The rear self-adjusting brakes only adjust in reverse. That is why the owner's manual tells you to apply the brakes in reverse. The issue is that if they are out of adjustment, you need to do it more than a few times.
A low brake pedal can be caused by the rears being out of adjustment. It can also be caused by air in the lines or in some cases there is play at the master cylinder.
First thing is to adjust the rears by hand not with the reverse method. You should be able to feel the brakes drag when you rotate the tire by hand. If you do not feel any drag, the brakes are out of adjustment. Also bleed the brakes to make sure there is no air in the lines.
I have owned my 90 GMC K1500 since new and I am the only one to work on it. I have learned to adjust the rear brakes by hand every tire rotation to keep the pedal adjusted properly. The self adjusters do not do a good enough job.
//2000CAYukon