Strap the end of the Y piece or cat pipe to the frame so it's solid.
Undo the end of the can so it's all free, then any or all use all of the following (some in conjunction with eachother) in any random order until it's free......
Tap joint with small steel hammer to shock the joint, whilst 'waggling' the end can
Tap the butt of the end can that faces the joint with a rubber mallet
Twist End can using a large damp towel wrapped around it and torqued up tight with a stick like a tourniquet
Enlarge the slot in the joining pipe with the widest flat blade screwdriver you can find
Heat outer pipe, chill inner pipe (athlete's injury spray or pipe frieze spray)
Use opposing forces.....EG tap one side, then the opposite, then top then bottom, levering the can in the opposite direction each time.
It's not stuck, it's just being stubborn
