they lose boost a little early I think, maybe they are all like that but I will check into it.

 

There are a few things that make up the HCU and any one of thm could be causing the issue, but more than likely it's a sticky valve.

You have the electric motor, the pump and the inlet and outlet valves.

Those valve operate in 3 modes. Pressure rise, pressure hold and pressure decrease.

Each one has a specific point that it opens and closes in one of those modes.

Do me a favor. Next time it happens, see if you notice if your grid heater is on. I'm thinking it may be the heavy current draw from that, the extra current to the trailer lights or brakes (if it has them), and the draw on the motor that may be reducing the power that the motor is getting and therefore applying to the HCU motor.

Only way to find out for sure is get it to the dealer and have it hooked up to the DRB and get the live readings.