I recently cleaned my bike. I wiped the rims with a clean wet rag and ever since the rear breaks squeek loudly.
Is this to be expected? Anything I can do to prevent the squeeking in the future when I clean the bike?

I had a similar issue once and cured it by scrubbing the rims lightly with a scouring pad.

Follow that with a wipe of rubbing alcohol.

I'd also rough up the face of the pads themselves. Sometimes they glaze, and exposing new material will stop the yelling.

It's funny, but sometimes refacing the rim's brake surface eliminates squeaks, sometimes it causes them. With Mathauser/KS salmon pads, it takes a day or two of riding to "bed in" the brake track and make the sqeaks stop.

Pick up some yard dirt and rub it into the pads. Andy

Feature: You don't need a bell on your bike any more ..

I had a loud noise coming from the front brakes, there was nothing i could do, tried everything, figured they hardened up over the years, so went to the parts bin and replaced them, problem sol-ved

Agreed that changing parameters will change the results. Pad material, contact patch/contour, age meets this change. But it doesn't always eliminate the squeals. Andy