Author Topic: K7 rear brake behaviour - is that normal? (Read 5333 times)

Erny · « **on:** March 26, 2018, 04:53:14 PM »

While doing first testing miles I realised unexpected behaviour of rear brake.
When braking on flat surface no issue, it brakes really perfect, fluent, high power.
But when braking on bumpy surface, brake pedal is moving up/down, braking is not fluent, I feel as brake drum is very oval...it blocks wheel easily

I also realised when doing setting of brake that all is OK when bike is on main stand, but when I put bike down from stand and sit on it, rear brake is braking!

Lookig at brake, wondering is that normal? I can imagine it is not very safe braking. I'm maybe oversensitive to it as I don't have front brake now (waiting new hoses to arrive).

754 · « **Reply #1 on:** March 26, 2018, 05:22:50 PM »

There has to be freeplay in the brake rod when the weight is on it.
Try this on the stand, loosen axle, press brake firmly and hold....without letting go, tighten the axle.
This centers the shoes to the drum.

Stev-o · « **Reply #2 on:** March 26, 2018, 06:51:40 PM »

Rear brake is not adjusted correctly, loosen it up.

Keep in mind rear brake is only suppose to be approx 30% of your total stopping power.

scottly · « **Reply #3 on:** March 26, 2018, 08:13:28 PM »

Quote from: Erny on March 26, 2018, 04:53:14 PM

I also realised when doing setting of brake that all is OK when bike is on main stand, but when I put bike down from stand and sit on it, rear brake is braking!

This isn't right. Perhaps, as Steve suggested, it's adjusted too tight? Can you take a picture of the brake linkage from the side? Maybe someone will see something wrong..

Erny · « **Reply #4 on:** March 27, 2018, 06:22:43 AM »

If I release it, brake pedal touches muffler easily when pushed down.
So, no too tight nor too loose..

I'll make pic.

But lookimg again way to assemble the rear wheel, I believe I did it right way

scottly · « **Reply #5 on:** March 28, 2018, 10:55:46 PM »

Update??

Erny · « **Reply #6 on:** March 29, 2018, 02:15:55 AM »

Yes. I did photo yesterday night.

Wheel is now not blocking when I sit on bike. But still the same during ride - when braking on flat surface it works very good. But when braking on bumpy or wavy sufrace, I feel pedal moving up & down, but I suspect this is normal due to rear brake construction, when brake pedal travel depends from swing arm movement?

Have a look at photos, do you see something wrong?

strynboen · « **Reply #7 on:** March 29, 2018, 02:36:05 AM »

is the kontra arm mounted in the korrekt place on the frame...it must be vorking in a parallel vay..not total perfekt geometri..but close ,,

Erny · « **Reply #8 on:** March 29, 2018, 02:39:21 AM »

It is

strynboen · « **Reply #9 on:** March 29, 2018, 02:50:00 AM »

can see your backhveel is backed komplet aut..as the chain is veared aut..can this max aut posision do somthing...?

but the brake arm have a forvard..posision..normaly they are posisoned a bit backvard ..your is a bit forvard....is the brake Shues veared..or is the adjusting at the brake rod shaft aut of line...there is a dot in the shaft..vho need to line up...thek the shop manual..

(edit i can see the dots lign up on the Photo)

Yamahawk · « **Reply #10 on:** March 29, 2018, 03:20:57 AM »

Something is amiss... There is something not right with the way your back wheel is set up, or somehow the brake stay is too short. the stay should be at an almost right angle to your drum mounting ear, and the arm that is pulled by the brake rod should angle back towards the rear, at a slight angle, so that when the pedal is depressed, the brake actuator arm is pulled at a right angle also. I would pull the rear wheel, remove the drum baking plate, and check your brake shoes for uneven wear front to rear. Check with some chalk on the shoes, mark then around the circumference, and then insert them back into the drum, and rotate the wheel applying the brake, to check the contact area of your shoes. If there is any type of uneven contact between the shoes and drum, it will show up doing this. Erne, do you know if the rear brake stay is the stock one? Or, has it maybe been swapped out at sometime? Sometimes, the PO may have taken the brake arm off the splined shaft, and re-installed it one spline off, that could give you a poor angle of pull with the pedal and rod. as has been stated, you need a 90 degree angle or as close as you can, when the brake pedal actuates the shoes against the drum. This is for good leverage.
Charlie

Erny · « **Reply #11 on:** March 29, 2018, 03:45:24 AM »

Shoes are stock original, bike has only 9000miles. Picture attached.
For me shoes lookes very good, no signs for irregular wear.
Only photo I have attached.

BTW chain is new, DID, OEM.

Yamahawk · « **Reply #12 on:** March 29, 2018, 04:24:10 AM »

From your picture, it looks like the top shoe (in your pic its the bottom one) has more meat on it at the back section, which means there is contact at the front of the shoe first. What Frank mentioned about centering the brake shoes by loosening the axle, applying the brake, and then tightening the axle would center the shoes in your drum. Most drum brakes are not centered, which allows them to wear in an off-center pattern. Your shoes are already wearing out of center, so if you can get some replacements, then do the chalk test to see where they are contacting, you might be able to get a better working rear brake. I know that RaceTech and a couple other brake shops offer a service to arc the drum and shoes together, but you can do pretty much the same by sanding the shoes yourself, after the chalk test to see where pad removal will give you contact all the way around the arc, instead of at only certain spots. Hope I am making sense here...
Charlie

Erny · « **Reply #13 on:** March 29, 2018, 05:56:02 AM »

Thanks Charlie,

I will soon replace rear tire so when I remove the wheel I'll check that again.

