Sticking Hydraulics

[Pale Rider]

Remove the pads and use a nail file to slightly lengthen the calliper aperture, or file/emery cloth the 'top and bottom' of the metal backing plate of the pads.

Before refitting, put a tiny bit of grease on the top and bottom of the backing plate.

 

[I like Skol]

Remove the pads and use a nail file to slightly lengthen the calliper aperture, or file/emery cloth the 'top and bottom' of the metal backing plate of the pads.

Before refitting, put a tiny bit of grease on the top and bottom of the backing plate.

Do not do this! The pads are a good fit in the calliper, Shimano build quality makes sure of this at this level. Also the grease WILL migrate on to the pads and cause contamination and depending on the grease type will attack and swell the seals making the calliper stick even worse.
There is something odd going on here as this shouldn't be happening. I just wish I could get a look at the problem, not saying I am any kind of mechanical genius but just that sometimes a second pair of eyes spot things that the first person has overlooked.

 

[Levo-Lon]

Fluid lock from an over filled system causes this but you Do need heat to expand the fluid..

System should be filled with pads fully retrac5ed and a bleed block in the caliper for correct fluid level.
you could bleed one lever pull off and see if it sorts it..
undo bleed nipple pull lever to the bar and HOLD nip up bleed nipple..just once only..try brakes.

do the pistons retract fully?..

I still think you have a master cylinder pisto issue..ie the piston does not fully retract after brakes applied..

 

[Pale Rider]

Do not do this! .

Oh well, this is an internet forum so I'd expect no less than instant contradiction.

The pads are not migrating fractionally away from the rotor.

Marginally reducing the size of the backing plate, or increasing the size of the slot by a tiny amount will probably allow the pads to move the tiny amount needed.

It may be the problem is no more than a burr on one of the touching surfaces.

A smear of grease on the edge of the plate will not migrate anywhere, because there won't be enough to do so.

 

[mick160]

Firstly, I have no experience of hydraulic brakes on bikes, but I have just had this thought (I am a fork lift truck engineer).

I have had a problem on a forklift with the master cylinder pressurising and not releasing the pressure. There needs to be some float between the master cylinder pushrod and the piston. On some designs of master cylinder if the piston is not fully released then the pressure just continues to build until the brakes are fully locked. The float between the pushrod and the master cylinder piston ensures the piston always returns fully.

I am not sure of the design of your master cylinder and whether any float can be adjusted but it might be worth investigating. How does your brake lever feel when the brakes are locked, does it feel solid with no/little give?

 

[Levo-Lon]

Firstly, I have no experience of hydraulic brakes on bikes, but I have just had this thought (I am a fork lift truck engineer).

I have had a problem on a forklift with the master cylinder pressurising and not releasing the pressure. There needs to be some float between the master cylinder pushrod and the piston. On some designs of master cylinder if the piston is not fully released then the pressure just continues to build until the brakes are fully locked. The float between the pushrod and the master cylinder piston ensures the piston always returns fully.

I am not sure of the design of your master cylinder and whether any float can be adjusted but it might be worth investigating. How does your brake lever feel when the brakes are locked, does it feel solid with no/little give?

the piston has to fully retract..to keep the system open when brakes are not applied..
im thinking this may be the problem too as i posted earlier..

 

[Levo-Lon]

@Kestevan
does the lever feel different when the lock occurs

 

[Levo-Lon]

If you look at the pic ,i think it may be the master cylinder piston return spring "33" that maybe weak or faulty..
still guessing but you can push the pistons back so the master cylinder piston Does move freely..but the spring must be ok for the piston to return to the open system postion.

Not sure you can buy master cyl parts with shimano...but id strip and check if it was mine.
shop replacement would be best...


https://www.google.co.uk/search?client=tablet-android-samsung&hl=en-GB&oe=utf-8&safe=images&q=shimano+master+cylinder+parts&source=browser-suggest&qsubts=1454453450084#imgrc=Erx7PuoWnCkevM:

 

[lpretro1]

if the op wants to get a warranty replacement then he needs to leave the damn brake alone or he will void that warranty

 

[Kajjal]

When the wheel locks is it both pads or just one?

A very quick test is hold brake on, loosen caliper bolts and while still holding brakes on tighten caliper bolts. Then pull brake on a few times. Then check gap either side of disc. You may need to adust by eye to get even gap both sides.

If this fails contact GT for warranty advice, don't damage the bike trying to fix it.

 

[Yellow Saddle]

You cannot buy parts for Shimano brakes. The smallest part you can get is either a complete caliper or complete lever. Also, if you have ever attempted to push pistons back you'll realize what a huge force it requires and how long it takes for the piston to move just 1mm. That little spring in the lever cannot do that and is just there to keep the lever from flopping around. There is nothing in a disc brake to retract the pads.

It is possible to have too much fluid in the system making it impossible for the piston to sit at it's intended level but that requires some stupid intervention such as bleeding it with the pistons fully extended and with nothing in between the two to simulate the brake disc. The OP suggests that this is not the case though, since the lock-up appears after a while. The scenario I sketched will see to it that the wheel is locked up from the start.

