Here's a complete guide on how to repair a leaking joint whether it wasn't soldered correctly or it just started leaking in time, this guide will assist you in repairing it the CORRECT way, not the quick-fix way :)

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How to solder the correct way:    • How to Solder Copper Pipe The CORRECT...  
How to solder in a wall:    • How to Solder Copper Pipe in a Wall (...  
How to solder with water in pipe:    • Soldering Copper Pipes With Water in ...  

Amazon affiliate links to all the products I used in this video:

TOOLS:
Bernzomatic Torch: https://amzn.to/3s4uF3e
IRWIN 10" Locking Pliers: https://amzn.to/2SuJSY3
Fitting brush 4 in 1: https://amzn.to/45ELin6
Flux brushes: https://amzn.to/3S1xsrK
Utility knife: https://amzn.to/2U4dKKP
Pencil reamer: https://amzn.to/2tBuhKW
Fire proof blanket: https://amzn.to/3tzkcQV

MATERIALS:
Lead-free solder: https://amzn.to/3ZT6Jzn
Propane gas(Blue bottle): https://amzn.to/3tpWAOo
Lead-free flux(water-soluble): https://amzn.to/2XjaYn9
150 Grit Sandpaper: https://amzn.to/2YbJq6D

DISCLAIMER: Got2Learn is NOT responsible for any damage done to a property of which the plumbing wasn't done by a professional, I do not recommend doing your own plumbing if you are unsure about what you are doing, always hire a LICENSED contractor when doing any type of plumbing so you can be covered by insurances if something does happen, these videos are for entertainment purposes only!