Author Topic: but i just rebuilt my brakes!!! ie: why are they sticking?!?! (Read 4502 times)

Greg · « **on:** June 20, 2005, 02:08:33 PM »

well i've put off putting this one to the forum bc i figured i could figure it out...but i could use some help here.
76 cb750f
tore apart the front caliper , cleaned it out, new square lip seal. got it sandblasted, and the guy blasted inside the piston bore, but the piston and seal still fit nice and snug so i thoght no problem. new stailes steel brake lines. didn't rebuild the mc...

rode around for about 600 miles with no problems, brakes great. then one hot day on teh highway i realized the front caliepr was seizing, pulled over, let it cool off, and limped home.

pulled the caliper off, cleaned it out, rebled the system -- still seizes
pulled the mc off, cleaned it out, rebled -- still seizes.
pulled the pressure switch off, cleaned out, rebled -- still seizes.

the air return hole in the mc seems to be totally clear, so i don't know what i am missing. however, even when ir elease pressure by loosening the bleeder valved the brake still seems to drag.

any ideas abot where i should start?

thanks guys!1

greg

TwoTired · « **Reply #1 on:** June 20, 2005, 02:17:14 PM »

Did you adjust the stationary pad for .006 gap?

The piston side pad must be retracted a bit for this adjustment.

Push the caliper toward the rotor with the palm of your hand (below the bleed nipple). Does it retract smoothly?

Greg · « **Reply #2 on:** June 20, 2005, 02:24:26 PM »

twotiered
thanks for the quick response.
i did adjust the pad -- didnt measure it, but eyeballed it.
if i push the caliper towards the rotor, nothign will happen. the piston will not retract into the caliper.
i know this implies that there is a blockage in the return? -- but i don't know where else to look/clean.
ideas? thoughts?
thanks!
greg

Bob Wessner · « **Reply #3 on:** June 20, 2005, 02:28:31 PM »

Don't mean to second guess you, but are you absolutely sure that little hole (closest to lever) in the M/C is clear? I thought mine was after cleaning, but had to take it out in direct sun light to be sure, and it wasn't.

Greg · « **Reply #4 on:** June 20, 2005, 02:32:05 PM »

bob
what we need more of around here is second guessing.
i **think** the hole is clear, but all symptoms point to clogged, right? i mean, what else *could* be wrong? brakes are prertty simple. so your vote is faulty mc cleaning job? i sprayed a bunch of brake cleaner through the hole, stuck some a piece of copper wire through, but maybe it need to be cleaned some more?
hhmm...
thanks
greg

Bob Wessner · « **Reply #5 on:** June 20, 2005, 02:35:28 PM »

If you used both cleaner and a thin wire, it should be clean. How about the piston being cocked and jammed in the caliper seal?

TwoTired · « **Reply #6 on:** June 20, 2005, 02:37:10 PM »

If the piston won't retract it's either hydraulic pressure or mechanical resitance. If Hydraulic pressure, loosening the bleeder or any fitting in the line should relieve the pressure. If so, then the master is suspect.
Check the master to ensure there is nothing peventing the piston there from retracting fully with the brake lever. Did you hone the master cylinder bore during rebuild? Are you sure you got the correct master kit for your master cylinder? Do you know the master on your bike is from the year you bought the kit for?

If when pressure in the lines is relieved and the piston still won't move, then I would suspect some of the blasting media remained during overaul and has now wedged into the space between piston & caliper.

Greg · « **Reply #7 on:** June 20, 2005, 02:41:34 PM »

bob
if the piston is cocked and jammed, wouldn't taking it out and putting back in nicely fix it? is there some trick to putting it in?
thanks for the ideas...
(i read your tutorial before cleaning the mc, it was a big help!! thanks!)

twotiered
loosening the bleeder allows the piston to retract some, but it still drags a bit. my 81 kz440's wheel spins *freely* whereas the cb750 spins, but with drag (when bleeder is loosened).

do you think it could be a blasting media problem when it didn't surface for over 600 miles?

i guess ill try and pull apart the mc again tonight and clean it out. i didnt have a mc rebuild kit, just cleaned it up. and no, i ddin't hone the mc bore -- it looked totally clean.

thanks guys!

