How to Push Caliper Piston Back in and It May Be Sized

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Today decided to replace the front pads on my 2006 Prius.
After removing the wheel, and folding up the brake caliper, I needed to start the car to put it in neutral in
order to rotate the wheel to service the brake disk a little, as the other wheel is still sitting on the ground.

Unfortunately, after that, when I placed the C-clamp to start pushing the piston back to full retracted
position, I found that it had pushed all the way out and was no longer in the caliper bore. It is still
in the rubber boot, but now some fluid is leaking out of the boot edges around the piston. I put the piston
back, but cannot push it back down again using the C-clamp.

Is there a recovery from this?

Thanks
Caliper pistons are very exact fits in their bores. There's a very delicate touch to getting them back in. You'll have to bleed the brakes afterward anyway, so you may as well open the bleed valve, unclip the boot, or both, and dump the fluid out of there so it is not in your way.
Then just start delicately with your fingers wiggling the piston around at the end of the bore. A properly sized piston to the bore is going to feel like it's just never going in there ever, right up until the very moment when you happen to hit perfectly aligned, and then it goes shoop and it's in. Force is not the key, and if by any chance you have tried force, it might be good to pick up a new piston now.

In any case, if you have a way to get to the dealer, this would be a great time to pick up the rubber kit for your caliper, and replace the internal seal as well as the boot, because you've given yourself this ideal opportunity. Also, once you do get the piston to shoop into the end of the bore, you still have to get it started past the rubber seal, which may involve a bit of judiciously-applied force. It will probably go better with a new seal than with 12-year-old cooked rubber.

Then just reassemble and bleed as described in the manual.

-Chap
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Front Brake Caliper Piston stuck out | PriusChat
It seems replacement is in order.

Or

In order for the brake caliper to fit over the thicker new brake pads, the caliper piston will need to be compressed backwards by using a "C" clamp.

Attach the "C" clamp to the caliper piston using the back of an old brake pad to evenly distribute the force across the piston.

Before you begin compressing the piston, move to the engine bay and twist off the brake fluid reservoir cap. Removing the cap will allow the brake fluid to more easily travel backwards through the brake lines.

Repeatedly check the level in the brake fluid reservoir while slowly compressing the caliper piston to prevent it from overflowing which may damage any painted surfaces. Compress the piston until it is flush with its rubber dust boot. Promptly replace the brake fluid reservoir cap since brake fluid is hygroscopic (absorbs moisture).

Push the brake pads flush against the rotor.
I've only done this once on a 63 Volvo Break job.
This is what I learned. The caliper piston has popped out beyond the O-ring seal (hence, your leak). It has not gone so far as to tear the dust boot.
When I did it, I realized I was so deep into this I better just disconnect the caliper and start clean up with some white vinegar on a bench before everything got FUBAR'd.
I pulled back the dust boot (not necessary) and saw the reason it wasn't returning into position was because I had it at a slight angle. I got a nice piece of flat bar that gave me better control, got it squared with the bore and pressed it back into place with about 25 lbs of even pressure from each arm.
I thought at the time I could have used a dowel the size of the piston cup and a dead blow hammer as an alternative as long as the dowel was very close to the piston size.
Hope you decide to use two jack stands next time and rotate tires next time.

iPhone ?
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Before you begin compressing the piston, move to the engine bay and twist off the brake fluid reservoir cap. Removing the cap will allow the brake fluid to more easily travel backwards through the brake lines.
Although this is done often, please don't. If you have an older 60s 70s car then fine, but pushing old fluid back through an anti-lock brake system is just asking for future problems. Just attach a small rubber hose to the bleed vent and let the old fluid flow into a disposable container. Add new fluid to reservoir to make up any difference if needed.
Best way really is to get a friend to come over to help. You give the friend all of your hammers and a gift certificate to a nice eatery at least half an hour away. That way, by the time the friend returns with the hammers, you've finished the job and weren't tempted to use them.

Having already ejected the piston, you've got a great opportunity to clean it up and the bore, and pick the old rubber seal out, and put in a new one from the rubber kit, freshly greased with the little packet of red grease that comes in the kit.

And a clean piston in a clean bore, with a new, freshly greased rubber seal, slides home with a pinch of your thumbs (once you get it lined up straight).

