Sticking Hydraulics

[Kestevan]

I've just got a new GT Grade with Shimano RS505 Hydraulic brakes, and I'm having repeated problems with sticking brakes.

Last Monday I got the bike out for the commute only to find the front wheel was seized solid. Dropped it off at the LBS and he cleaned the pistons and everything was OK for rest of week.

Bike was cleaned and washed on Saturday (although I didn't strip down the brakes, just washed everything with warm water and a sponge). Come this morning front wheel is again locked up, and back is somewhat draggy.
Guessing I'm going to have to dismantle and clean the pistons tonight... Is this level of maintenance normal with road hydraulics? cos if it is, it's going to make using the GT as a daily commuter a much bigger PITA than I need.

 

[Yellow Saddle]

Dirty pistons don't cause a solid lock-up. Something else is wrong.

 

[I like Skol]

This surprises me. I swapped from Tektro hydraulics to Shimano XT brakes on my commuter due to sticky piston problems and apart from a leaky calliper on the front after about 4yrs and 10,000 miles they have been completely trouble free, rarely need adjusting and don't get cleaned. In fact I often put them away damp with road spray and winter salt. I would expect the road bike brakes you have to be of a similar quality to the XT MTB counterparts. Maybe washing your bike then putting it away is a bad thing to do but surely can't be any worse than putting it away full of salt.

You are not storing the bike leaned against a hot radiator are you? Just wondering if the heat expansion is affecting the system in some way?

I think the answer is to give it to me, everyone knows you can't commute on carbon

 

[Levo-Lon]

id look at the pads and the securing pin.
if the pads are tight in the caliper they will stick on..

remove the pads and check the metal edge for rust or damage..give them a light sanding with wet dry paper.
check the caliper for a burr where the pad fits..

clean the pad pin and apply a thin film of coppa slip

 

[Kestevan]

Pads are not damaged/rusty (or at least not last week).
Bike is kept in an unheated garage.

Cant say I ever had this issue with my MTB either, that just worked.... and that thing and "maintenance" were only on very distant terms.

And no, @I like Skol you can't have it..... stop asking

 

[lpretro1]

Very unusual for Shimano pistons to stick - Avid yes but not the Shimano. But it does happen occasionally. If the bike is new I'd return to the LBS - we had this on a brand new Ghost bike and it was a faulty brake - replaced under warranty

 

[Levo-Lon]

Ahh its a new bike..so as above faulty master cylinder may be the prob with it locking on

 

[Supersuperleeds]

Might need bleeding? I had air in tubes and it caused the brakes to come on by themselves.

 

[Yellow Saddle]

Might need bleeding? I had air in tubes and it caused the brakes to come on by themselves.

No and no.

This stuck piston cannot be fixed by bleeding the brakes. Bleeding is not the panacea of all things hydraulic. Further, there is no way that air in tubes can cause the brakes to come on by themselves. The cause of that problem of yours is paranormal activity, not bubbles.

 

[Kajjal]

If it is locking the wheel solid even if after going back to the shop it looks like needing a warranty replacement.

Dragging a bit can be due to a misaligned caliper or pad damage. Check nothing is stuck behind the pads as disc brake tolerances are much lower than rim brakes.

 

[Kestevan]

Just stripped it down and pushed the pistons back into the break caliper. No sign of any undue gunk, and the pistons moved back OK when pushed firmly with a steel rule.

Replaced everything and initially it appeared OK, however after repeated use of the brake it has now started to drag a little again. It looks like the piston is simply not retracting fully and the problem gets worse each time the brake is applied.

If this was a "normal" bike I'd take it back to the dealer and start there... trouble was this was a competition prize and came straight from the distributor/manufacturer. I've email the imported (Cycling Sport Group) and will have to see what they come back with (if anything.... there are some "interesting" accounts of GT's level of customer service...)

 

[Yellow Saddle]

Just stripped it down and pushed the pistons back into the break caliper. No sign of any undue gunk, and the pistons moved back OK when pushed firmly with a steel rule.

Replaced everything and initially it appeared OK, however after repeated use of the brake it has now started to drag a little again. It looks like the piston is simply not retracting fully and the problem gets worse each time the brake is applied.

If this was a "normal" bike I'd take it back to the dealer and start there... trouble was this was a competition prize and came straight from the distributor/manufacturer. I've email the imported (Cycling Sport Group) and will have to see what they come back with (if anything.... there are some "interesting" accounts of GT's level of customer service...)

Just remember that there is no mechanism in a disc brake to retract the pads. The only reason they retract (if they do) is because of a bit of friction between the piston and the rubber piston seals and thus the seal's elasticity. It is only the seal's reluctance to make the piston slide that last bit that makes it retract slightly when the lever is released. It isn't even design in, it is just luck that it happens. On car brakes they pads make constant contact with the discs.

You need to find the cause of the dragging. Aim down the pads' gap and spin the wheel. Is the dragging intermittent or permanent? Describe exactly what you meant by "seized solid" in your original post. Could you really not move the wheel? Did this disappear with time? Can you move the pads back far enough so that there is a large initial gap that requires several pumps of the lever to go away?

 

[Pale Rider]

Slightly bent rotor?

 

[Kestevan]

After initially pushing the pistons apart the wheel spun with no rubbing at all. Brakes then worked a few times and rubbing started, becoming steadily worse each time brake applied. Wheel now stops spinning after 1 or 2 rotations. It's not an intermittent rubbing more like brakes are constantly on.

Rotor is straight and it's a through axle so not misaligned.

By seized solid I mean when I pushed the bike normally the wheel didn't rotate at all, simply skidded.
I could rotate the wheel by hand, but it required a fair bit of force and wouldn't spin at all unless forced.

 

[Tim Hall]

Disclaimer: I have no disc braked bikes in my fleet.
<wild handwaving>If the system is sealed and the temperature of the fluid increases, the fluid will expand, with the effect of driving the pistons towards the disc. Is there a vent somewhere in the sytem that isn't working?</wh>