I don't think you will see anything but dirty oil by removing the back cover. It has to be something in the brakes. Make sure the return springs are intact. It could be that your wheel cylinder is not retracting when you let off the brakes. Do you have a little free peddle on the master cylinder. When you took the drum off and replaced it, did you step on the brakes before you tried it again? If not, there is a possibility you have a warped drum.
