Topic: Front brake caliper question

[zzcoopzz]

I've been working on the front breaks because they're pretty mushy and are working at about 45%, not very conducive for canyon runs.

I've ripped it all apart, bled the lines, changed the master cylinder, replaced the old lines with new ones but still the same mushiness.  I ripped the calipers apart again and now am thinking that its something in the calipers.  The piston is real tight, maybe too tight, plus I have to pump the breaks to get them to maybe 75%.

the next step is to rebuild the calipers and replace the pistons.  I guess my question is, should the piston be this tight?  I cant move it with the piston with my fingers.

thanks,
Cooper

[csendker]

I believe there need to be a just little bit of play in the piston as the only retraction force is the rubber seal & a bit of suction. My piston can be moved in & out with just my fingers.

[nomadwarmachine]

Oxidation can build up over time on the race of the seal groove, making the seal tighter than normal and requiring more force to move the piston.  When I rebuilt my front brake, I took a plastic Dremel brush and ran it along the race to scratch off the oxidized material WITHOUT scratching the aluminium itself.  What ever you do, make sure you use a material that is softer than the metal, otherwise you will be hunting for a new caliper.  Scrape out the gunk, lube it with brake fluid, and pop in a new seal.  Should be a nice smooth operation afterwards.

[ofreen]

I guess my question is, should the piston be this tight?  I cant move it with the piston with my fingers.
thanks,
Cooper

The answer is no, it shouldn't be that tight.  Have you had the caliper apart?  If not, it likely there is a lot of crud on the piston and caliper bore.  One thing I don't see mentioned much is the necessity of making sure the piston is spotlessly clean when replacing the front brake pads.  I don't know what bike you have, but on the old 750s, there is no dirt seal on the piston.  As the brake pads wear, the piston extends further and further out of the caliper.  The piston collects crud.  It has to be cleaned thoroughly before installing new pads, or else you are pushing all that crud up into the caliper.  The piston will stick after that.  A good indication that is happening is when your rotor turns brownish-blue during a short ride and the skin melts off your fingers when you touch it.

When I replace brake pads, I always disassemble the caliper and clean it before installing the pads.

[eldar]

that crud sucks. it is not only dirt and road grime but also dot 3 attracts moisture. the best thing to do is probably clean you caliper every year before you start riding. The dremel works good, I uses a brass wheel and that worked very well, the plastic is sometimes not strong enough to get out all the crap. Pull out the rubber seal and clean that groove!

[ProTeal55]

Not sure if you replaced your brakeline with another rubber piece or not, but a stainless steel line will make your brakes allot better. Even though they are both rubber lines, the stainless steel outter coating prevents the hose from expanding under pressure,which makes your brakes "tighter" .

[csendker]

I just re-read your post.  Two sets of hoses & two MC's.  My bet is that there's air somewhere in the system.  Unless you have a leak, mushy = something's compressing (or expanding).  A leak is obvious, so eliminate that.  With two different sets of MC's & hoses, I'd guess at least one set was good - this should eliminate 'expanding'.  That leaves compressing, which means air.  I just repalced mine last night.  I bled them until there was no air, and they were still mushy.  I wired the handle closed for a couple of hours (air bubbles rise...) but got impatient and re-bled them.  Bingo! a zillion micro-bubbles came out and now they're nice & hard. And that's with crappy old original hoses.  Keep bleeding, or try some different bleeding techniques.  I'll bet you still have air trapped somewhere in the system.

[csendker]

I used this gizmo from Advance Auto for around $7.  The bleeder 'peg' could be better - it kept popping out, but overall it worked OK.

[eldar]

yeah I used one of those too. Works ok but I dont feel like I get everything out with that so  I use a clear tube and open/close the npple.

[zzcoopzz]

I just re-read your post.  Two sets of hoses & two MC's.

I have a 1977 CB750 F, two calipers on the front, one line for each heading into a splitter, then up to a 2000 SV650 master cylinder (I like the external reservoir).

the lines have been replaced with the sv650 lines so I'm assuming they are pretty tight.  next step will be to get steel breaded lines, but right now other parts take priority.

I'm going to try that wiring the handle closed trick and see what happens.  I have to say though I bled the lines for a ridiculous amount of time yesterday.  I have a system where I have the bled tube go right back to the reservoir so I don't have to refill the cap every six or so pumps.  I can sit there all day long and open and close that nipple tell the cows come home.  hrm...

every now and again would pump the line and would get that burst of tiny air bubbles to release, however I bled the lines for a good 45 minutes without seeing any air bubbles so I'm assuming I got them all.

that crud sucks. it is not only dirt and road grime but also dot 3 attracts moisture. the best thing to do is probably clean you caliper every year before you start riding. The dremel works good, I uses a brass wheel and that worked very well, the plastic is sometimes not strong enough to get out all the crap. Pull out the rubber seal and clean that groove!

yeah, I found that gunk and took a copper wire brush to it.  looks like I got all of the crud.  the piston is still real tight however.  I'm guessing that that seal is just hard rubber at this point.  I'm ordering a rebuild kit, we'll see if thats the trick.

I did check the piston without the seal, though maybe the caliper got hit and misshaped the cylinder.  it slides nicely... its got to be the seal.

thanks everyone for the help, I'll let you know how it goes when I get the rebuild kit.

coop