Here's the post on pressurizing using a brake fluid pressure bleeder:
https://xoutpost.com/bmw-sav-forums/...ystem-e53.html
On other cars of mine, I've made an interface for the brake system out of an old brake fluid reservoir cap, so that's what I started with here = a coolant reservoir cap. But the bleed screw interface is a lot easier on these cars. And as
80stech mentioned above, the bleed screw option allows testing of the reservoir cap, if you're brave enough to pump to 2 bar (30 psi). I partly did it for that reason, but did not go past 20 psi. 20 psi with the cap not leaking was enough for me to conclude the cap was not faulty.
In general, it will be much easier to check for a coolant leak when the system is pressurized. Cold, not dangerous, take your time with a flashlight, disassembling as needed until you can pinpoint the source.
Consider yourself lucky that the coolant is leaking externally vs. internally as many of us eventually face.