Degassing andHuma reg install Fx SL .25

I recall reading a recent thread in which Chuck and others helpfully walks folks through the process. I am in the same situation as you having received the reg but just not got round to installing it in my streamline. From memory the thread title referenced wildcats and streamlines. A quick search should locate it. Let us know how you get on. 
 
Don't fire it empty something will break when the pressure gets low enough. I've not touched a streamline but the Huma website seems to suggest the block is similar to the wildcat in that there is a port on the side that will let you bleed to air (not officially the right way but def the easiest). Specifically, they sell a gauge to fit in either of those rifles to measure the regulated pressure. Pull the stock and see if there is a brass grub screw.

This is the picture (of a wildcat) but the page is for the streamline too. See pic for where the gauge would screw in.

https://www.huma-air.com/FX-regulator-tester-gauge

The rest is super simple once you do it the first time and the Huma 'fitting instructions' will help. Getting the existing reg out of the tube will be the challenge. The method I use has been removed from youtube so I'm hesitant to discuss here but if it's like other fx regs you might be able to grab it with long needle nose pliers. 
 
To all it may concern,

To let the air out of your Streamline you simply remove the dust cover from the air gauge, then loosen the gauge about 1/2 turn or more. The air will come out of a small hole in the neck that the gauge is screwed into. To do this you will need an 11mm open end wrench, but it has to be a very thin one or you will have to make it thinner using a belt sander or grinder, because there is not much room between the bottom of the gauge and the neck it screws into.

You will need a stout pair of needle nose pliers to pull out the FX reg, a micro torch to soften the Loctite on the hammer spring nut, and a chronograph to shoot over while adjusting your hammer spring tension. Be careful not to use too much pressure on the Allen wrench as you turn the hsn...you do not want to strip out the hole in the center.

Once you remove the stock and let all the air out just grab the air tube firmly with both hands and unscrew it from the receiver block. To remove the FX reg just grab the red nut with the needle nose pliers and pull it out! Now all you have to do is slide in your Huma and the plenum ring, reassemble your rifle, and fill it with air! The Huma reg goes in with the brass adjustment screw facing forwards towards the muzzle. You will also be inserting a 7mm plenum ring into the airtube at the rear of the Huma that faces the breech block.

All that is left is to shoot over a chronograph and adjust the hammer spring tension to the pinnacle of performance. Simple!

Best regards, Chuck