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How far does rear brake piston retract?

11K views 22 replies 12 participants last post by  Norm Peterson 
#1 ·
Hi,

Trying to replace the rear brake pads on a 2014 Mustang GT with the Brembo brakes. This is the second time i'm replacing the pads and i've run into an issue that i didn't have the first time around.

I'm using the brake caliper tool you can rent from Autozone, as indicated in the attached photo. The piston retracts easily at first but once it retracts to the red line it stops. No matter how hard i twist the tool the piston won't retract any further. I even bent the T handle part when i put a pipe over it for more leverage.

My question is, does the piston go in further? Does it go all the way to the yellow line? Meaning the metal face of the piston sits flush with the caliper?

As is, I can't get the calipers over the new brake pads, i didn't have this problem the first time around and I don't remember how far the pistons retracted the last time.

Tried with Ebrake up and then down, tried releasing the bleeder screw and tried both left and right side rear pistons and they both pretty much stopped at around the same depth.

Help? Thanks
 

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#5 ·
Pop the seal around the boot a ltitle bit and pour some brake fluid in the piston to lubricate it. Work it in both directions.

Those pistons have a long way to go.

After that, buy new calipers. My passenger side was unusably bound up after 80k miles.
 
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#6 ·
I had the same problem as you are experiencing. I tried just turning the piston in and it was difficult and wouldn't really retract. Then I disconnected the E-brake and opened the bleeder screw and it still wouldn't go in as it should. So I thought I'd try removing the brake line too and take the caliper to the bench to see what was wrong. Once I got it to the vice, it turned in easily - so I'm guessing that removing the brake line was what allowed the piston to turn in. I was planning on bleeding the brake lines anyway so taking the brake line off was no big deal.


Occasionally I hear what sounds like the rear brake pads still rubbing on the rotors when I'm driving so I don't think they are retracting as they should when there is no pressure on the pedal. They are not 'on' but not retracting as the should and I get a TSHHHH noise occasionally. Almost like there is a check valve somewhere in the brake line from the master cylinder of perhaps the ABS module. SO - try disconnecting the brake line and see what happened with the piston.
 
#7 ·
Just making sure you are taking the cap off of the brake fluid reservoir and open the bleeder(with a tube and bottle attached of course). This is important because the master cylinder multiplies the pressure in the brake system to compress the calipers with great force. When you are trying to compress the piston, you are fighting this high pressure if you keep the system closed tight. Fighting to compress the piston can turn a 15 minute job into head scatchin' butt kickin' hell!
 
#8 ·
The piston can be pushed flush to the caliper housing. Also, there are "guide" pins on the back of the pad. There are slots (quite wide) those guide pins slide through. If those pins and slots don't line up, the pad won't go in. Or worse, you'll push the piston in, put in the pad and then go for a drive. You'll hear all kinds of crazy noise (not squeaking) coming from your rear brakes. It's the piston riding on top of those guide pins and rattling. Ask me how I know. :grin:
 
#11 ·
Are you certain that the rotor is properly seated up against the flange? I usually snug the rotors into the correct orientation with a couple of lug nuts (and short spacers if the lug nuts aren't open-end style).

Incidentally, it's much better to open the bleed fittings when pushing/screwing the pads back into the caliper bores. Dumping the dirtiest fluid is much better than forcing it back into the ABS hydraulic control unit.


Norm
 
#12 ·
When trying to retract the piston back into the cylinder, the caliper has been removed from it's mount, the old pads are removed and the caliper is just hanging (with a coat hanger) there not mounted. Three hands would be handy - one to hold the caliper steady, the 2nd to hold the tool in the piston and the 3rd to turn the ratchet. Is there another way which involves having the rotor in place? Wouldn't the rotor get in the way of turning the piston back in?
Agreed - when retracting the piston it is better to open the bleeder screw for the reason you mentioned - however I always bleed the brake lines after doing a brake job so any dirty fluid that might get pushed back into the line, would get bled out during the bleed process anyway.
 
#13 ·
Well, we removed the rotor so we could install the calipers back onto the bracket for more leverage. It's tough trying to hold onto the calipers AND operate the caliper tool. We slipped some pipe over the T handle on the caliper tool and was able to spin the piston in another 1.5 turns. That did the trick for the driver side.

We tried doing the same thing on the passenger side but the crappy T handle started bending too much from the extra leverage of the pipes. We stopped because we didn't want to destroy the rental tool.

In the end we removed the metal shims on the pads and scrapped the pads over the concrete to take off just a tiny bit of material and the caliper was barely able to slip over.

We did disconnect the E brake cable on both sides and popped the bleeder valve open on both sides but that didn't seem to help at all.

I think if we had a better tool we probably could have rotated the piston more. If only we could have removed the sliding T handle and put a heavy duty screwdriver through the hole or if the tool was designed to work with a ratchet instead of the t handle...

Not sure if i'm using the correct terminology, when i say "T Handle" i mean the section circled in red in the attached photo.

Well, thanks for the help guys, it didn't go perfectly but we got them in at the end.
 
#15 ·
Why not loosely bolt the caliper in place with only one bolt so it can swing out for tool access to screw the piston back in? You'll probably need to match the bottom hole in the caliper up to the top hole in the bracket, but it'll be that third hand you're looking for to hold the caliper from completely falling.

Yes, you can bleed the old fluid back out of the ABS unit, but there is still some risk that not all of the suspended crud will get flushed out.


On edit . . . my son-in-law used to work in the repair industry, so I now have access to that whole kit.


Norm
 
#17 ·
I was busy swapping pads and bleeding brakes for a track session. I finally looked at your pic. I think your piston popped out of the cylinder. That's why it won't go in any further. I can't imagine the piston moving that far out, even with fully worn pads. Considering the amount of force you applied, you may have galled the cylinder wall.

If this happened to me, I would replace the caliper assembly.
 
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#20 ·
People who don't understand the value of bleeding whether or not they open the bleed screw when pushing the pistons back in shouldn't take wrenches any closer to their car's brakes than the lug nuts that hold the wheels on.


Norm
 
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