rebuilding regulators?

I have 3 leaky regulators, I'd like to combine 2 or 3 of them to be a multiple pressure setup so my questions are:

how do I combine them, just a brass nipple pipe and thread tape?
They all leak through the hole in the bonnet. is it worth rebuilding them, where should I look for parts? I took the black and blue ones apart and cleaned them, there was white chalky stuff in there, the rubber diaphragms seemed okay, put them back together and they still leak. How tight does the bonnet need to be?

Here is what i have:
black:Norgren model r81200lnka, co2, USA
blue: North American Dispense systems 70012, co2, nitrogen, air, USA (co2 mount)
light blue: RegO 1573L, USA

northernbrewer.com and midwestsupplies.com both carry regulators and parts. Try there and good luck.

I have an old leaker which I took to LHBS (Steinbarts). They have a fully staffed repair department and the guy took it apart and told me it was fine and did not leak. It still leaks and I decided not to waste any money on the repair kit. But I do think it is easy to fix in most cases. Some sources for parts are,

http://rapidswholesale.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=8P490&Catego ry_Code=Faucets

http://www.chicompany.net/regulator-repair-kits-840.html