TwoTired · « **Reply #8 on:** June 20, 2005, 02:42:57 PM »

another thought..
Did you clearance the new pads to move freely in the caliper bore?

Are you using the nylon ring behind the pad?

Greg · « **Reply #9 on:** June 20, 2005, 02:47:45 PM »

twotiered

i wasn' using the ring before, after rebuilding the mc i put it in there.
what do you mean by "clearance" the pads?
with the caliper all mounted up i tried pushing the piston out with a c-clamp to ensure plenty of space for the ring and newer pads, but even with the c-clamp i couldnt get the piston to retract.
also put brake grease on the back and edges of the pads so they could slide around in there.
i'm leaning towards a missasembly of the mc, or some problem with the piston in the mc getting caught/bound.
thoughts/
greg

TwoTired · « **Reply #10 on:** June 20, 2005, 03:12:52 PM »

Quote from: Greg on June 20, 2005, 02:41:34 PM

twotiered
loosening the bleeder allows the piston to retract some, but it still drags a bit. my 81 kz440's wheel spins *freely* whereas the cb750 spins, but with drag (when bleeder is loosened).

do you think it could be a blasting media problem when it didn't surface for over 600 miles?

I don't know about your kz440. But, the CB750 brakes don't have a mechanical retractor. It relies on the rotor itself and riding vibration to move piston back into the bore.

Well, loose media would float around and act like sediment. IF it eventually collected in one place, then yeah, 600 miles later, maybe.

But, fix the pressure release issue first. Then, if you still have retraction issues revisit the sediment possibility.

I was just tossing out ideas. I'd never heard of blasting the caliper internals. I would think that's too aggressive and would leave too rough a surface. Perhaps not.

Quote from: Greg on June 20, 2005, 02:47:45 PM

twotiered

i wasn' using the ring before, after rebuilding the mc i put it in there.
what do you mean by "clearance" the pads?
with the caliper all mounted up i tried pushing the piston out with a c-clamp to ensure plenty of space for the ring and newer pads, but even with the c-clamp i couldnt get the piston to retract.
also put brake grease on the back and edges of the pads so they could slide around in there.
i'm leaning towards a missasembly of the mc, or some problem with the piston in the mc getting caught/bound.

Some pads come with "bonus material". And, they won't fit into the caliper body smoothy. You have to file around the circumference so they will.

What were you doing with the C clamp? How do you push the piston OUT with that? If you are saying you couldn't get the piston INTO the caliper bore with a C clamp? No wonder it won't retract now!

What brake grease?

Did you test that the piston would move freely in the caliper bore dry, without the seal, to verify smooth freedom of movement? It should drop in a fall out freely.

If the piston in the MC bore is sticking, you ought to be able to feel excessive play in the brake lever.
Oh Wait...Did you change levers? Some levers have different "piston pusher nubs". If yours is too long, it would block the full retraction of the mc piston.

Bob Wessner · « **Reply #11 on:** June 20, 2005, 03:13:01 PM »

How old and what kind of shape were the M/C parts in? I guess I would have opted for a rebuild kit rather than reuse parts like that. About all I can offer is get a correct M/C rebuild kit, go back through everything with a fine tooth comb, as they say. You have a lot riding on it.. no pun intended.

Greg · « **Reply #12 on:** June 20, 2005, 03:19:48 PM »

twotiered
thanks for the ideas. i agree with you about blasting the caliepr internals -- the guy nodded and said "uh huh" when we talked and then blasted away like an action movie.
thanks for tossing out ideas, that is frequently how you realize soemthign that you overlooked.
i'm goig to try and pull apart the mc again and then ill report back

bob
parts look ok, but if after pulling it apart again it still doenst work ill probably spring for a kit.

thanks both of you for the ideas!!

greg

bryanj · « **Reply #13 on:** June 21, 2005, 05:56:50 AM »

OK now for the question nobody seems to have asked. Does the caliper mounting swivel freely on the pin holding it to the fork leg mounting braket?

MRieck · « **Reply #14 on:** June 21, 2005, 06:27:20 AM »

Did you overfill the reservoir?

MikeDeB · « **Reply #15 on:** June 21, 2005, 11:55:10 AM »

Quote from: Greg on June 20, 2005, 02:08:33 PM

got it sandblasted, and the guy blasted inside the piston bore, but the piston and seal still fit nice and snug so i thoght no problem.

