My method for leak detection was to pressurise the pipe work to 5 bar with compressed air and use a gas detection spray from screwfix on all the joints, as an extra precaution I left pressurised overnight and checked again. I suppose I could have used washing up liquid in water but felt more comfortable using the spray.
RM Solar have a purpose built filling station that will fill the system with the Glycol mix, bleed the system of any air and pressurise. I had originally intended to use a Rothenburg pressure tester to fill and pressurise the system and to bleed the system of air using two additional bleed valves I had installed on the flow and return pipes at the top of the panels. (The RM supplied panel connectors have integrated bleed valves however these are inaccessible once the side flashing's have been installed) in the end I decided on a home spun pump and pressure vessel.
Once all the air was out the system I set the flow rate of the glycol around the system to 1 litre per minute
This is an old pump connected to a 5 litre plastic container I used as a reservoir for filling and for the return pipe to feed into, I connected the outlet on the top of the pump to the pump station using washing machine supply hoses as it had the same connectors as the pump station and kept topping the reservoir up with glycol as it filled the system.
I then ran the pump for 10-15 minutes to flush the air out of the system this could be observed by the Glycol in the reservoir turning from milky blue to clear blue, I then shut off the return to the reservoir,this pressurised the system to about 2.5 bar which is when I shut off the feed. I then used my homemade pressurisation vessel to pressurise to 3.5 bar.
This was an old water filter I had on my house water supply; a tube comes from the bottom of the vessel and connected to the pump station, the filter was filled with Glycol, I opened the connector at the pump station slightly and then pressurised (foot pump or compressor) the vessel to force the air out of the tubes, Once this was done it was easy to open the valve on the pump station and pressurise the system to 3.5 bar, you need to make sure that the vessel does not run out of Glycol otherwise it will put air into the system. This sounds quite convoluted but it wasn't and worked perfectly.