Hydraulic disc brakes mysteriously locked up

[Saxif]

I know this is a bit of an old post to resurrect but has anyone solved this problem? I have a Whyte with avids on it, not sure which Dot fluid but it has locked up. I thought it was because summer heat caused the fluid to expand, I couldn't get the back wheel off to sort out my flat tyre. It has loosed up recently but I am thinking of taking it in to change the fluid.

Being able to avoid this would be great, in previous bikes with disc brakes I have been able to make slight adjustments to avoid wheel rub but I have never had hydraulic before or come across this ...

[bobc]

I had this on an old ford escort....
On the car, the problem was that the mechanism allowing the pistons to retract was a wee hole in the master cylinder that was only uncovered when the plunger was completely "off". A bit of detritus (or carpet or something) was preventing the pedal going fully back so the pistons were in one way only mode, leading to a couple of nightmare journeys with front brakes on all the time. Hydraulics (on a car) should be self adjusting, but this can only work if the master cylinder pistons are adjusted correctly w.r.t. their "off" position.
Are pushbikes the same? (I've never worked on a bike system...)

[Indeboat]

As you have already found out the oil in the system had built up a pressure,normally the brake master cylinder(Brake lever) has to have a small amount of free play to enable the piston to return and alow the oil to return back to the hydraulic/brake fluid reservoir.
It could be that both brake levers or pistons in the brake master cylinders are sticking due to slight oxidisation,with use they might get better,providing that there is the free play,this is the very first part of the lever movement.

[Brucey]

I had this on an old ford escort....
On the car, the problem was that the mechanism allowing the pistons to retract was a wee hole in the master cylinder that was only uncovered when the plunger was completely "off". A bit of detritus (or carpet or something) was preventing the pedal going fully back so the pistons were in one way only mode, leading to a couple of nightmare journeys with front brakes on all the time. Hydraulics (on a car) should be self adjusting, but this can only work if the master cylinder pistons are adjusted correctly w.r.t. their "off" position.
Are pushbikes the same? (I've never worked on a bike system...)

The drilling you refer to is a 'compensation port'. Bike brakes come in 'open' and 'closed' flavours.

The latter have a pressure-sealed hydraulic system, usually with a gert great adjuster knob on it.

'Open' systems work just like car brakes, with a compensation port into a reservoir etc, but the system is usually sealed in the top reservoir (which only ever sees atmospheric pressure) by a compliant diaphragm.

In both cases if the system corrodes internally the pressure can rise inside the system; in the case of closed brakes, immediately, and in the case of open brakes it happens once the travel in the diaphragm seal is used up (at which point it will be hard up against the reservoir cover).

If the compensation port doesn't open it will start to behave like a closed system brake (not good).

External or internal corrosion can cause the slave piston to go sticky; once it doesn't retract easily it will drag and once it drags it will heat up and the dragging gets worse.

cheers

[TonyR]

In addition to that there are two other factors. Temperature and air pressure. A system last used and put away on a cold (winter) day can have the brakes jammed on when next taken out on a warm (summers) day. More prevalent on closed systems but as Brucey says if the compensation port is blocked it can happen on open systems too.

[Skiddadale]

Me too, had problems right from day one with the rear pads binding all the time. Spent an absolute age stripping them down replacing dot 5.1 to no avail.
Bought a brand new rear set, Pre bled and ready to fit.

Had the bike in a warm sunlounge and rear pads locked solid, not front though.

It can only mean that it is the length of the pipe and the fluid expanding , just like it does in the front set, but not as exaggerated.

Solution for me was to let some fluid out the rear bleed valve

[Gattonero]

Me too, had problems right from day one with the rear pads binding all the time. Spent an absolute age stripping them down replacing dot 5.1 to no avail.
Bought a brand new rear set, Pre bled and ready to fit.

Had the bike in a warm sunlounge and rear pads locked solid, not front though.

It can only mean that it is the length of the pipe and the fluid expanding , just like it does in the front set, but not as exaggerated.

Solution for me was to let some fluid out the rear bleed valve

Have you replaced the diaphragm? Often is a cause of trouble, and given the low cost is one of those things that should be replaced anyway.
I would also recommend some specific grease (the Castrol Red Girling has helped me many times) on the pistons: sometimes is enough to expand the pistons as much as possible and smear the grease, in severe cases you need to buff the ppistons from corrosion/contamination and grease the quad-rings (this of course means to purge the whole system and bleed again)

[peetee]

I have seen a lot of problems with Avid brakes. They seem to be prone to lever piston seal problems. With every one I have replaced you can hear the piston suck when the lever returns. I guess it's air getting into the system which doesn't always effect the power and response of the brake but can pressurise the system to lock the brakes on after lying idle. presumably the air sits in the reservoir and doesn't advance to the brake line.

[Gattonero]

I have seen a lot of problems with Avid brakes. They seem to be prone to lever piston seal problems. With every one I have replaced you can hear the piston suck when the lever returns. I guess it's air getting into the system which doesn't always effect the power and response of the brake but can pressurise the system to lock the brakes on after lying idle. presumably the air sits in the reservoir and doesn't advance to the brake line.

