Is my booster busted or is this normal?

It's-a me, mister wagon man with the crappy manual brakes.

I ended up installing an 8 inch dual diaphragm booster from MBM in conjunction with a 1 1/8 GM truck MC.

I am still using my old pedal ratio, but I am wondering if I might be getting full-whack boost out of this thing from the get go. Basically I barely need to rest my foot on the pedal to start braking, and pressing into whoa-crap zone doesn't take much more effort.

I have tested the booster's check valve (works). When the car is running I get a steady 20+ inches of vacuum on a gauge. The gauge hardly moves at all when I press the pedal - if I pump it it will jiggle a little but that's it. Shutting the car off gives me an assisted pump or two as it should. Checking the stored vacuum level with a gauge shows it drops about 5 psi per press/lift.

However, I can't get it to hold (much) vacuum with a mityvac. I have to pump pump pump pump to get the needle to settle somewhere off zero. Then keep pump pump pumping to keep building vacuum.

If I release the mityvac and then pump again from 0, the needle goes straight up to the last point where it wanted to settle the last time, but will then not want to pull much more on the next pump. E.g. pump it to 5 with the needle jiggling the whole time, release, next pump gets straight to 5, pumpy pumpy, get up to 8, release, needle swings steady to 8 before jiggly time again, etc). It gets harder the higher up you go. Again, I have a running-engine indicated 20+ of vacuum, but I can't pump this thing up even to 15 by hand.

But to be clear, it holds vacuum - both attached to the car and off of my mityvac alone. I just have to really work the mityvac to get vacuum to build, which seems to create some kind of rising set point as I go at it. The highest I can pump to will remain steady, even if I just go pumpy pump and give up at 5" - it'll stay there until I try for more.

Is this normal behavior for loading a booster with a small handheld pump or does this indicate something internally goofy?

I have never successfully tested a brake booster with a hand vacuum pump. What you are describing sounds like normal vacuum behavior. It also sounds like the pushrods may be adjusted a little too long. Try shortening one or both a skosh and see if that gives you a more linear pedal.

Your pedal ratio is way wrong.

Trying to pull the 40 or so cubic inches of air out of a booster with a mityvac hand pump is like draining a bathtub with a tea spoon.

The mighty-vac produces it's vacuum at a fraction of the volume that a running engine does, and I find that it's fittings never seal that that great. I'm not surprised that it's giving you trouble trying to operate a power brake booster.

Haha, okay - with the kinds of oversights I tend to make on first goes I figured it wouldn't hurt to ask.

I have the booster pushrod adjusted just out of contact with the MC at rest. I originally thought I had too much free play, but I ended up adjusting it out to the point where the fronts were clamped from the get-go.

The pedal ratio is currently 6:1 with the factory setup - if I drop the pivot hole an inch I'll get down to 4:1.