So I reached out to injectorguru. He was helpful, pointed out the problems with Bosch nozzles, and pointed me to sources online for Monark nozzles. Time was more of an issue than price, so I went with domestically available Firad.

I decided to re-rebuild my injectors because, as it was my first time, I did not have great confidence that I had done it correctly the first go round.

4 of my injectors went together easily and with re-shimming all pop right around 1675psi. Leakdown is 350psi @ 30 seconds post pop, so I think I'm good there.

My last injector has a serious pre-pee issue. First shim had it popping at 1800psi, but peeing from 1600. Second shim try dropped pop to 1550, but pee began around 1300psi. I did not disassemble and clean the nozzles (boddy/pintle), just dropped em in. Reading some more on pre-pee it seems 1) I should have done that for all of my nozzles, and 2) this could be the issue with my pee.

Thoughts? I'll give cleaning it a go in the morning. If that fixes it, should I pull and clean the others at this point?

Any other suggestions on pre-pee?

I'll also say that I'm happy to pay injectorguru to rebuild these if I can't make it right. I just want to give it one more shot on my own.

For me:
satisfaction of doing it myself >>>> $$$ + peace of mind knowing a pro did it

My personal preference with new nozzles is to disassemble them and wash with a strong solvent like lacquer thinner. They're shipped with an oil to protect them and it can screw up things during your pop calibration. Dip both parts in clean diesel and gently reassemble before putting in the holder and assembling the finished injector. Remember that the injector nozzles are precision machined parts, don't mix and match pieces, keep them as matched sets.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Just read your suggestion and this is what I had been up in the shop doing (though I sprayed with carb cleaner to clean).

I still could not get this new Firad nozzle pin to gravity-drop all the way into the nozzle. Nor would the nozzle spin on the pin when held horizontally by the top of the pin.

Just to see what would happen, I swapped out the new Firad in this nozzle for the Bosch (that I had driven about 20 miles on). This one gravity-dropped & spun perfectly. Put that in and I STILL GET pre-pee. This suggests to me that it is not the nozzle that is allowing/causing the pre-pee.

I had actually done a good bit of lapping on this nozzle (600/1200/2000) and am hesitant to remove more metal. It looks smooth...though there is some pitting in the bottom face of the spacer.

I don't concern myself with the "gravity drop" test on a brand new nozzle. The tolerances are *SO TIGHT* that they frequently don't pass that test. The pin and nozzle *MUST* be lubricated with diesel fuel before assembly, if you don't, it'll score and will be permanently damaged.

For a nozzle holder with chronic issues, check the lip where the injector nozzle sits in the lower half. If there is excessive rust, carbon buildup, or corrosion, it needs to be cleaned and dressed so that the nozzle sits square in the holder. If it is cocked even slightly, you'll get the pin binding up and poor spray pattern or "pee" problems.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thanks so much for the advice @Diseasel300.
I've checked the nozzle seat in the lower half and cleaned with a wire brush. There was no rust and no obvious carbon build up. It is now soaking in thinner.
I did notice that this surface is not flat/level. It seems it is 'stepped' down from out-to-in. I can feel it with a small punch/point.

I checked one of my other 'good' injectors and it too is not flat, but seems to be less pronounced than my 'problem' injector. Ive tried this injector with 2 different new nozzles and the same problem plagues both. I'm hopeful the soaking and another good cleaning will do the trick...but I've already ordered a Bosch reman injector to be on the safe side.

Just to follow up. I replaced the problem injector with a rebuilt Bosch (put my new nozzle in there instead of the Bosch it came with). Popped perfectly and held great pressure (held solid at 1500 PSI long afterpopping).

Car runs well now, but a bit more smoke than before. A bit more rattle on startup for sure. Settles down after warming up.

One thing I wonder about is the leakdown on the other 4 injectors. They all pop perfectly but after 30 seconds they have all dropped down to about 1250 PSI. Thoughts? It seems a small price (~$200) to get rebuilt injectors if leakdown means that much to the running condition of the car.

(FYI-I'm soon to perform my first purge and replace the motor mounts.)

Fuel lubricates the Injector Nozzle and that means fuel passes through the clearances and why it is not going to hold pressure forever.

The test to see if the pintel/needle inside of the Injector Nozzle is seating well is to manipulate the handle on the pop tester to be sure to get all of the air out of the Injector and then slowly bring the pressure up to 200 psi less then the pop pressure.
If you have an excellent injector nozzle no fuel at all is going to come out. If you have a less then excellent nozzle some fuel is going to leak out.

It does not appear to be in the Factory Service Manual but there was a spec that for x amount of seconds at 200 psi below the pop pressure you were allowed X amount of drops. When I worked in a fuel injection shop I asked my Boss until I had seen enough of them to know good from bad.

The more important issue is after the air is out you work the handle extremely slowly and you can watch the pressure gauge. As you approach the pop pressure an excellent nozzle will not leak any fuel out of the end and a bad nozzle will instead of suddenly popping will pee out the fuel when it reaches the pop pressure.
You would like the see the gauge needle rise up to the pop pressure and then suddenly loose pressure and the needle fall.

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84 300D, 82 Volvo 244Gl Diesel