disc brakes start squealing after just a bit of use The rear disc brake on the Jitsie Varial 24&quot; Hybrid that I recently got (for Christmas, so about two weeks ago) started squealing and losing its power. This was also a problem on the hardtail mtb that I was riding on before.
I've cleaned the rotors with isopropyl alcohol, sanded down the brake pads, and bedded them in by riding around my neighborhood and slightly pulling the brake lever. This would fixed the squealing noise and loss of power for about a day then it would come back. When that didn't work, I tried replacing the brake pads. That would fix it for slightly longer, but it would still come back after maybe a week.
I have tried to be very careful about not contaminating the pads. I haven't washed my bike with anything yet, and I wear rubber gloves when touching the rotor. Before riding trials I never had any issues with squealing brakes. Is it normal for that to happen continually and after such a short period of time? If not what could be the problem? I did notice that both bikes were running Shimano B01S Resin Brake Pads. I'm not sure if that could be a problem.
A couple things to look at:

Perhaps try Brake Clean to clean the rotor and spray on a rag then wipe the pad surface. I successfully use it on rim brakes when they occasionally squeal and it is not due to the toe angle of the pads. However be careful with it as it is stronger than rubbing alcohol so do not get it all over the place.
It could be possible the rotor is out of true. In which case, you should go the shop you got the bike at to get that addressed.
You could possibly have a bubble in the brake line which is causing a lack of pressure to close the caliper and that in turn is causing vibrations as you slow along with the loss of braking power. Again the shop that you got the bike at should be able to address that for you pretty quickly.

Hope that helps.