Rear brake doesn’t grip/slips through and doesn’t stop the bike sharp or at all I cleaned my bike about a month ago and since then I tried to fix both of my brakes.
I have hydraulic disk brakes (Tektro Auriga). After I replaced my pads, I was bedding them for over three days of consistent biking and braking on low and then high speeds, but it didn't work. I bled my bike more than five times, and it didn't help. I sanded those pads with 200 grid sandpaper and rubbed them with isopropyl alcohol. It fixed two of my pads - I guess because the front brake works fine (70-90% of the strength it used to provide), however, the rear ones don't.
The brake feels sharp, not spongy or anything (I bled it twice specifically), but it feels like it's contaminated with oil because when I brake, no matter hard or not, the wheel slides through the brake pads and stops very slowly. In rare cases, after 20-ish minutes of active braking (bedding in-like process), it starts squealing and bites just a tiny bit more, however, the very next day it appears from the very start e.g. close to no grip.
The only thing I haven't don't yet - burning or baking the pads.
BTW, I ordered new pads, but I still want to investigate why this occurred and how to fix it, because the pads which are used right now are two weeks old.
This sounds like a classic case of pad contamination. You are well advised to try to determine the cause to avoid it happening again.
My first suspicion would be your washing process. How did you wash the bike? Is it possible you got a cleaning product on the pads?
I'd definitely inspect the calipers very carefully to make sure you do not have any leaks. Clean the calipers and rotors thoroughly with brake cleaner, then with the old pads or a brake block in place try squeezing the lever hard a few times. Recheck for any fluid leaks.
If you are getting good resistance when squeezing the levers and there is no 'sponginess', it's unlikely that bleeding is required. You said you bled the brakes more than five time, I do have to wonder whether you are maybe casing contamination when you are performing the bleeding.