I just installed a new sink in my bathroom and the water pressure was fine and there was no sign of a problem. However maybe two or 3 days later, there was a lack of water pressure of more 50% from both hot and cold water. The toilet is fine and so is the shower and kitchen sink. It's a new sink, but old pipes. What could be causing this problem? Is it debree that needs to be flushed out or something more costly? Any help here would be appreciated. Thanks. : )

The most likely event is debris broke loose in the supply lines and is clogging your faucet valve. Tell us about the faucet, does it have cartridges?

Try taking off the aerator and see how pressure is then. If good, clean the aerator and reinstall. Should be all set. If not, let us know.

PS. The aerator is the part at the end of the spout where the water comes out. Is just unscrews.