Pumped-Out Pistons on Hayes Hydraulic Disc Brakes (Trek Liquid 20)



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Michael Slater

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So the pistons got pumped out on both the front and rear brake of my wife's Trek Liquid 20. This
bike has Hayes Hydraulic disc brakes. When the pistons get 'pumped out,' there is continual friction
between pad and disc.

Reading through the service manual and the online manual
(http://www.hayesdiscbrake.com/pdf/General-45-14575A.pdf), the short answer is to remove the pads
and then 'walk the pistons back into their bore.'

Well, much easier said than done. The pads come out easily, but getting the pistons back in the bore
seems to be impossible. I can get them in, and the rubbing between pad and disc is much reduced, but
it's still there. The rear brake is more difficult than the front.

The instructions suggest using 'the box end of a 10mm end wrench.' I used exactly that instrument,
and it was not clear to me at all how this is the magic implement for doing this. I still am left
with either a tiny lip of the piston still overhanging the bore, or else one side shoved in
adequately and the other side poking out terribly. Either way, there is still friction.

Before I drag this thing back to the shop to get it corrected 100%, does anyone have any suggestions
about how to do this exercise more effectively?

It's a really alarming problem, because it's not that unlikely that the brake lever will be thrown
while the wheels are out (like in the back of my car, for example), and I didn't notice any
lock-outs on the brake pads or levers to prevent it.

regards

[email protected]

http://karavshin.org
 
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Huw Pritchard

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On Sun, 30 Mar 2003 06:57:04 +0000, Michael Slater did issue forth:

> Reading through the service manual and the online manual
> (http://www.hayesdiscbrake.com/pdf/General-45-14575A.pdf), the short answer is to remove the pads
> and then 'walk the pistons back into their bore.'
>
> Well, much easier said than done. The pads come out easily, but getting the pistons back in the
> bore seems to be impossible. I can get them in, and the rubbing between pad and disc is much
> reduced, but it's still there. The rear brake is more difficult than the front.

I've never worked on a Hayes brake, only on Hopes, but I'd imagine that the same techniques will
still work. If you use a spanner as suggested, try twisting it instead of pushing it. This way you
should be able to put pressure on both pistons, not just one.

If this doesn't do it, it looks like you might have to let some fluid out. Open the bleed nipple
just a little bit to ease a tiny bit of fluid out, do it back up and try pushing the pistons back
again. Keep doing this until you get to where you want to be. Hopefully you'll have let out a small
enough quantity of fluid that you won't have to rebleed the system.

> It's a really alarming problem, because it's not that unlikely that the brake lever will be thrown
> while the wheels are out (like in the back of my car, for example), and I didn't notice any
> lock-outs on the brake pads or levers to prevent it.

I always pack the caliper out with something if I'm going anywhere with the wheels removed.

--
Huw Pritchard Replace bounce with huw to reply by mail
 
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Mike Jacoubowsk

Guest
Best bet is to bring it into the shop and have one of the mechanics show you how to re-set the brake
adjustment. Please note that a small amount of drag is normal with disc brakes but, having said
that, your problem might be that the mechanism is slightly out of alignment. That's an easy thing to
fix; you loosen the two bolts holding the disc mechanism to the dropout, and, with the wheel in
place, squeeze the brake lever. Tighten the bolts while the levers are being squeezed, and the
mechanism will now be aligned (and possibly exhibit less drag).

--Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com
 
T

Tdwfl

Guest
>Huw Pritchard" bounceCHECKSIGBEFOREMAIL wrote: >I always pack the caliper out
with something if I'm going anywhere with
>the wheels removed.

Or one can wedge something against the handlebar to keep the brake levers from depressed.

tim
 
C

Chuan Chew

Guest
In article <[email protected]>, [email protected] (TDWFL) wrote:

> >Huw Pritchard" bounceCHECKSIGBEFOREMAIL wrote: >I always pack the caliper out
> with something if I'm going anywhere with
> >the wheels removed.
>
> Or one can wedge something against the handlebar to keep the brake levers from depressed.

A box of Prozac?

--Chuan

> tim
 
D

Doug Huffman

Guest
It has been a while since I've been in my Hayes Brakes manual since my set seems to be trouble
free. IIRC...

Two situations seem to be being addressed here.

The pistons may be inadvertently extruded from their bores while the wheel/rotor is removed. ISTR
this requires a knife-edge to make the first slight separation, followed by a pair of cards and
finally the 10 mm boxed wrench. I avoid this problem by wedging a small piece of sheet metal in
place of the rotor as soon as the wheel is removed.

Disks will touch the rotor during normal use and previous posters have addressed this and its
minimization.

I have had high ambient temperatures cause a thermal lock. While the bike was parked on a 100+F day
and in the sunlight, the master-cylinder heated enough to lock the slave-cylinder. While momentarily
disconcerting, venting a bit of fluid - from the vent screw on the master-cylinder - cured the
problem. I try now to do my annual fluid flush and fill on a hot day.

"Michael Slater" <[email protected]> wrote in message
news:[email protected]...
> writeup of the solution...
>
> http://karavshin.org/blogs/black-coffee/archive/000384.html
 
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