Those familiar with Eichler-built homes are likely in-the-know about the dreaded black mastic. You know, the goo that remains on the floor even after the affair of removing the floor tiles themselves -- the stuff that has to be removed before new flooring is installed. Fortunately, in our case, the tiles had already been removed, leaving only the concrete floor with mastic residue.
While there's sure to be may different techniques (like the one listed on their website), we had great success swabbing it on with a stiff acid-brush (on a handle -- wearing suitable shoes as I did have to walk across the surface), letting it sit for an hour-or-so then scrubbing some more -- you can actually see spots not yet softened when you're scrubbing around, so you get a good sense of when to stop.
After it loosened the mastic, I flooded the area with an alkali-based "purple" degreaser/water solution, scrubbed some more and suctioned it up with a Rigid 12-gallon/5HP wet-dry vac -- so far one of the best purchases yet. The Bean-e-doo emulsifies very quickly, even in plain water. We also tried a few other techniques that involved cat litter, squeegees, scrapers, etc.
The floor came amazingly clean, but just to make sure, we flooded again with a slight bleach solution (in the hopes to neutralize the cleaner as well as to help guard against mildew/mold/odors)... so far, the results are amazing considering how simple the process is... see below.
The downside is that while it was great at removing the black mastic, it didn't put a dent in the white/tan carpet glue in some spots on the floor and there are still stains left from the mastic... and the room smells a bit like Italian dressing, but it could be worse.
The process is, however, time consuming and it isn't removing all of the stains not matter how many applications, techniques, etc. Given how much we have to do, we might need to resort to a quicker method, but this is certainly a good option for many folks. Our friends Darren and Leah rented a wet-grinder, so that's something we might check into as well.