Ok guys I've got this very weird problem

Yesterday I tried to bleed the brake system with my father.
In the beginning the brake pedal wasn't coming back automatically when released.
After a while it did and all air seems to be out of the system.
Now the weird part: I hardly get any braking power ... The brake cilinders at the back wheels move a bit but only one side of the drums (the left Side moves a bit).
Any idea what could be the problem here?
Thanks!