Power brakes, fine. alternator - just tested friday and tested good, its fairlynew! (unrelated problem) which belt is the vacuum belt, which is the vacuum pump?

 

look for duble pullys mine vpump is under altnaer

 

As stated vaccum pump is under your alternater, and is driven off the double pully. If the belts are tight you might want to do a vaccum test of your Vaccum Regulator valvue black plastic thing on driver's side of IP has two vaccuum line running to it. I found the test on this site and did it on my 88. Mine wouldn't shift into 2nd, I adjusted it and it works fine now.

 

Anyone have a link on how to test it? tools needeD? etc...thanks!

 

first of all does it shift normally from 1st to second
if it does it isnt a vaccum issue
how is reverse , is it normal or does it slip in reverse
several internal issues can cause the loss of third gear
a c-6 in a 1990 is not that uncommon ans is normal on commercial units especially uhaul they specified c-6 to reduce the cost of the truck and they dont worry about your fuel economy when renting