• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

1980 cb750k drum brake problem

leo750k

CB750 Enthusiast
Messages
44
Reaction score
0
Points
6
Location
central florida
Replaced the brake shoes on my rear drum as part of rear wheel service, (all new bearings to) all went together fine. This is on a bike I have been restoring for the last 2 months. Finally took it out for a short ride, motor still needs some tuning but all in all it ran pretty good. Had to replace the foot brake shaft. During the installation of the brake rod I noticed that when I spun the wheel in the forward direction and applied the brake, (with only my hand) the wheel stops instantly with a sharp click/clunk sound. release the lever and the wheel turns nice and free. spin the wheel in the reverse direction and apply the brake, and you get what you would expect, a light brushing sound as the shoes stop the wheel. hold the brake on ever so lightly and you can turn the wheel, try to turn it forward, and clunk, it stops dead. It feels like you threw the ratchet lever on a winch. Had it apart twice now, all looks fine, hell there is not a lot in there to be out of place. Without the brake applied wheel turns freely in both directions.
I'm stumped, on my one ride I did not use the rear brake, and now I'm afraid if i do I will lock up the rear wheel and flat spot my new tire

I get all the weird problems, has anyone experienced anything like this? Please help.
Thanks
Leonard
 
If you changed wheel bearings did they fit tight and did you restake the screw-in retainer so that it does not back off to loosen bearings? You remove that staking when you remove the part.

Brake shoes must be tight on their axles to move freely but with no slop. The brake linkage to pedal must be the same, any looseness beyond working freely needs to be cured. Make sure the lever on brake hub is tight to shaft that splits the shoes to stop. Assuming you have spring(s) on the shoes. The SLIGHTEST amount of grease can be used there on the two axles and the main shaft can handle a bit more, they tend to stick. Consider all grease flinging off and where it will go, you CAN use it but the word sparing really applies say to the shoe axles. I use like a drop spread between fingers to thin it out before using fingers to lube the axle.

Main wheel axle must all be tight as well as wheel itself. Linkage at the pedal support up front can do it too. Look at the point the brake rod ends at front, the folded over plate used there to attach to pedal shaft, it can stick there to come loose with a noise sometimes if switching directions on brake activation.

Look and lean in close to listen as you make brake come on to follow your click while looking for something moving, sounds like you have something loose there, the norm is the same action forward as rearward and reverse sounds like it's supposed to.
 
I did re-stake the bearing retainer very well. I will go over it again with a fine tooth comb paying close attention to the things you pointed out. Did not actually have the peddle hooked up when I noticed this noise, I was actuating the brake with my hand and the long pull rod disconnected.

Did just re-lace this wheel, have not rechecked spoke torques since the one ride(which I need to do), but this noise is coming from the hub, I'm almost certain.
will reply back after I look into all you suggested, thanks
 
video of my problem

I did re-stake the bearing retainer very well. I will go over it again with a fine tooth comb paying close attention to the things you pointed out. Did not actually have the peddle hooked up when I noticed this noise, I was actuating the brake with my hand and the long pull rod disconnected.

Did just re-lace this wheel, have not rechecked spoke torques since the one ride(which I need to do), but this noise is coming from the hub, I'm almost certain.
will reply back after I look into all you suggested, thanks

hope I'm replying correctly.
Well went over everything again, spokes are all good and tight too. I shot a video with sound. If I can figure out how to upload it it really shows whats happening well, (a picture is worth a thousand words anyway).
Thanks everybody for your help,
Leonard
trying to post an .avi video file, looks like I have to upload it to YouTube or Facebook, and then link it to this site, is that correct?

OK new data. I just went out to look it over one more time, and I just discovered that when i rotate the wheel forward, and apply brake, and it locks up, if I hold forward pressure on the wheel and release the brake peddle, the brake pads remain locked to the drum. As soon as I let up on that pressure the shoes return. The clunking noise is being caused when the shoes contact the drum, but only when the drum is rotating forward, as soon as the pressure from the forward rotating drum is removed the shoes return to position. But none of this occurs during backward rotation. something is going on with those darn new aftermarket shoes. when I first got them I laid them on top of the original shoes, they matched exactly, of course I didn't get out the calipers or anything like that, and i didn't check the width, just compared the radii.
 
