I'm an idiot!
I didn't look at your post closley enough. Diesel engines don't use vacuum! Look at the following quote.
"HYDROBOOST POWER BRAKE SYSTEMS - Diesel engines do not create enough useable vacuum to actuate the vacuum power brake booster. The alternative is a hydraulic-assisted power brake boosteror hydrobcmst. This system is currently found in the1 1/4-ton CUCV and the 3/4-ton CUCV Blazer, both powered by the 366 cubic inch V8 General Motorsdiesel engine. Both units are found throughout the NCF and at some public works stations. The hydroboost uses hydraulic pressure developed by the power steering"
Check the fluid first. If it's full the pump may be shot