Diesel 2010 SMB 4x4.
I did find this post on the automotive forums.
http://www.automotiveforums.com/t521713 ... iesel.html
In a "normal" vacuum assisted situation, you have the maximum vacuum available during braking with the throttle plates closed on a gasoline engine. Well these (and most) diesels do not have any throttle and cannot make engine vacuum at all.
When you depress the brake pedal, a small valve is opened allowing stored vacuum to be vented to a diaphram. Doing this lowers the vacuum in the storage "tank" that is part of the booster assembly. In a gasoline engine, there is so much vacuum pressure and volume available during closed throttle deceleration, that you never even notice any vacuum was "used" as it is replentished immedaitely by the engine.
Diesel engines installed in vehicles with vacuum assisted brakes all require some form of vacuum pump to be fitted. The most common way is to drive them off the engine belts where the amount of vacuum is related to speed and time (it takes some time to produce or replentish used vacuum).
So, when you have a truck like yours with an engine driven vacuum pump, and you are aproaching a stop, you remove your Rt foot from the accelerator (GOOD thing), the engine speed drops (reducing the vacuum pump capacity), and you depress the brake pedal (which causes the vacuum in the booster to drop). As you are slowing down, the vacuum pump continues to run and replentish the used vacuum, THIS causes the pedal to "sink" because as vacuum is replentished there is ever-more "assist" available to the brake system.
The brake booster is basically a canister with a diaphram inside that is sealed on one side, and vented to the atmosphere on the other (brake pedal) side. The sealed side is placed under vecuum by the engine or vacuum pump in this case. The assist available (in PSI) is the area of the diaphram times the pressure difference between the two sides ofthe diaphram (A*Pd). Since the area does not change, but the Pressure does (and does so MUCH more in a system with a vacuum pump) you will have more assist as the engine driven pump replentishes the used vacuum.
Per the FSM, if the vehicle stops "normally" DURING braking, and the pedal does NOT SINK DURING braking, but ONLY while sitting stopped, there is not "problem".
Sounds like the issue I was having.
Will test again today in low, high, and 2WD.