Because diesel engines lack the vacuum that gasoline engines produce, they are commonly retrofitted with vacuum pumps to produce the necessary amount of vacuum to assist in braking.

There are pretty much two ways to produce vacuum for braking on a diesel engine, an electrical or mechanical device.

1) Many engines are equipped with belt driven pumps.  Mercedes Benz has a small belt driven pump that can be retrofitted to a belt system easily.  Many Toyota engines have alternators that also serve as vacuum pumps.  They have an oil feed & return line for lubrication of this unit. 

2) Other engines have "bolt on" gear driven units, such as Isuzu 4BD1T's and Cummins 4BT & 6BT Engines.  These engines have both gear driven vacuum pumps and power steering pumps. 

3) You can use an electrical device like this.  Seen in the picture is a Gast Vacuum Pump # DOA-V120-JK.  They come in either 12 volt or 24 volt systems.  You might want to look into something like this.  You can also use a device like this (see ** for information on it) to turn the pump on and off:

Taken from this pump which appeared on Ebay (not sure how long the link will be valid): 

**This is a used vacuum operated electric switch which turns your vacuum pump on and off.  If this switch malfunctions, it can cause your pump to run all the time, run erratically, or not run at all.  This is located on your firewall under the plastic shroud, just under the hood seal.  It has the wiring pigtail and the rubber boot enclosure.  This is for the diesel model only.  

These are getting pretty hard to find.  This will fit 1984 to 1987 Ford Escort and Tempo, Mercury Lynx and Topaz with 2.0 Litre diesel engine.  This may also fit the 1984/5 Mazda 626 2.0 diesel, but you check for sure before you bid. Switch can be used for other applications need ing a device to turn on and off a vacuum pump.