Never heard of sand/beadblasting a piston bore before. I would think that that would leave the surface too rough for the piston to move back and forth. With a clean shiny bore and a little brake fluid for lube the piston should settle in the bore very smoothly. You may consider using a caliper hone to smooth the inside of the bore. Now for the caliper arm. Did you disassemble the arm, clean and relube the pin? These things are always sticky when I come across them and will cause the brake to sieze. Take it apart and clean it. Don't loose the two o-rings that fit at the top and bottom of the arm. Lube with some white lithium grease and reassemble. The arm should pivot on it's mount freely with no binding. If it is clean and with lube it will.

Greg · « **Reply #16 on:** June 21, 2005, 09:06:10 PM »

hhmmmm
just cleaned the mc again last night, new pads, mounted everything, and bled like crazy. on the center stand seemed to work fine, then on the road within 5 minutes was dragging bad again.
sooo....
mrieck:
what is too full? the mc is at an angle, so i fill it as much as i can, it reaches one rim of the canister, but comes nowhere close to filling the whoel container.

the caliper pin swivels smoothly, though since it is an easy fix i might try cleaning it.

i mght try honing the piston bore. one concern though: if the beadblasting took out some material, will honing it take out too much more?

Bob Wessner · « **Reply #17 on:** June 22, 2005, 02:08:39 AM »

Don't know if this will help, but sounds like you might be overfilling the M/C. If all you have is the side stand, try to have someone hold it upright while you unscrew and look inside the M/C. On my older bike, there is a line scored around the inside of the M/C, this is the full/fill line. It is not to be filled to the upper lip of the chamber, just to this line.

MRieck · « **Reply #18 on:** June 22, 2005, 07:42:20 AM »

Quote from: Greg on June 21, 2005, 09:06:10 PM

sooo....
mrieck:
what is too full? the mc is at an angle, so i fill it as much as i can, it reaches one rim of the canister, but comes nowhere close to filling the whoel container.

Take some fluid out. Remember once you put that rubber diaphram in you are taking up a lot of area and brake fluid expands when it gets hot. In reality you don't need that much fluid. Also, did you lube the piston and seal with brake fluid before assembly? What kind of pads are you using? I have seen aftermarket that are to thick which don't present a problem cold but are a problem once they get hot.

Greg · « **Reply #19 on:** June 22, 2005, 09:17:31 AM »

hey guys
ok, i'll try taking some fluid out and see what happens.
also, mrieck, i'm using eb (

) pads (can't remember exactly) and they are pretty fat. if it is true that they expand i could *imagine* them being a problem. maybe i'll try sanding them down....,
thanks for the ideas guys!
greg

MRieck · « **Reply #20 on:** June 22, 2005, 09:49:58 AM »

Quote from: Greg on June 22, 2005, 09:17:31 AM

hey guys
ok, i'll try taking some fluid out and see what happens.
also, mrieck, i'm using eb () pads (can't remember exactly) and they are pretty fat. if it is true that they expand i could *imagine* them being a problem. maybe i'll try sanding them down....,
thanks for the ideas guys!
greg

EBC pads. I have run into problems before with SBS pads we were using in a GSXR750 race bike. Pads were to thick and we had to sand them down. Make sure you do it on a flat surface (i.e. sheet of glass) and go in a figure 8 pattern. Good luck.

Greg · « **Reply #21 on:** June 28, 2005, 10:37:16 AM »

to all those that offered some advice...

problem solved!

As Bob suggested, the problem ended up being that the piston in the caliper was getting cocked by the lateral motion of the pads when braking. It was able to be cocked because of the sandblasting of the piston bore, which removed just enough material to allow the piston to cock slightly. And it still fit real nice and tight, but it was just enough...the annoying part was that with the old caliper bench tested on the centerstand fine, but when i took it out to test ride it would seize. This was because on the centerstand the braking action wasn't hard enough to cock and jam the piston...

in any case, thanks all for the help!

Anyone have a front caliper for a 76 750f? I'm using a borrowed 750k caliper right now....

Thanks again!

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Author Topic: but i just rebuilt my brakes!!! ie: why are they sticking?!?! (Read 4502 times)

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