I replaced front and back brake 2 days ago. Yesterday i was practicing wheelie and at some point the back brake died on me.. then 5 min after back to normal then 15 min after died again. Pressing the back brake and no braking.....Any idea on what to check? It is the first time i experience this problem since i replaced all the pads

 

I'd try the simple fix first and flush the brake fluid again.

 

Make sure everything is tightened correctly too.