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Rear brake issues... piston seized? Or not enough pressure?

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AC_Schnitzel

AC_Schnitzel

Well-known member
Joined
Jul 11, 2018
Messages
132
Location
Bay Area, CA
Went to change my pads -- it took a lot of force to push the piston back in. More than what I think would be normal.

Upon reassembly, I added new fluid and bled the line for any air that might have gotten in.

Pressing the brake lever, the piston doesn't move outwards at all...

Is my piston seized? Or am I somehow not maintaining pressure in the master cylinder?
 
You probably still have air in the piston. Hydraulic force through those braided brake lines should easily push the cylinder OUT.

Did you clean off the piston when you had it apart? Most every XB I’ve owned has sticky pistons, especially up front.
 
34nineteen said:
You probably still have air in the piston. Hydraulic force through those braided brake lines should easily push the cylinder OUT.

Did you clean off the piston when you had it apart? Most every XB I’ve owned has sticky pistons, especially up front.
Click to expand...

Nah -- I just pushed it back in. Now the pedal isn't pushing the piston back out.

I don't have a compressor or grease gun -- how else could I get it out?
 
Is the lever going to the bar? If yes, then you have an air bubble. Keep bleeding.

If not, then something is stuck.
You could always take it to a bicycle shop and ask them to blow it out with compressed air. Usually they are easier with that sort of stuff than an auto/mc garage. Make sure to stuff a shop rag in there the pads go to catch the piston and make sure it doesn’t damage anything when it comes flying out at high speed.

Or I will sell you mine for $20 + shipping
 
One other option but it will cost more is a hidden rear caliper set up. Also, get yourself a syringe, maybe a turkey baster and push fluid from the caliper to the master cylinder. Once you have pressure, you can do it the old fashioned way using long strokes while bleeding at first, then use short fast strokes until satisfied. While the bike sits overnight, put a zip ty around the bar and the lever nice and tight. In the morning it should be like new again. Presently, if it's not leaking it's probably ok.
 
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AC_Schnitzel said:
My neighbor has a compressor -- might ask him if I can use it to push the piston out.

I'm also going to rebuild my rear MC first and see if that fixes the issue.
Click to expand...

Wait? So the master cylinder isn’t working? Why are you messing with the caliper if the master cylinder isn’t working? Or are you just guessi.... ohhhh never mind.... i get it now. Maybe try a TPS reset while you’re at it?
 
My question still remains. How do I discern between failing MC vs seized/corroded caliper piston.

Symptoms:
- brake lever doesn't build enough pressure to push piston out (though it can push fluid through the bleeder)
- caliper piston was difficult to retract (reservoir was off)

I took the entire assembly off the bike and will rebuild both since the parts are relatively cheap.

34nineteen said:
You asked for the part number for the assembly, not a rebuild kit.
Click to expand...

My mistake, I meant the rebuild kit for that model caliper (not available w/ them).

EDIT: I will try to bleed one more time -- maybe there is air stuck at the top of the brake line near the MC.
 
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Take m/c, hose, and caliper all off the bike so you can hang them to float the air pocket out if needed.


Is the pedal stopping because of a “hydraulic lock” or is the piston in the m/c at it’s mechanical end point?

If it locked “hydraulically” then you should be able to push the piston out with that force. Unless it’s been sitting in seawater, it will break free under your weight. Also, make sure your piston actually has room to move to free itself. You may need to remove it from the rotor, remove the pads and shove a shop rag in there to cushion the blow when it breaks free. You may want to wrap it in a old towel or something to catch the brake fluid from flying anywhere.

You can also loosen the hose at the top of the m/c to see if the m/c is actually pushing fluid.

If it’s just reaching the end of the throw, you have an air pocket, which is cushioning the hydraulic force from moving the piston.

I used 1oz plastic syringes for my bleeds. I prefer to push the fluid back upward towards the reservoir.
 
the MC piston is at its end point. Thanks for the tips. I will try those things tomorrow, starting with bleeding at the banjo bolt at the MC.
 
I’d recommend taking off the heel guard and tying a shop rag around it to catch spills.

Good luck !

Ps. I actually use a 2 oz (60mL) syringe for my bleeds. YMMV, but that’s what works for me.
 
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