I suspect that the o-rings are making a seal when the tank is filled to high pressure, but that as it gets down to lower pressure it is not enough for it to make a tight seal. The air tube has a vent hole (or breath hole as it is also called) that allows the regulator to compensate for atmospheric pressure. The regulator sits in a sleeve called the regulator "seat"...on the regulator seat there are two large o-rings. There are two smaller o-rings on the regulator itself. The vent/breath hole of the air tube is centered between the two o-rings of the regulator seat when the regulator is installed in the rifle.
I am going to suggest a test...fill your air tank up to 230 bar and put some soapy water on the vent hole to see if it makes bubbles... if so then one or both of the o-rings is not making a good seal. Also put some soapy water where the air tank screws onto the rear end cap on the breech block to check for a slow leak. Remove your regulator assembly, pull the regulator out of the regulator seat and inspect the two small o-rings for nicks of any kind, then inspect the two large o-rings for any nicks or damage. Even if they look perfect they may not make a perfect seal if they have gotten hard. I recommend you order new o-rings and install them and throw the old ones away. When you inspect for damage if you see what looks like a small half moon shaped "bite" out of one of the o-rings let me know and I will tell you what causes that. Otherwise just replace the old ones with new ones...and make sure you use a little pure silicone grease on each o-ring to lubricate it.
Here is the Exploded Parts Drawing:
http://www.fxairguns.com/wp-content/uploads/2016/12/Streamline-Exploded-drawing-pdf.pdf Just to add a little confusion it calls for two different size o-rings for the regulator seat...reference location G16 & G1. You will see in the parts table it calls for: G16 19497 O-ring 23x2,5 NBR 70 & G1 19497 O-ring 23,3x2,4 NBR 70. That is two different sizes and makes no sense at all, and so it is an obvious typographical error. I happen to have the Streamline and Wildcat both, and they both use exactly the same FX regulator and regulator seat; and therefore the correct size o-rings for the regulator seat should be Part Number 19497 O-ring 23,3x2,4 NBR 70. Neither the Streamline nor the Wildcat parts diagram shows what size o-rings to use on the regulator itself, but the people at FX USA should have them. If they don't then AOA probably does. The o-ring for the rear end cap where the air tube screws onto is Part Number 19485 O-ring 23x2,5 NBR 70. Not a bad idea to replace it while you're at it!
Over time o-rings lose their elasticity and become hard, and no longer make a good seal which leads to slow leaks. They must be changed periodically so I recommend you order plenty of each so you have some spares on hand. Hope this info helps solve your problem.
All the best, Chuck
