[Richard]

Ok, if you pull the plug from the SUPPLY side you will get air.

Look on that valve for more ports. They should only be under pressure when the service brake is supplied.

Now, if all that is futile. Can you detach the brake supply line at the can? If so you should be able to use brass NPT fittings to install a tee. Of course, use DOT fittings for the tubing.

You really only have three choices. Tap in at the relay. Cut the tubing and insert a tee. Insert a tee between the tubing and the brake can.