Rear brake problems

I have a 1998 1/2 ton 4WD GMC suburban and my problems lies in the rear brakes. About once a month I have to take off the rear wheels and loosen the rear brakes shoes because the self- adjustment keeps over-adjusting the rear brakes shoes. Does anyone have any ideas as to why this is happening and how I can resolve the problem? Thank you in advance for any help!

Have you measured the distanct that the adjusters are at before backing them off to see if they are returning to the exact same measurement from the previous time?I'm asking because I'm wondering if the brake fluid in the wheel cylinder is not being allowed to return to the reservior.Each time you depress the brakes,a small amount of fluid remains and eventually the shoes end up against the drum.Is it possible the rear flex line is partially collapsed?Are the return springs weak?Just some ideas to help with your problem.
chuck's

I agree with the previous reply, these are notorious for under adjusting the rear shoes. Sounds like a bad master cylinder