I hope that this illustration of how the holdopen works will assist you in repairing your Luger. The basis of this diagram are the excellent illustrations in the
Luger Mechanical Features E-Book by Gerard Henrotin. A fine book that will explain much. It only costs about $7 US to download and will provide much explanation about your Luger. I highly recommend this book to new Luger owners.
You can obtain this E-book here:
http://users.skynet.be/HL-Editions/ebook/future.htm
Now onto the explanation:
The top illustration shows the holdopen device installed in the Luger frame (red portion does the work, the spring keeps it in the down position when there is not an empty magazine in place.)
When the last shot is fired from a magazine, or the toggle is drawn back with an empty magazine in place, the magazine buttion pushes upward on the holdopen device placing the end into the notch on the underside of the breechblock. Once the magazine is withdrawn you "should" be able to pull back on the toggle and release the holdopen device so that the breechblock can move forward and close. If this is not happening, perhaps your holdopen spring is broken or missing or improperly installed.
Open the takedown lever... then Holding the frame in one hand and with the other hand pull back on the toggle as far as it will go and
with your THIRD HAND press down on the exposed part of the holdopen with a pencil or screwdriver(not your finger).
The holdopen is shown in red in the top illustration) and you should be able to lower your toggle and disassemble your Luger.
Good Luck and let us know how you make out with this... I hope this explanation has been helpful.