There are a couple of possibilities. To install new pads, the pistons had to be pressed into the calipers to make room. They get run back out later by stroking the brake pedal. The most common mistake is pushing the brake pedal all the way to the floor. That is likely to damage the master cylinder. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal over half way runs the lip seals over that crud and can rip them. The most common symptom is a slowly-sinking brake pedal, and that often does not show up for two or three days.
Also consider that if the reservoir was allowed to run empty, and the truck has anti-lock brakes, you may need a scanner to command the computer to open some of the valves in the hydraulic controller so two chambers can be bled. If the hydraulic system was not opened, there is no need to bleed anything. It is a good idea to exchange the fluid periodically to get the old moisture-laden fluid out, but few of us actually do that. That would not introduce any air into the system if the reservoir never ran empty.
When bleeding is necessary, a lot of people think it has to be done with a helper pushing the brake pedal, and that helper usually pushes the pedal to the floor. That is again, a good way to damage the master cylinder.
Monday, June 26th, 2017 AT 3:19 PM