I know that this should probably be somewhere else but nobody elsewhere seems to know. My rear brakes do not work. The shoes are good and adjusted properly. My front brakes work great the rears are not. When the truck is not running the pedal goes almost all the way to the floor. If I pump it it will tighten some. When the engine is running the pedal is still kinda squishy. I can jack both rear wheels off the ground and start the truck and put it in gear and not hardly stop the rears from yurning at idle no matter how hard I push the pedal. Is it possible for half of the master cylinder to not work. Please help and thank you!

check your wheel cylinders in the rear end. I had this problem with my fathers 99 burb. they got rusted up over time and would not move when you pushed the peddle.

Ryan

You bring up an interesting topic. The rear brakes on my 97 Sub have never worked well. I have changed the proportioning valve to one thaqt was suggested has giving more to the rear and installed over sized cylinders in the rear brakes, and still no notisible braking in the rear as noted by the fact that the rear shoes last forever. I had always thought the rear brakes reacted to the master cylinder first and then the fronts.

I to would like suggestiions.

The front disks, where all the weight is and shifts forward to, do most of the braking, all the time.

The rear wheel cylinders may be frozen, but that would cause the pedal to be stiff, not go to the floor.

Your master cylinder has two seperate 'compartments', one for the front brakes, one for the rear brakes - the cups\o'rings for the rear brakes are leaking.

If your Sub has the rear height-sensing valve, it may need adjustment - rear attitude corresponds to weight.

If not, make sure the rear shoes are the same brand and quality as the front pads.

Yes, it is possible to kill half of a master cylinder and have the other work fine (rare, but still possible). Brake fluid is hydroscopic (attracts water) and over time (especially if there hasn't been routine brake fluid flushes) will create that rusty grunge that ends up at the bottoms of wheel/caliper cylinders and the bottom of the MC, it also one of the largest culprits of ABS failures (all of them solinoids/gears and such really dont like FOD either). This grunge is very abrasive on the plunger and its corisponding seals. If you are max traveling you brake pedal (all the way to the floor), chances are that you have started cutting grooves in the MC seals and letting fluid blow by and not creating the pressures needed.
Like GMCTD stated, if you have plugged wheel cylinders or a kinked/glogged line you will have good pedal pressure. I would tend to look at the MC and then thoroughly flush out the system with fresh brake fluid (you'll waste about 1 1/2 gallons if you have a real dirty system).
I personally flush my system out once a year to keep the boiling point at specifications (low water absorpsion by the fluid). With doing this I have noticed that the resivoir almost never gets dark in color and I havent had a brake fade issue.
For my flushing/ brake bleeding I use the snap-on, blue-point brake pressure bleeder ( one could probably make one out of a hudson sprayer that was resealed with brake fluid compatable seals.)