Toyota Repair: Toyota Sequoia 2001 Back Brakes, rear disc brakes, toyota sequoia

Question
Ever since we bought our Sequoia the one weakness I found was the brakes (4 wheel disk anti-lock on a 4 wheel drive).  They seem to working fine and next you now the front brakes are doing all the work.  Now I know that most of the stopping power % comes from the front breaks by design, however there comes a time when not even the Parking brake works (no hold).  I take it in and Toyota they adjust the back brakes and all works great for a few weeks and the process starts all over again.  The Sequoia is not driven hard, so I cannot understand what is going on…..I truly believe this is a design flaw. We have over 63k miles on the Sequoia now, but this has been an issue from day one.  Question how are the back breaks adjusted and is this normal?  Yes, I do feel this is a safety issue due to the breaking power decreases when the back brakes don't do their share of the work.  Tired of going to Toyota for quick adjustments and not fixing the issue from the very start.  

Answer
The rear disc brakes are not adjustable, only the emergency brake is. The e-brake is seperate from the discs, in other words the disc brakes do not activate when using the e-brake, there is a small drum brake on each disc that is used for the e-brake and it is adjustable.