Your going to flush the system anyway, so just open the bleeder screw and push the pistons in using a puddy knife,, block of wood etc..
You really don't want to push old fluid back into the system anyway.
I do not know anything about trikes but i do know a wee bit about brakes
If there is a parking brake system on these brakes then the piston needs to be screwed back in as the parking brake adjuster prevents them from being pushed in
the two sides need to be screwed in opposite directions to wind the piston back in you will need to apply pressure while winding the piston there are special tools designed for this sort of job
A quick search brought me this Brake Service Tools