It is also possible to get vapour lock from overheated brakes but that takes some doing and the OP would have reported that he went down a very long steep hill and dragged his brakes 'till they smoked and his arms ached.

There is something else amiss and as the OP drip-feeds us with clues, it will emerge. A skilled mechanic would remove the caliper and lever assembly in one piece, clamp it on the workbench and worked back and forth with the lever and pistons to see if the problem can be replicated. Most however, just send the assembly back to the agent for replacement. Shimano's guarantee/warrantee system is superb. Shimano brakes are the best of the very best but factory faults do occur.

 

[Levo-Lon]

The spring in the mc ensures the piston returns to open postion. Slave seals just move the pistons a mm or so after force is relaxed.
if the piston moves or stays a couple of mm + down the bore then the system will be closed and the caliper will lock up or cause the binding brakes effect..after a few applications. Or heat

new complete brake would be best result as op seems to have checked the usual

 

[Globalti]

I have the same problem, unresolved, with an old set of Hope Minis on my little-used mountain bike. The way hydraulic brakes work is that it's the flexibility of the piston seals that allows the piston to move and press on the pads. The pistons don't slide through the seals and as @Yellow Saddle writes, it takes huge effort to push the piston though the seals as they are extremely tight. As the pads wear however the pistons do creep gradually through the seals to take up the slack. We used my MTB for a C2C ride with slicks and my son grabbed a handful of front brake then found the wheel partially seized; he had pushed the pistons through the seals and there wasn't sufficient flexibility left in the seals to retract the pistons. I did service the brake, cleaning everything and even polishing the pistons and replacing the seals but I was given some old unwanted seals and suspect they may have been too old and stiff because the brake is still sticking. I'm going to get brand new seals from Hope and see if that resolves the problem.

Dismantling an hydraulic brake is a pain because you have to apply air pressure to pop out the pistons, which then shoot across the workshop in a spray of brake fluid if you don't catch them in a cloth. I've developed a technique of using the plastic lilo inflator that came with my Joeblow track pump, screwed hard into the entry or the bleed port in the calliper half where it cuts its own thread and stays tight enough not to get blown out. I then apply steady pressure to the pump until the piston comes out with a loud pop. While doing this you have to close the transfer port between the calliper halves with a finger pressed on hard. Reassembling the two calliper halves has to be done in very clean conditions or you will get fluid leakage from the seam.

 

[Yellow Saddle]

I have the same problem, unresolved, with an old set of Hope Minis on my little-used mountain bike. The way hydraulic brakes work is that it's the flexibility of the piston seals that allows the piston to move and press on the pads. The pistons don't slide through the seals and as @Yellow Saddle writes, it takes huge effort to push the piston though the seals as they are extremely tight. As the pads wear however the pistons do creep gradually through the seals to take up the slack. We used my MTB for a C2C ride with slicks and my son grabbed a handful of front brake then found the wheel partially seized; he had pushed the pistons through the seals and there wasn't sufficient flexibility left in the seals to retract the pistons. I did service the brake, cleaning everything and even polishing the pistons and replacing the seals but I was given some old unwanted seals and suspect they may have been too old and stiff because the brake is still sticking. I'm going to get brand new seals from Hope and see if that resolves the problem.

Dismantling an hydraulic brake is a pain because you have to apply air pressure to pop out the pistons, which then shoot across the workshop in a spray of brake fluid if you don't catch them in a cloth. I've developed a technique of using the plastic lilo inflator that came with my Joeblow track pump, screwed hard into the entry or the bleed port in the calliper half where it cuts its own thread and stays tight enough not to get blown out. I then apply steady pressure to the pump until the piston comes out with a loud pop. While doing this you have to close the transfer port between the calliper halves with a finger pressed on hard. Reassembling the two calliper halves has to be done in very clean conditions or you will get fluid leakage from the seam.

This is a nice description of problems typically experience with bicycle brakes using DOT brake fluid instead of oil. You won't find any brake manual that admits this but bicycle brake calipers just don't work that well with DOT brake fluid. The components are too small and the fluid not lubricious enough to make the little pistons work as well as they should. Brakes using mineral oil, such as Shimano, Magura and some Taiwanese brands (I cant remember the brand names now), suffer less from this problem. the worst of the pile are brakes with Bakelite resin pistons and DOT fluid. It is not even worth attempting to polish these pistons, just replace them from time to time. Hope Mini is particularly bad.
Your lilo pump trick is a nice tip for DIY repairs. In the workshop I just trained the mechanics to wrap the caliper in cloth leaving a little aperture for the compressed air nozzle and bam! The cloth is now contaminated and should be used for that brake only and discarded. Having said that, we have on occasion had to take a ladder and search the rafter storage area for a.w.o.l. pistons. Apprentice training has its moments. But then again, none as exciting as shock overall training.

 

[sidevalve]

New seals - if you can't get them new master cylinder and or calliper - if bike is under warranty fine - if not your gonna have to shell out.