Compressing pistons

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Let us say I am doing a front brake job on a regular car. I remove the caliper and pull the pads, THEN I unscrew the bleeder and push the piston in and then put it back like new so the new, full pads will fit the rotors.


My question is will I run into problems with fluid contamination, air in the system, or anything else that I cant think of right now?
 
Yes. You'll get a tiny bit of moisture in the brake fluid and the system will have to be bled.
 
Shouldnt be that much of an issue as long as it didnt get any air in the caliper with the bleeder open....I usually use a big [censored] c-clamp to push the piston in and do it without opening the bleeder....
 
When I did my Honda Fit brakes 20,000km ago (I also learned this in Auto Mechanics from a teacher):

Unscrew the bleeder before you push in the piston, this prevents copper oxide and gunk from being forced back into your ABS pump.

If you gravity bleed when you are done, with a vinyl hose attached, your brakes will be great.

If you can hit the brakes on gravel to cycle the ABS pump afterwards as well, you will have solid, firm brakes.
 
In years past I would just push back the piston. No ABS back then. Today, I'd think it'd have to be a very large piston to drive fluid all the way back to the ABS pump. Might as well try to crack the bleeder. I don't think just cracking the bleeder will get air in, not unless if the piston were somehow pulled out in the process, or you waved the caliper around extensively with the bleeder cracked--I'd hold it steady while pushing in, then tighten back down while it's still in the same spot.
 
Never push the funky old Brake Fluid that is in your Calipers back through the ABS & Master cylinder.

Open the bleeder, Push the piston in, Install your Pads & reinstall the Caliper, Pump the brake pedal a couple times, Open the bleeder & whatever air is in the caliper will gravity bleed out.
 
I cracked the bleeder before pushing the piston back, but I attached a tube to the bleeder first.

That way no air got back in because the tube was filled with old fluid and I closed the bleeder after the piston fully retracted. I have a nice firm pedal and it's been 3 months since I did the brakes.

I did a full fluid exchange last year, so I didn't bother bleeding the fluid after I was done.
 
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