That MC is a flawed design, and them pistons often have friction. the red grease can mitigate the latter.

[DaraParsavand]

I came across this thread after having 3 out of 4 disc brakes (2 bikes) locked up after storage of 6 months in an area that does get damp. These were both Avid brand brakes on 2016 Salsa Mtn Bikes. After seeing this thread and watching https://www.youtube.com/watch?v=RKlr6dc5nus I was determined to give at a try. Two different bike stores, one where I got the bikes told me they would do nothing and recommended I get new brakes as this old a brake from Avid wasn't very good. It was super easy to slightly loosen the fluid screw with a torx (star) wrench and then spread the pads with a moderate size flathead screwdriver (I didn't have the $20 Park tool or equivalent tools). Rode yesterday for a few hours with absolutely no problems on either bike. I'm quite disappointed in both my local bike shops for not clueing me in very well.

When this fix doesn't work anymore - either because too much air got in the line (I feel no sponginess so far) or it won't unstick, I will be replacing with TRP Spykes which seem to be the best cable actuated disc brakes (since they use dual pistons) under around $100 per wheel including levers and cables (if you look for them on sale and install yourself). I assume they are available in UK (I'm in CA, USA). I got burned by the take the wheel off without using a chuck problem and now this with hydraulics - I really despise this technology now and feel it was a big big mistake for the market to move to hydraulics for anybody but racers - it's a worthless idea for people who like to work on their bikes as I used to. Servicing a shock is one thing - brakes need to be bombproof - not causing my headaches.

[Brucey]

I have seen a lot of problems with Avid brakes. They seem to be prone to lever piston seal problems. With every one I have replaced you can hear the piston suck when the lever returns. I guess it's air getting into the system which doesn't always effect the power and response of the brake but can pressurise the system to lock the brakes on after lying idle. presumably the air sits in the reservoir and doesn't advance to the brake line.

FWIW the MC shouldn't 'suck' hard but it definitely will do if the caliper pistons start to bind. Thus a MC seal failure and/or air entering the system via the MC seal can be symptoms of sitcky caliper pistons.

Corrosion in the piston bores, crud ingress, swollen seals, and foreign matter in the hydraulic system can all cause the pistons to go sticky. The seals in a DOT system will (I think) be made of EPDM and this swells up like crazy if any conventional oils come into contact with it. This means that all kinds of 'maintenance sprays' etc are absolutely verboten around these (or any other ) DOT fluid brakes. The only lubricants that are safe around DOT seals (that I know of ) are castrol red rubber grease (which uses a vegetable oil base) and some silicon greases/lubes. Hope recommend a silicone spray used by plumbers as a piston lubricant; I don't see why that shouldn't be used in other DOT fluid brakes.

DOT fluid brakes coming on by themselves in storage (i.e. wheels going draggy without the brakes being applied even once, which might be sticky pistons) can occur via

1) gas evolution within the system (eg via corrosion) and/or
2) the fluid volume within the system increasing.

The latter occurs when the fluid absorbs atmospheric moisture. DOT fluid is incredibly hygroscopic, and water diffuses into the fluid via the seals. The net effect is that the total volume of fluid increases and if the system is already 'full' the diaphragm cannot accommodate any further volume increase and the brake comes on. Note that this also signals that the contaminated fluid now has a depressed boiling point, so ought to be changed.

FWIW I think hydraulic brakes feel great to use and folk usually love them, right up until the moment when they go wrong. Then they turn into a major PITA. A frequent response to this PITA-ness is that folk say 'just not worth the aggro' and for most purposes I'd have to agree. Mineral oil brakes are probably more reliable than most DOT ones but these are not 100% problem-free either.

FWIW in cable-operated brakes I'd choose BB7 over spyke/spyre; the latter have an unnecessarily complicated mechanism inside that is vulnerable to corrosion damage, pad adjusters that move under their own steam and the idea that both pads moving is useful is simply not that important.

cheers

[peetee]

.

FWIW I think hydraulic brakes feel great to use and folk usually love them, right up until the moment when they go wrong. Then they turn into a major PITA. A frequent response to this PITA-ness is that folk say 'just not worth the aggro' and for most purposes I'd have to agree. Mineral oil brakes are probably more reliable than most DOT ones but these are not 100% problem-free either.

cheers

When I am asked to fix hydraulic brake problems I invariably just say that I would rather replace than repair. I want people to ride off on a safe bike and the only way I can be sure that they will have working brakes is to fit new ones.

[peetee]

.

FWIW I think hydraulic brakes feel great to use and folk usually love them, right up until the moment when they go wrong. Then they turn into a major PITA. A frequent response to this PITA-ness is that folk say 'just not worth the aggro' and for most purposes I'd have to agree. Mineral oil brakes are probably more reliable than most DOT ones but these are not 100% problem-free either.

cheers

When I am asked to fix hydraulic brake problems I invariably just say that I would rather replace than repair. I want people to ride off on a safe bike and the only way I can be sure that they will have working brakes is to fit new ones. Last summer it was blazing hot for weeks and i had a lot of bikes in with fluid-related brake problems. Every one had been stored in a shed.