It'd be better to post a video while I don't know how to do so just past a photo here. There's a short 2mm wide crack locates at the inner side of the L junction, which is closed to another T junction. It's hard to cut and change the pipe here I suppose. I want to try a simple fix with roof sealant to seal the crack first, then put bandage wrapping. Posting before doing that to check if anyone has better idea or similar experience. Thanks.
The pipe is for my rooftop solar heating panels BTW.