-Chap
For next time: don't even think about starting the car, disconnect the 12 volt battery negative cable at the outset, and after everything is reassembled pump the brake pedal multiple times, then reconnect battery cable.
The reason for pumping the brake pedal is to eliminate any gap between the brake caliper pistons and the brake pads before power is restored to the skid control ECU.
I don't think this has been mentioned, so I'll mention it. The reason your piston is out of the caliper bore is because the Prius pressurizes the brake system when you start the car (even a little before that, when you open the driver's door). If the caliper is off the rotor at the time of said pressurization, the piston is just pushed out. To prevent that from happening you could as mentioned, disconnect the battery, but also the old low tech trick of putting a piece of plywood similar in thickness to the thickness of the rotor while the piston is just hanging there is always a good idea. IF the brakes decide to cycle, they'll push against the plywood (or other material of your choice) and prevent the piston from going too far.
As for remedying this situation, I honestly do not know how to go about it after the normal caliper rebuild (which was already covered above). I think Techstream is required to bleed the brakes, but perhaps not on Gen 2? Maybe someone can describe the brake bleeding procedure on Gen 2. It may be straight forward, like on the non-hybrid. Two person job of coordinating pedal presses and bleeder valve opening/closing.

Good luck and please keep us informed as to how it worked out.
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I heard here at PC that you could accomplish without the Techstream by disconnecting the 12v battery during the process.
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I used to hone out and with a set of stones on a drill rebore the clutch cylinder and brake master cylinder regularly on my 56 all wheel drive international scout, but today.
I want factory fresh brake calipers, I'd replace the caliper.
It's my understanding the Prius pistons screw in like older Saab's did.
I would think the fluid evacuated when the cylinder popped out, good time to replace all the brake fluid.
The rears do (some generations, some markets). It's a parking-brake thing.
-Chap
(sorry for the underline, can't shake it)
Third
gen Repair Manual has a procedure for bleeding brakes without Techstream. It doesn't require battery disconnect, uses a chicken dance that has the car "On", not "Ready", and in "invalid" mode. @NutzAboutBolts has a video on this as well, pinned in the 3rd gen maintenance forum.
Second gen I'm not sure. There's no sanctioned brake bleed without Techstream in the Repair Manual though.

The attachment is third gen:
Attached Files:
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Thank you all for your replies. Yes, this is all the result of rank stupidity. I did the job correctly before on my other Prius 3 years ago. But back then I had reviewed the manual beforehand and remembered the 12v disconnect part first (and didn't need to move the wheel).
As a result of the delay now I decided to just pull and replace the rotors and start off completely new as they and the pads appeared to have been original from 2006.
So now I'm going to just replace the whole caliper for $45, because I'll need to have it bled anyway.

My only question now is after the work is done, will the car start so I can drive it to the mechanic so he can use his techstream to bleed it?.... Or will the computer disable it in my driveway because of air bubble codes?

thanks guys,
The chain of event is comical.
The car will start and scream at you about the brakes.

YOU WILL NOT HAVE PROPERLY FUNCTIONING BRAKES.

IS THE COST OF A TOW WORTH A 100% PREVENTABLE ACCIDENT?

Just like people who get nailed with a DUI/DWI, they chose to be completely idiotic and drive themselves instead of taxi, rideshare, or sleep in their car.

You already admit to being stupid once. Break the cycle. Please don't drive w/ non-functioning brakes.

Al Bundy and Lucifer like this.
Always worth taking a look at hobbit's site, seems like he has worked on his Gen 2 brakes without Techstream and lived to tell the tale (and he has a pretty good track record for making sure what he thinks is going on is what is going on, before he goes on about it too much).
-Chap
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Your friend hobbit mentions a "hydraulic-only backup mode" for bleeding the front brakes, but he doesn't elaborate any further.
Is that the same thing mentioned in this posting by zak.kapoor where you jump the 4 and 13 pins together, remove 2 relays and then do a gravity bleed of the front brakes?

Replacing brake fluid and bleeding brakes on a GEN II Prius without Techstream | PriusChat

That may let me drive the car to the mechanic where he can do the techstream bleeding.

thanks;