Last edited:
If the clunking happens at the contact point even very slowly increasing brake then evidence of something loose somewhere there. A clunk is too loose evidence; closer will not clunk, there is no action spacing (increased clearance) for a hit to make noise.

If the brake hub is in the OEM location with torque arm hole at 4-4:30 and the lever around 8 o'clock then the self energizing shoe is the top one. If the brake surface inside is rough and groove wear lines in it then that can hang onto the shoe to not let go. It may need some sanding to smooth it up. If the shoes at the end of shoe friction material end in a 90 degree square end then not right, the OEM shoes drop off in a slight angle on both ends and important to not have the shoe hang to the wheel. A 90 degree angle locks onto the friction surface to stick. The main brake shaft splitting the shoes MUST be greased and spinning smoothly before grease or it adds to sticking. Needs to be tight in its' hole yet still able to spin easy. Any corrosion works against all of that. The self energizing shoe (top) will be the troublemaker with wheel rotating forward. That switches in reverse, the bottom one then becomes the energizer. If anyone had replaced shoe springs to lesser springload that can do it too. Or weak ones. May need to smooth up the wear point at the splitting of the shoes to open up, that cam can wear at the rocking corner to hold the shoe in place rather than popping off with the slightest decrease in pressure on the outer lever.

The problem there should be easy as spit to find. It could be helpful to work the brake backing plate out in the open independent of it being in the wheel to see what potential sticking could be happening without it even in the hub.
 
well i pulled it apart again this morning before work, did some sanding on the drum, but it rely wasn't that bad. compared the chamfer on the ends of the linings with the old pair i replaced, big difference, the old linings tapered back about a half inch, the new linings barley trimmed the corner an eight of an inch. so i got out my file and made the new ones look exactly like the old ones. reassembled everything, same darn noise. so i really started comparing every little detail of the old to the new shoes, I found that the shoe's castings are not symmetrical, there is a left and right hand shoe, (on the old pair) on the new pair, I have either 2 right hand shoes, or left hand shoes, dam aftermarket crap.
When i get home later i will post some pictures i took of the old vs new shoes so you can see what i mean. im new at motorcycles, but it sure looks to me that i was sold HALF of TWO pair of shoes
 
Uh, the shoes are supposed to be identical.

https://www.partzilla.com/catalog/honda/motorcycle/1980/cb750k-a-750-four-k/rear-brake-panel

Look at part #5, there are 2 listings for it and JB or ASK but those codes mean different vendors only of the same part, the important thing is to note the required amount way to the right, or 2 of whichever one you choose. Means they are identical. If they weren't both would not be labled as part #5 either.

Pretty much all the bike brake shoes are identical, although there may be some that are not, I've never run across it though. Double leading shoes may be but those are not that type, they are single leading shoe only. Why only one of the pair is self energized, on double leading both are self energized. On single you simply flip one shoe over to use it against the other.
 
Last edited:
View attachment 9059View attachment 9060View attachment 9061View attachment 9062View attachment 9063View attachment 9064View attachment 9065View attachment 9066View attachment 9067View attachment 9068View attachment 9069

