[BonzoHansen]

I know I did not imagine this.

Can I damage my braking system when I install new pads if I don’t open the bleeders to push in the pistons?

Yes! Opening caliper bleeders and using line locks before pushing caliper piston into bore helps prevent needless replacement of master cylinders. Pushing caliper pistons back into the caliper bore without opening the bleeders could damage master cylinders or ABS components. Opening the bleeder will allow the piston to be pushed back without forcing all the fluid into the master cylinder. However, sending some contaminated fluid to the master cylinder is still possible with the bleeder open. Using a line lock will assure you that this doesn’t happen.

Brake fluid inside the caliper is full of dirt and sludge. Using mechanical force to push a piston back will force the fluid through the ABS and the vent port on the master cylinder, which is about 0.015” in diameter. Without the bleeders open, this dirt could plug the vent port, causing brake drag or lockup when the fluid heats up. Furthermore, many of today’s ABS units have one or more filters inside. Dirt and sludge will plug or restrict these filters, causing problems with ABS operation.

Back when I was in the service business, I read a number of bulletins on this. I file this info under the better-be-safe-than-sorry file. Of course, my Dad had a saying. There is a right way, a wrong way, and my way.

Edit: More

Some Volvo site
11.When servicing disc brakes, open the bleeder screws and vent the brake fluid, if it is necessary to push the caliper pistons in. There is sediment that naturally collects in calipers over a period of time. This sediment, if allowed to flow back into the master cylinder along with the brake fluid, can possibly damage the ABS hydraulic unit. [Editor's Note: see above notes on clamping the rubber brake lines.]

About.com Auto Repair
When forcing the piston back into the caliper, open the bleeder valve first. Corrosion and other deposits accumulate inside of the caliper. If the bleeder valve is not opened when the piston is forced back into the caliper, these deposits will be forced back through the brake lines and into the master-cylinder.

Americas Pride Online – sellers of auto equipment
It is extremely important to open the bleeder screw when compressing caliper pistons or wheel cylinders during routine brake maintenance. When a caliper piston is pushed in, brake fluid gets forced backward, up into the system. Since the caliper is the lowest point in the system, dirt and corrosion naturally accumulate there. When this grime and dirt finds its way into the HCU portion of the ABS system, it can cause valves to stick. This can lead to: a vehicle pulling one way or another; accumulators sticking open, which leads to low pedal; compensator ports plugging, which leads to dragging brakes; and of course, it can make the ABS light come on due to all of the above.

If technicians get in the habit of opening the bleeder when they compress the caliper piston, as well as selling customers a brake fluid flush, these problems will more than likely be prevented.