The dot 3 fluid is no good, the boiling point is way to low. There is an adjustment rod connected to the brake peddle that you can extend some. It really sounds like you have a pad and fluid issue. DOT 4 racing fluid is a very good idea, do not use dot 5. I would be interested in the brand of pads you are using. Also the rears really do not need to be adjusted to the point the drum is hard to remove. With the drum on, you can reach in the back with a brake spoon and tighten them all the way, back them off about ten cliicks or notches on each side. You really may be overheating the pads, shoes and or fluid.
Dennis Knox
Buckeye Legends