Water supplies were affected in Pontypridd, Treorchy, Tonypandy and Pentre following the incident on Friday.
The company said supplies had returned to normal for the majority of customers by Saturday morning.
Only Clydach Vale is still experiencing problems due to "air locks" in the system.
Welsh Water said it had delivered bottled water to distribution points and more than 600 "vulnerable customers", such as those with medical conditions and the elderly, had received a supply.
Hundreds of customers were left without water after a pipe burst in Rhondda Cynon Taff.