OKI have tried to attach enough pics to show what i mean. the first pics, with writing on them are the original Honda shoes, as the illustration shows in the factory manual, chapter 14, page six at the bottom, you can notice that on both sides you can see the casting ports/plugs on both sides are outwards. there is an oblong logo on the bottom right shoe, and the number 3 near the top. on the left shoe is a round logo, and no number at the top. near the middle of both shoes is the number ASK A7 GG. if you flip both shoes over on the back of the shoe that was on left has the oval logo with number 4 on top, the back of shoe that was on right has the round , no number on top. both have the ASK numbers in mid., and you notice both have a blue ink dot, also there are no casting ports at the outer edge of where the lining goes. and Honda clearly shows them on the same side in the manual.
now look at the shoes with linings, these are my new aftermarket shoes.they cannot be installed as shown in the manual. they were both cast from the same mold, the Honda linings were caste with two different molds.
could the aftermarket shoe that has the casting ports to the rear be causing my interference? they would be the ones on the upper shoe, which i believe i learned from you would be the "energizing shoe" when braking while moving forward, is that correct?
sorry for being so winded. i know i am returning these shoes, and buying OEM Honda.
i also did some measuring, both the pins are measuring .548 thousandths ea. way within .001, holes in Honda parts .551 within ,001, holes in aftermarket between .555-.558 with a few thou taper. width of pad surface =Honda 1.534 at 2 plcs on both shoes, aftermarket =1.560 - 1.562. thickness at pin hole and cam lobe: Honda = .780 - .784, aftermarket = .778 - .783

thank you for the time you are taking to help me with this. hope my pics post, and very soon I will post all the before and after shots I took. this bike sat in the Florida rain, and heat for the last 12-13 years. Its been a real challenge, but also a lot of fun and a great feeling of accomplishment. look forward to being part of the Honda CB750 comunity, and helping others in the future
 
View attachment 9059View attachment 9060View attachment 9061View attachment 9062View attachment 9063View attachment 9064View attachment 9065View attachment 9066View attachment 9067View attachment 9068View attachment 9069

OKI have tried to attach enough pics to show what i mean. the first pics, with writing on them are the original Honda shoes, as the illustration shows in the factory manual, chapter 14, page six at the bottom, you can notice that on both sides you can see the casting ports/plugs on both sides are outwards. there is an oblong logo on the bottom right shoe, and the number 3 near the top. on the left shoe is a round logo, and no number at the top. near the middle of both shoes is the number ASK A7 GG. if you flip both shoes over on the back of the shoe that was on left has the oval logo with number 4 on top, the back of shoe that was on right has the round , no number on top. both have the ASK numbers in mid., and you notice both have a blue ink dot, also there are no casting ports at the outer edge of where the lining goes. and Honda clearly shows them on the same side in the manual.
now look at the shoes with linings, these are my new aftermarket shoes.they cannot be installed as shown in the manual. they were both cast from the same mold, the Honda linings were caste with two different molds.
could the aftermarket shoe that has the casting ports to the rear be causing my interference? they would be the ones on the upper shoe, which i believe i learned from you would be the "energizing shoe" when braking while moving forward, is that correct?
sorry for being so winded. i know i am returning these shoes, and buying OEM Honda.
i also did some measuring, both the pins are measuring .548 thousandths ea. way within .001, holes in Honda parts .551 within ,001, holes in aftermarket between .555-.558 with a few thou taper. width of pad surface =Honda 1.534 at 2 plcs on both shoes, aftermarket =1.560 - 1.562. thickness at pin hole and cam lobe: Honda = .780 - .784, aftermarket = .778 - .783

thank you for the time you are taking to help me with this. hope my pics post, and very soon I will post all the before and after shots I took. this bike sat in the Florida rain, and heat for the last 12-13 years. Its been a real challenge, but also a lot of fun and a great feeling of accomplishment. look forward to being part of the Honda CB750 comunity, and helping others in the future.
Thanks,
Leonard
 
my first but not my last

this is my first motorcycle restoration. IMG_20181110_200852.jpgIMG_20181110_200120.jpgIMG_20181110_200937.jpgIMG_20190613_132042.jpgIMG_20190613_132034.jpg
 
Pics of shoes are not showing. Website says 'invalid attachment' FWIW.

Yes, top upper shoe is the self energizer.

The parts can be cast from way more than one mold and still be 'identical', markings literally mean nothing, only the shape matters. You ignore all numbers, codes or logos to focus on shaped difference that makes them not the same and it cannot even be casting flaws as those can be cleaned up to again make part identical.

When the official part callout in parts list says the parts are the same they will be, at least as far as function goes.

Getting around to your casting port thing...........

