Topic: Front brake woes

[alterego]

So, with the weather turning nice again I figured it's time to try and sort out the disc brake once and for all... a little history: it's never worked properly, the lever would be way too hard to press with hardly any braking happening, bleeding the system did nothing at all, then had the piston seal changed and braided hoses put in which made the brakes work for exactly 5 minutes before getting stuck again with a near solid lever. The mechanic (supposedly specialises in older bikes) didn't mention anything, just put in whatever I gave him and charged me for the work and after the brake stopped working again I couldn't be bothered going back there.. At some point after that I noticed that there was brake fluid leaking out as well and then winter came so nothing got done...

Now indeed the pad inside is completely soaked with brake fluid. It had been seeping before the new seal was put in as well but nothing this bad. So we took it apart ourselves by pressing the lever until the piston popped out (it would come out but not retract when pressed). The piston looks mostly clean but does have a few little pits in it, the calliper on the other hand is covered in pits on the inside.

So the big question is: how much pitting is too much? I tried rubbing the piston with a mild abrasive cleaner to see if I could smooth it out but one pit in particular is quite noticeable when rubbing a finger over it, even if it's just about 1mm square. Could that be the culprit? There are a few little scratches on the other side of the piston as well which can be felt by touch. As mentioned, the rubber seal is brand new so ideally that should be fine, I hope. I'm going to try and get it out without damaging it to see if the channel inside the calliper is clear or if there is dirt in there which might break the seal. And while it's all apart I'll check that the little hole in the calliper is clear. Could pitting inside the calliper body cause issues or is that less important if the piston and seal are perfect?

Is there anything else I could try or just order a new piston and hope for the best? I'll try to post up pictures later today.

Sorry for the potentially stupid questions; I just really want my brake to work again (on a budget; I feel a bit ripped off by that mechanic who changed the rubber seal but potentially never checked the piston itself. Assuming that's what's causing my trouble). So now just trying to do it myself.

Thanks

[HondanutRider]

Its all been said before in this forum how to clean and rebuild these brakes....although severe pitting may cause the piston to "hang up", CLEANLINESS is the key.  In particular, the groove where the o-ring sits, INCLUDING the recessed lip on the leading edge - must be positively clear of any debris.  It is the action of the squared o-ring seal rolling up into this lip while clinging to the moving piston, that pulls the piston slightly back when the pressure is released.  Any dirt causes problems.

Having said that, your symptoms imply that you may have other problems and would be better off buying a new caliper.  David Silver has replacement ones I believe, for some models of SOHC4. http://www.davidsilverspares.co.uk/CB350-FOUR/part_44054/  That or a suitable used unit might be a better solution than trying to fix your leak and hard action.

[Lostboy Steve]

I have been through all of my components and switched to stainless lines. I also used countless different recommended methods of anti-squeak agents. I still get a ton of squeaking.

[prov431]

To cure the sqealing you need to file the edges of the moveable (piston side) brake pad - bevel the edges all around, make sure the pad moves freely in it's socket and doesn't hang up. Clean the brake rotor as well - go ride it...should cure it!

[flybox1]

pitting prevents a proper seal between it and the piston seal
this irregular edge can allow brake fluid to seep through, as you've apparently experienced, as well as it can prevent the piston from retracting properly. 
rebuilt properly, and with correct caliper arm alignment, you should not squeal.
get a new phenolic piston, a drilled rotor, and anti-squeal pad from godffery's garage.
http://www.vinmoto.org/gallery/v/VinMoto-Garage/album03/Godfferys-Garage-123/album185/

get a new piston seal (be sure to adequately clean the groove it sits in for it to properly function), and use the appropriate lubes on its different parts.  there are many caliper rebuild threads with great information. 
read this one before you start. 

http://forums.sohc4.net/index.php?topic=104349.0

There are multiple threads about this same topic:

http://forums.sohc4.net/index.php?topic=56544.msg610718#msg610718
  Lets' review.
There are four products needed to restore a caliper.
1. Brake cleaner
2. Brake parts assembly lube for the caliper seal only.
3. Dow corning High vacuum grease.
4. Brake fluid

Each has it own specific application and needs for the task to be performed.

#1 is used to get everything clean.  (given the confusion, it had to be said)
#2 is used ONLY on the seal and must be compatible with #4, as some of it will invade the brake fluid chamber.
#3 is NOT used on the seal, as that would put silicone inside the brake fluid chamber.  This is a bad thing as the compounds are incompatible.  It is used behind the brake pads and a thin coat is used on the parts of the calipers and piston that are exposed to the elements and subject to water ingression.  The Dow Grease will not mix with ANYTHING.  It is a simple barrier with the very important property that it will NOT melt when the parts get hot and then run/creep into the brake pad friction material and provide lubrication between pad and disk rotor.
#4 is what makes the hydraulic part of the brake function. (I hope that part is clear.)

Cheers,

[Dave Voss]

All of the advice regarding cleanliness is super important, but if your caliper has significant pitting inside, then I would suspect that the groove for the piston seal is no longer smooth enough to properly support the seal to prevent the piston from sticking, or brake fluid from seeping, and so I would suggest replacing the entire caliper assembly with new or good condition used that can be cleaned and rebuilt with a new seal.  A lot of time can be spent trying to work with corroded calipers and pistons, but in my experience it just isn't worth it.

[mono]

my piston was pitted  just in a couple places and would not retract properly.  replaced the piston and the seal -- bam.  works great.  i got a stainless replacement piston for $35 to fix the pitting problem for good!

[alterego]

Thanks for your responses. Finally I've taken some pictures. And upon further inspection of the caliper I can see that the edge along where the seal sits is kind of chipped away in places. I'm guessing that's not good. Hopefully the pictures are clear enough... I've taken the seal out as well and it seems reasonably clean, it's just that the metal is pitted around it.

For the moment I'm not that concerned with squealing that much, I just wish I had kind of functional front brakes.

Guess I have some shopping to do... And also get yet another set of pads because these are soaked with brake fluid now.. again..

[70CB750]

I would replace the housing and the piston too.  The pitting is not bad, but somebody used plumbers tool don the piston seems like.

The housing is quite deteriorated.

[bryanj]

The piston is shot, the housing has no corrosion where the pisto fits so could be reused as long as the seal groove is cleaned out properly. The corrosion you have where the pad fits needs to be removed then remove the paint from the outer steel rim of the pad so the pad fits real easy in the housing and when fitting used the correct grease.

Another sieze ponit is the pin that the caliper bracket pivots on and this needs to be free as a bird.

Also if the tiny hole you can not see in the master blocks the brake drags on---plenty of post on that one

[mono]

yeah, definitely new piston time.  yikes.

as far as the housing goes... it's probably seen better days, but i'd say clean it up and try to reuse it.  if it works, great!  if not, get another.