Post by anon on Feb 1, 2007 20:06:33 GMT
From: rowlf Sent: 10/09/2002
An excellent posting by scracha in response to another members queries about brake calipers.
Q/ With the pads out it's possible to *fully* depress the lever without the pistons popping out. Surely this can't be right.
A/ It's fine. You have to pump the brakes to build up the pressure. After messing about with pistons (even if you've not taken them out fully), you have to pump the brakes.
Q/ With the lever fully depressed the pads move maybe 5 millimeters out the when it's released they spring back maybe 4.
This is GOOD.
Q/ It would be possilbe to 'pump' the pistons through the full range of their movement. I didn't (I don't want to do a full rebuild)
A/ Keep the calipers connected to the brake lines and just pump them out just over half an inch and clean them up, saves having to bleed the whole system and muck about with seals. Only pump the pistons right out and fully clean if they are really bad (ie. try the first method and 9/10 times they'll not stick. There's no point in pumping them right out every time as carp can't get in there to the back of the pistons.
Q/ What's the problem? Air in the system or siezed pistons? I've tried to clean them with brake fluid.
A/ There is special brake component cleaning spray available but to be honest I just blast them with WD40 and let all the carp drip out of them. Then use fine wet n' dry, wire brush (watch the paint on the caliper when using this though), a rag and a soft screwdriver (gently, don't score the pistons or burst seals) to get stubborn bits. Remember to grease the pistons before you shove them back in (preferably using a G clamp), and use high temp grease and wipe off the excess as you don't really want the stuff getting on discs or pads.
Q/ They are in no way smooth. One has a rusty patch. I freed them to the point there they moved easily with a large pair of pliers but still can't move them with my hand.
A/ Doesn't matter. Only very clean brakes move by hand. As long as they are not sticking once you've put them back on the bike then don't worry. After a winter's riding I normally have to use G clamps to get the buggers moving. You've already said that they move back 4mm when you release the brake/lever. To test, stick them on the bike, go a nice gentle run, put the bike on the centre stand and rotate the wheel(s) by hand. IMHO a (very) slight rubbing is acceptable.
Q/ Am now bleeding the brakes. There was lot's of air in them before but no obvious sign of leakage. It strikes me that the rusty patch on the rear caliper *must* leak.
A/ Nope. The rust patch will be just the bit that's been exposed (the more your pads wear the further the pistons come out....waiting till pads get down to 2mm is a bit of a false economy). Behind the rust patch will be a dust seal and further down will be the proper seal that stops the fluid coming out. Believe me...if any fluid was leaking you'd know about it very soon as bike's don't have a huge fluid resovouar. Bleed them again in a couple of days as you'll be unlikely to get all the air out the first time.
Q/ Am still hoping for the best. May well need to get new pistons and an overhaul kit.
A/ Not unless absolutely necessary.
Tip:- One other tip that very few people do. Brake dust gets EVERYWHERE. The drilled holes on your discs are there for a reason. Get a screwdriver or pipecleaner or similar and clear all the brake dust out of the little holes (give the discs a wipe afterwards as they'll now becovered in the carp you've just taken out of the holes) and notice the difference in wet weather and under extreme dry weather braking.
ANYONE can do a quick strip of calipers. It will take you about a couple of hours the first time but after that you'll do it in 20 minutes. I'm sure there'll be someone in this club in your area who could take the time to show you how to do it if you don't have a haynes/clymer manual.
YOU NEED
12ml (or was it a 14??) ring spanner,
an 8ml ring spanner (or decent socket set),
a wire brush,
high temp grease (most are nowadays),
Copper slip stuff,
WD40,
a small G-clamp (although 2 makes it a lot easier),
soft (flat) screwdriver,
philips screwdriver,
hammer,
some rags and maybe some very fine wet n' dry and an old toothbrush.
MY METHOD
1) Get soft flat screwdriver and attempt to prise apart brake pads from disc. Make sure not to score discs.
2) Use 12ml ring spanner to undo two bolts holding caliper on.
3) Swear a lot and wangle caliper off the disc. (You may end up whacking caliper with hammer using a block of wood so it's spreading the impact). The caliper will be attached with the brake line but you'll have enough space to work on it.
4) Take philips screwdriver, unscrew screws on brake fluid resevoir on handlebars. You will probably find the screws are made of cheese. Find a better fitting screwdriver or use a socket. Failing that, knock screwdriver into screw with hammer, grip with both hands and unscrew it. (replace screws with them hex headed non rusting ones used with allen keys is much more sensible....I recommend getting a small bucket of anodised bolts from Mr Fastner...they're very handy for loads of things).
5) Force open sliding mechanism on the caliper (with nice long screwdriver(s), pads will fall out.
6) Check fluid level, put cover back on fluid resevoir (without the screws) and wrap rag around it.
7) Clean EVERYTHING as per posting above.
Keep eye on brake fluid level when pumping pistons out and also when pushing them back in.
8) Replace pads if necessary.
9) Regrease sliding mechanism
10) Reassemble using copper slip stuff, especially on the two big caliper mounting bolts.
11) Bleed brakes as per my last post and then put the screws (or allen bolt thingies) back in the brake fluid resovuar.
(the rear is similar except you have to bleed both sides and the pads have a retaining pin that has to be taken out....newer divvies have twin disk setup which means bleeding both sides)
If you really honestly can't do it then find a good friend who can as 99% of bike shops just don't take the time and do the job properly which is why you'll find your calipers sticking again so quickly.
Many people hose (just a gentle trickle, you don't want to strip paint) the calipers down after riding in bad weather but I can't say I've ever been bothered to try this.
Trial-error knowledge ;D