Caliper lacks mechanical force

[23jayhawk]

What causes a caliper to go bad? I've been suffering a gradual decline in braking power on the front of my '02 KDX, and through a long process I've concluded the problem is in the caliper.

However, I removed and completely disassembled the caliper and there appears to be no problems with it. The seals are in good condition, the cylinder walls have no scoring or corrosion, the slide pins were greased & worked fine. Basically, I cannot see any problems with the caliper, so I don't know what I would gain by installing a rebuild kit.

Any ideas?

For a rundown of what I've done so far, read on:

-- bled brakes conventionally on bike
-- installed new brake pads
-- removed system (intact) and bled conventionally, with the caliper elevated above m/c
-- swapped m/c's with my son's '03 KDX (mine still lacks power, and his can do 2-finger stoppies in the driveway with my old m/c)
-- bled system with a vaccum bleeder with caliper elevated
-- removed & disassembled caliper, cleaned & reinstalled
-- reverse bled the system from caliper to m/c ( at this point, if I am certain of anything, it's that the system is free of air)
-- installed braided s/s brake line
.
Other than a slightly firmer lever due to a better evacuation of the system, I am no further along than when I started. Pushing the bike along the garage floor and grabbing a handful of brake lever, you can (almost) lock the front wheel. That is on a slick garage floor, so that gives you an idea of how weak the brakes are.

So what exactly is there to go wrong with a caliper? I am wondering if they can start developing some flex in the caliper frame that you can't see, but is absorbing the hydraulic pressure instead of converting it to mechanical force.

 

[CaptainObvious]

Is the system leak free?

Sounds like you've done everything right...to the caliper. I'm thinking it must be the slave cylinder (the part that the brake lever attaches to).

 

[reelrazor]

Brake lever attaches to the MASTER cylinder, which Jayhawk says he has tried replacing with one off his son's bike. But, he doesn't say whether or not that cureed his problem.

He has tried new pads too.

Myself, I would be giving the rotor a good looking over. If it has worn thin(below spec) it may be that there isn't enough fluid displacement to push the pads hard against the rotor. Also, new pads need to be bedded in and worn shiny rotors DO NOT help that any.

 

[23jayhawk]

The system is tight, no leaks. I'll swap front wheels tonight to check out the rotor, but that seems like a long shot. I've cleaned it several times with no improvement.

 

[CaptainObvious]

Opps, I didn't catch the "m/c". I've experienced a warped rotor on a friends bike. It did in fact make the braking force weak, but it also pulsed at the lever.

 

[Patman]

I had a heck of a time purging the air from my front brake after replacing the hose just last week. I did everything you didand I finally cranked the Mitevac up to 26in hg and pulled 1.5 bottles of fluid through it until it was bubble free while hanging caliper high. Also put some grease around the bleeder threads just to be sure you are not sucking any air.

 

[David Trustrum]

Yeah I was going to say swap the wheel if the rotor has 'coned' you might be flexing it. Another idea is the sliding plate may have got bent if you smacked the calliper in a rut, thus getting sideways deflection when you squeeze the lever, you could try swapping that over.

Brake can lines swell when they get old but was never a prob on my ancient kdx.

 

[matt-itude]

just reading throuh all this and thought I would mention to you guys that a really easy way to bleed the brakes (if you arent needing to replace the fluid such as if the bike got laid down and the lever pulled while it was laid down) is to just push the caliper piston in with the bike in the up right position. this will push fluid and air back up and through the master into the resivore where the air floats to the surface and typically works well because it is all up hill. then just pump the brakes back up.

 

[23jayhawk]

It's not the rotor.. swapped front wheels, and I can still do stoppies on my son's bike, with my old m/c and my front wheel. So it must be in the caliper assembly.

I've removed & disassembled the caliper again. Looking it over it's kinda hard to see if any of the parts are tweaked. There are no flat surfaces or straight lines anyway, so not sure how I'd tell if it was torqued out of shape.

The pistons slide in the caliper bore super smooth with no seals in place, but it is a fairly snug fit with the seals installed. Perhaps they have swelled and are causing a restriction on the piston sliding through the caliper bore. Guess I'll order some new seals and try that next.

 

[Rcannon]

I wonder if the caliper hanger might be bent? I had this happen on a CR 80 a few weeks ago. The brake would drag quite severly.

Is your front lever ok? Does the lever put enough pressure on the m/c itself? If it uses a screw, can it be adjusted?

The only other idea I have is this. I once had troubles on my old DR 350. No matter what I did , I could not get the front brake to work very well.

I finally decided to replace everything with brand new oem stuff. Everything. I was so pissed I did not care if it was 1000.00. I just had to fix this. It was saturday night so I could not order parts until monday morning. For the heck of it, I pulled the brake lever in and duct taped it to the handlebar. I became sidetracked and left the lever taped all night. When I removed the tape the next morning , the brake was good again. There must have been trapped air inside the m/c that left it while being depressed.

Good luck!

 

[marcusgunby]

If it feels firm it must be something other than air in the system-some systems with air in can still work well.I would look at the caliper, i used one from a 2003 KX125 on my 04 rm125 and it improved it but its still not as good as a 2003 kx125, however i think the KX has a 10mm larger rotor, maybe for you its time for a big rotor kit-they dont cost too much.