Inspect the right rear wheel cylinder. After the truck sits for a while, even days, you should see a very small spurt of fluid when you open the bleeder valve. The systems are designed to maintain about 10 psi fluid pressure to keep the lip seals standing up and sealing throughout changes in barometric pressure. Increasing barometric pressure can put the hydraulic system into a relative vacuum which would lead to air seeping into the wheel cylinder.
The residual check valve is the one responsible for maintaining this pressure on any drum brake system. It is normally placed inside the drum brake outlet port of the master cylinder. If you have rear wheel anti-lock, which I suspect, I believe you have a single line going to the rear brakes. The fact you're not getting air out of the left wheel cylinder is proof the air is coming in through the right wheel cylinder.
Any chance someone replaced the steel line(s) at the master cylinder and mixed them up? If so, the residual check valve will be holding 10 psi of pressure on the front disc brakes so they will not fully release, and the rear brakes will draw in air during changes in barometric pressure.
I have a real simple trick that allows replacement of the master cylinder without the need to bleed at the wheels. It saves a lot of time.
Monday, April 27th, 2009 AT 5:05 AM