Disk brakes don't work well after taking out pads and putting them back I checked my disk brakes for the first time last night (i've never had disk brakes before).  The front ones were definitely worn out, and I replaced them.  They work great, now.

With the rear ones, I decided that they still had quite a bit of life left in them, and put them back in, rather than replacing them.  Now, they hardly work at all.  They were working fine before.

I think that I was careful not to get them dirty while I took them out and looked at them, but I guess that it's possible that I got some dirt on them while I took them apart and inspected them.

The other possibility is that maybe the pistons which push the brake pad onto the rotor got pushed back too far out, when I was putting the pads back in, and so now they're not pushing the pad onto the rotor with much force.

Is this a known gotcha?
If you set back the pistons (which is actually unnecessary if you are just inspecting the pads), pump the brake lever to move the pads back onto the rotor.

You can also clean the rotor with some solvent to make sure it is not contaminated with any grease or oil.

Update:

You can also check to see if you have a sticky piston in one side of the caliper that is not advancing properly. Unsticking them is easy, and guides on how to do so are easily found.