Tossing in some ideas.
1) Per above perhaps pull your rear drums, inspect if good, put back on and adjust the shoes till they barely drag. Confirm when you push the brakes they are working.
2) I would pull the front tires and inspect your front brakes too. If your rotors are all pitted with rust or your pads are thin these can effect your stopping. Confirm when you push the brakes they are working.
3) Per above confirm what your vacuum pump is putting out good vacuum, don't guess. A $20 vacuum/fuel gauge at the auto parts store will be all you need to know.
4) Generally the feeling of having power brakes comes primarily from the brake booster helping you push the master cylinder. If not working or poorly working you will not feel like the truck has good brakes. If all checks out this is likely your issue.
5) If the truck has old, worn, heat damaged (Glazed), thin rotors or drums, and prior say rusty brakes the contact surface is dimished and the heat build up may be adding to the problem. The only thing to do is replace.
6) As I said above, make sure the brakes are working. I helped a friend get his mom's VW Vanagon back on the road a couple of years ago. The front brakes would not engage, only the rears. Against my logic I concluded after testing it must be the rubber hoses. Whammo, two new rubber hoses and brakes where fixed. (Prior mechanic put a master cylinder in it thinking it was that.) So if one of your brakes is not moving when pushing the pedal crack the brake line connectors starting at the master till you determine where you are loosing the brake pedal pressure.
7) Crappy/cheap pads, shoes, rotors, and drums all diminish your brake system. Only buy high end brands when you do rotors or drums and always go with high end Semi Metalic or Ceramic pads. Let them eat your rotors and drums if that is even of concern for you. I always tell people replace your rotors and drums each time. It always leads to less braking and steering wobble down the road if you do not. So if everything is working (Master Cyl, booster, vacuum pump, calipers and wheel cylinders.) I would redo your rubber brake lines, calipers, wheel cylinders, pads, rotors, shoes, and drums with good parts. (Lines, calipers, and cylinders because these trucks are getting long in the tooth.) I would bet your braking will dramtically improve!(Again if all else is working as it should.) I have seen it a half dozen times myself, going with upgraded brake parts brings 15-30% improved braking over the aging stock brakes. Even better results when replacing a cheap Midas, Goodyear, and Pepboys style advertised brake special brake job. (Cheap pads/shoes and just turned (Maybe not) the rotors and drums.)
8) Many have done a hydroboost upgrade and have been very happy.
Also keep in mind these are 80's-early 90's technology. So they are not going to stop like a modern F250. So just know these do stop well when working, but don't expect them to stop like a car or a modern day truck.
My truck was stolen last year. I replaced with a used truck and it stops but not like it should. So I will be doing a complete brake job front and back this summer to improve what was likely a brake job of cheaper chicken pads front and rear.
My local parts supplier has a product from DFC that I have used on my Cadillac and former F250. This brand is very well made, very stought, and the braking is very smooth and fast. So I plan to source those parts and put that on my new used truck. I would imagine 20-25% better braking and more predictable braking.
Hope that helps give you some ideas of what to test and how to move forward.
Let us know what you find, did, and the results!
