How to pre clean a very dirty LP before Ultrasound Machine?

Drumbeat, your US cleaner cannot clean 750 filthy records unless you want to change bath at $3 or $4 a pop. As bad as you state condition of records you cant start with agitation. You have to blast it off,if scared of steam you need to break surface tension with chemical let soak flood with distilled water then lightly vacuum until you can use a brush and finally get it in your machine.Living in NYC (my brother lives there) may not have DIY access. You could order vpi wand, $15 shop vac and f****in duct tape, a $10 lazy Susan to spin record. Music direct has chemical and wand

I also been using steam cleaner with VPI and DIY ultrasonic for years. Be careful if buying one, don't buy the cheap ones at the end of there life span they will start to over heat the water and will or could damage your records. You could also look at Merrill record cleaner. I made a DIY one and it works very well, on bad mold dirty records,better to use it outside. It's a little messy. If you do DIY be sure to make a rubber gasket under pvc cap to protect label.

Sorry, it's called GEM Dandy record cleaner made by Merrill. 

Amazon,eBay 

Reagent Lab water about $9 bucks a gallon at Carolina Biological where I bought it. Or distilled water at any pharmacy

$ 9 water is final rinse on vacuum machine, not ultra sonic, I found steam clean works well for mold. I have a shop vac with pvc pipe I cut slit with drimel put mofi cleaning pad on it (DIY wand) put album on old broken turntable with cork mat and then steam and vacuum for nasty dirty mold albums. I did 90 nasty albums few months ago, I found this method goes pretty fast. I don't want to put these albums on my machine or its wand. Just a warning your house will smell like mold may want to go outside.

Will do sleepwalker, looked it up your correct. For me if album has mold spots I through them away. If just dirty and mold smell, steam, machine then ultrasonic.

enzyme cleaner, audio intelligent, mofi 

I don't know if you can control temp, try to keep it a couple inches away. I use distilled water in steamer. There are you tube videos out there. I'm not trying to make it complicated but you still need a way to remove dirty steamed water. Only way I know is to vacuum. i would think a cloth rag will scratch album and you would need a lot of rags. Vacuum wand may scratch also but if you flood album with distilled water with spray bottle over steam water and vacuum lightly puddle of water not record your odds are better for less damage. This process is going to be a pain in the a$$ but there is a clean pristine record under all the crap.

Forgot to add apply your choice of enzyme cleaner to record with spay mist bottle and let sit for 5 or 10 minutes before steam. 

Mobile Fidelity enzyme cleaner $29, Audio Intellagent enzyme $37, both at Music Direct, there both good, 750 records I would go cheap, be sure to rinse off enzyme cleaner off with water or a record wash cleaner. Music direct has 50 record sleeves a brush and record wash for package deal of $39. That would through u into free shipping deal.