'Honda clearly shows them on the same side in the manual.
now look at the shoes with linings, these are my new aftermarket shoes.they cannot be installed as shown in the manual. they were both cast from the same mold, the Honda linings were caste with two different molds.'

All that means is that Honda casted the parts in pairs that APPEAR to be left and rights but they ARE NOT. Their casting machine obviously had a bigger bed to cast larger molds or multiples of smaller ones. If the aftermarket vendor only has a smaller bed machine then he can only cast one part at a time instead of two or multiples (4 or 6?) even of that, then his parts will all appear to be one sided where the Honda cast in pairs isn't.

The 'casting ports' don't matter as to which side, at least they better not. Not casting ports anyway, they are part resting points for the ejector pins that punch the part out of the mold (the close proximity there makes part stick tight in the mold) after it is molded and the machine opens up to drop the part out. The actual casting hole entry for molten metal is usually only one spot and I could find it likely if I looked at one.

https://www.plastopialtd.com/ejector-pins-marks/

If you think the ejector pin columns are making shoe drag on something on backside then take part to a grinder and remove some of it slightly to match the other side, done with some sense it shouldn't hurt the part at all. Just copy the shape and form on the other side. The ejector pins can be on either side of a part, done correctly they do not effect the use of the part at all.

You have an OEM 'left-right' set and a separate set of aftermarket 'right-right', or 'left-left', whatever you want to call it, and thinking they are different when they all are the exact same ONE part there. The manufacturing method used there is different and has thrown your mind off. If the holes and centers are the same, and width of pad and curve matches then the parts are the same.
 
Last edited:
What may well be your noise or clunk that started all this............

'...both the pins are measuring .548 thousandths ea. way within .001, holes in Honda parts .551 within ,001, holes in aftermarket between .555-.558 with a few thou taper.'

Honda clearance there .003" and the aftermarket .007"-.010" and likely the noise. 3 won't make any noise but 7-10 can as the shoe rotates to whack at the end of the clearance takeup. Like valve noise when they get loose. Extra clearance beyond needed always makes more noise. Only need 2 or 3 there max. Taper will tend to rock the shoe sideways out of plane too making even more noise.
 
agreed I was grabbing at straws for an answer, I dont see any abrasion or marks from rubbing, but its not the noise its that the drum and wheel lock up hard when brake is applied going forward, even just rolling out of my driveway. I have to go buy a set of 12" calipers so I can check the drum dia. to see if it is past 7.1". could a drum that is out of spec cause the pads to bind and lock up the wheel? I'm really stumped on this one. Tonight I will lay the new shoes against the drum to see how close the radii are to each other.
if you have any other thoughts on this please let me know.
Thanks,
Leonard
 
Pull the shoes off backing plate and simply put each into the drum to check by hand, the most direct way to check curves.

'...the drum and wheel lock up hard when brake is applied going forward, even just rolling out of my driveway.'

Having trouble with that as I expect it to, and the self energizing force on one shoe will exaggerate it and even more likely since it has a loose pivot hole.
 
Longshot..................................Make absolutely sure all pins and such that hold shoes are very tight in backing plate.
 
I will check the pins, thanks.
I did just take the shoes and laid them into the drum just to check the fit, (how close the radii match) and they fit/match great.
drum did have some minimal wear lines, I sanded them out with some 120 paper, haven't put it back together yet.
I also have noticed I am missing a rubber washer that goes on the "brake stopper bolt" before the brake stopper arm, I will order one today
thanks for all the help,
Leonard
 
well, bought a pair of OEM Honda shoes, installed them, problem gone. I think it was the taper in the over sized pin holes of the aftermarket shoes. i think they were racking sideways, and the edge of the shoe was binding. but the bright side is that after having the rear wheel off and on 7 times, i'm practicley an expert at it now. LoL
Thank you everybody for your help.
now as soon as i ditch the pods, and reinstall the OEM air box, its off to the races.
does anyone have an aftermarket source for the air box boots for a 1980 CB750 K A ?
 
Back
Top