I don't own a Milwaukee regulator, but I have several Ideal 52-1-12 needle valves. They have female 1/8" NPT ports on each end which require male 1/8" NPT adapters at each end. The safe way to set it up is to use a 1/8" NPT check valve between the Milwaukee bubble counter and the Ideal needle valve. Between the solenoid and the Ideal needle valve, you can use a 1/8" NPT hex nipple. You can get one at Lowes, Home Depot or a plumbing supply.
These are the ones at Lowes and Home Depot made by WATTS:
No. 122 Hex Nipple
122-2
1?8" MIP Both Ends
A-714
You can get a male 1/8" NPT check valve from Clippard, Swagelok, Parker, eBay, etc. Get a low cracking pressure check valve.
This wouldn't be my first choice, but it will work. Here is an eBay Clippard female 1/8" NPT check valve with a 1/2 psig cracking pressure. If you get this, you will need 3 of the 1/8" NPT hex nipples. Two will go on each side of the check valve and one will go between the solenoid and Ideal needle valve.

MORE NEWS