I think I covered all the replies. A million thanks to all of you for your comments and feedback. And especially to TwinTurbo for writing up the procedure. Taking the time to put together such a document just for the sake of helping others is really generous, considering you're already weighed down with the huge undertaking of a valley pan repair. Its got to be one of the most complicated jobs.

Here's one that is a nice set:

Cooling System Tools Astro 7858 Astro Pneumatic 7858 - Universal Radiator Pressure Tester

The key with these, is that there are many different sized radiator caps...so you need the right adapter for your car. AST (Assenmacher Specialty Tools), is well know for making good quality adapters, and you could buy just the one you need, (they run about $50 ea.). Both Stant and Snap-on sell the pump/gauge thing with only one or two basic adapters. Blue point sells a set of adapters. But the set referenced above does appear to be pretty complete, and is the one I used to test mine, (it came off a MAC Truck).

Here's the Snap-on version:
Pump:
SVTS262C, Tester, Cooling System

Adapters:
SVTA17000, Set, Cooling System Adaptors, 17 pcs.

Just the adapter for BMW:

TAB10345, Adaptor, Radiator, (for SVTS262B Cooling Tester)

As you can see, the adapters can get VERY costly. And the above Snap-on one's are metal/plastic. The AST are all aluminum, and a bit nicer. Both USA made. The first set linked are all metal as well, and pretty nice IMO, but totally Chicom.

Another reason to test the system cold is that sometimes it will only leak cold....

Pump the system up to about 15lbs, and leaks should show up. Except if it's leaking into a cylinder. Mine was pretty much holding pressure, but a slight defect in a hose showed up as a VERY small dribble from a connection point. If the system does not hold pressure, and there's no sign of leaks, time to pull the intake. Assuming there's no sign of smoke in the exhaust. There are ways to test for bad head gaskets, (chemically test for cross contamination of oil to water), such as this:
GDCT16, Tester, Combustion Leak

As tight as the V8 cars are in the area near the firewall, a fibre-optic camera is not a bad tool to have on hand as well; not to mention a lift.

I've got mine from HarborFreight:
Radiator & Cooling System Pressure Tester Kit

strange, it's rather well made unlike most stuffs HarborFreight carries.

If you don't lubricate those O–rings there's just a chance that by pushing the pipes in you would push O-rings from their grooves further into accumulator.
While I was dealing with a multiple leaks which appeared after VP gasket and rear collector gaskets replacement,I found the way to release a bit of pressure after each drive,so if it leaked only because of excessive pressure which pushes through a weaker point.Undo the expansion tank cap briefly and tighten it up.
I saved a lot of coolant by doing that.
Another thing to consider is....did you refill the coolant to correct level? Was your expansion tank cap replaced.
If the level is too high and a cap isn't working properly at releasing the excess pressure,pressure is gonna escape through some other spot.And probably could damage one of the hoses or so on...there are many plastic spots etc.

Obviously Chicom, as with everything at HF, but are those adapters plastic or metal?

The adapters are injection-molding plastic, very nicely done w/ good tolerance. The nipple are metals as you can see. Yeah not as good as the tools you have but for the price with many adapters for different makes/models it's something to consider for ocassional uses.

not the only way I am sure, but to assembly the twin pipes, this is what I do. I lbricate the o-rings with mild soap, some rubber just react differently with mineral base lube. use very little of soap really.
the tricks for me is:
1) Seat the pipes to the manifold ends first.
2) I Dremel the head off of long hex screws from Home-depot, put three of this (there are 6 of them total screw holes for water-pump assembly), so now I have a guide where the WP can be put on and will only glide into place perpendicular to engine block. For me this is the trick, no more pinch o-ring cause WP will not goes into place in an angle. Put 3 screws onto remaining screw holes, then remove the homedepot screws, and put the original screws back in their place.

Hope that make sense.

FANTASTIC INFO. Many thanks. I wish I had money to burn. I'll look for cheaper alternative...

Yeah, I clearly see the point of lubing the o-rings. A most painful lesson if that's the case. I bought some coolant dye and UV light. I'm really, REALLY, hoping its coming from somewhere else now...like a hose. Otherwise, I guess I'm going back in...for two days.

That's what eBay is for...used profesional mechanic's tools at a fraction of new cost! You can get a Stant coolant pressure tester for about $35-40 used, and then all you need is the BMW adapter. The AST adapter you need will come up once in a while. Or you could get the Snap-on tester used for about $65.

I have a VERY well equipped garage, and most of the tools came from eBay or Craig's List.

FOLLOW UP
 

UPDATE:

I got the Staton cooling system pressure tester (with BMW adapter) and it worked beautifully. Much to my utter disappointment, it helped me determine that indeed my valley pan cover was leaking AGAIN. I made the mistake of using RTV Black gasket to make my own gasket for the original valley pan cover. Perhaps not such a bad idea, but you really have to get it on there just right, and more importantly let it set up properly before you install it. I don't think I got the cover back on there right and/or it was too cold (40 F or so) so the RTV never got a chance to cure before I ran the vehicle. Lesson learned.

I'm back in there, and will be installing a factory BMW valley pan cover with gasket. I'll be using a torque wrench this time too. Wish me luck...