Preparing for new shoes purchase - what is the best, should I take OEM Honda? On Partzilla they have 2 versions, don't know what is the difference? https://www.partzilla.com/catalog/honda/motorcycle/1977/cb750k-a-750-four-k/rear-brake-panel

Or better look at aftermarket ones? For example ECB?

Yamahawk · « **Reply #14 on:** March 29, 2018, 06:47:09 AM »

Quote from: Erny on March 29, 2018, 05:56:02 AM

Thanks Charlie,

I will soon replace rear tire so when I remove the wheel I'll check that again.

Preparing for new shoes purchase - what is the best, should I take OEM Honda? On Partzilla they have 2 versions, don't know what is the difference? https://www.partzilla.com/catalog/honda/motorcycle/1977/cb750k-a-750-four-k/rear-brake-panel

Or better look at aftermarket ones? For example ECB?

EBC make good quality pads and shoes, you can't go wrong with them. Now, NOS Honda parts, they are good also, but depends on how long they have been on the shelf, too.
If your brake shoes are original, here's what happens. The brake lining on the shoe detaches from age, the bonding glue deteriorates, and then you have a real problem, as the loose lining will wedge between the other shoe and the drum. This will lock the rear wheel, and if it happens at speed, you are not going to like that. I change all my old bike's brake shoes when I buy them, as you don't know how old they are, and whether this could happen to you. I have taken apart the drum from old wheels, and found the lining just SITTING in there, waiting someone to have this happen... On bikes with front drum brakes, this is a major consideration, as it is then the FRONT tire that locks solid... not good. So, if you have no knowledge of the age of your rear shoes, please replace them, and have peace of mind.
Charlie

Stev-o · « **Reply #15 on:** March 29, 2018, 06:49:30 AM »

I'm wondering if he is feeling the movement of the swingarm? That can fell a little unusual if you havent been on a bike before [or for awhile]

strynboen · « **Reply #16 on:** March 29, 2018, 07:09:47 AM »

renember that there is splines in the brake pedal too..so it can be vrong mounted..and then to kompensate....byt brakes is not a part orginality is a qst..thange to new Honda or a good kvality..is not the problem..but to get some parts vho are fabric new is the importment..old "new " parts vho are 40 years is not the vay..in tires brake lines and parts vith limited lifetime

754 · « **Reply #17 on:** March 29, 2018, 07:52:00 AM »

It appears to be correct swingarm for the wheel.
But brake arm to swingarm seems shorter, maybe it or mount tis changed? Or its mounted to frame ?

Bodi · « **Reply #18 on:** March 29, 2018, 08:10:33 AM »

The brake stay arm looks weird with the slash cut, it should have a square end: original has a long flattened part - if the slash was cut just at the original hole I think it would probably look just like that. The spline shaft should lie directly in line with the swingarm centre.
I don't know if that would make much difference though. It will put the brake actuator arm further from the swingarrm centreline and exaggerate the suspension changing the brake - but how much?
If the brake stay attaches to anything except the swingarm then braking will ge very weird on bumps but it looks like the arm is in the right place, just a bit shrter than normal.
The brake pedal pivot isn't exactly at the swingarm pedal so some brake pumping on bumps is normal. Shouldn't be very noticeable though. Have you moved the brake pedal or swingarm somehow?

Don R · « **Reply #19 on:** March 29, 2018, 09:13:51 AM »

I agree with Bodi, something has been modified. You shouldn't feel the brake pedal move when the swingarm does. I wonder if the pedal pivot was incorrectly installed and someone modified the other parts so they would work like that. I'm betting when the swingarm pivots the brake linkage is hitting something.
I'd put it on the center stand, remove the shocks and move the rear wheel through it's travel while checking the linkage. Use blocks or a ramp under the wheel to raise and lower it.

Erny · « **Reply #20 on:** March 29, 2018, 09:30:35 AM »

I checked again today, it is assembled properly, I did as recommened, release axle, press brake, tighten again. Then, still having bike on mainstand I adjusted brake. When I put bike down from main stand, it blocked brake... so I had to release brake to have enough free play and avoid braking and swingarm moves.

Punches on both brake arm and brake pedal are aligned..

Brake arm legth - thats good question. Maybe one I have is from another bike / another Honda model??? No idea how it should look like, how long it should be. But looks like original, color etc.

Can somebody measure its length ? It is K7. Hole to hole.

Additional photos attached

Don R · « **Reply #21 on:** March 29, 2018, 09:50:26 AM »

My proposal is, the splined shaft the pedal mounts to has an arm connected to the pull rod for the brake. That arm isn't in the proper location and is contacting the frame when the swingarm pivots. Someone has shortened other parts to make it work while incorrectly assembled.
I've had trouble fitting this part back into place myself, it's difficult to do.

Edit , I couldn't see the pictures. The pedal dot looks like the part is correctly installed which brings me to the angle cut on the brake panel stay. It didn't come like that.

Erny · « **Reply #22 on:** March 29, 2018, 10:03:14 AM »

Quote from: Don R on March 29, 2018, 09:50:26 AM

... The pedal dot looks like the part is correctly installed which brings me to the angle cut on the brake panel stay. It didn't come like that.

I do not understand - what do you mean by angle cut on the brake panel stay?

754 · « **Reply #23 on:** March 29, 2018, 10:09:41 AM »

I think you have an earlier pedal and spline from 76 and down on your bike.
Probably not clearing the pipe and giving full travel.

Erny · « **Reply #24 on:** March 29, 2018, 10:17:55 AM »

This I modified today by adjusting screw on the pedal, it was almost on the end of its travel before (pedal went up, now no issue touching muffler) - see picture of pedal above

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Author Topic: K7 rear brake behaviour - is that normal? (Read 5333 times)