Topic: 1978 CB750K Front Brake Rebuild

[Rosinante]

Intend to rebuild the front brake system on this bike and I have the parts shown in the attachment.  With this thread, I am asking for advice on how to proceed, tips and suggestions.  What are the things I am likely to do wrong?  For example, on other brake systems I have had good luck using the intact hydraulic system to push the piston out of the caliper, and wonder if this will work.  I also have compressed air, but that method can sometimes be more exciting than I prefer.  I have the Haynes and Clymer manuals but am unsure how well they describe the procedure on this particular bike.

The caliper is not retracting.  That is....it is hard to push this bike around in my garage because the system is pressing the pad against the disk.  On other vehicles this has led me to replace the rubber brake hose.  Below is a link to a hose which appears to be characterized as appropriate for this bike.  Any concerns about this product or suggestions about where else to source this hose?

https://www.ebay.com/itm/292288650158?hash=item440dc2dbae:g:cLIAAOSwDshZ3Rk0

[willbird]

One thing you need to do is make sure to really thoroughly clean the groove in the caliper where the rubber seal goes. The groove is supposed to be wider than the seal by design, the seal creep caused by that extra width is what retracts the piston. over many years that grooved will grow "stuff" that makes the groove the same exact size as the seal, then your piston will not retract. I really had to dig the "stuff" out....it took some digging to get that groove clean. Once I did that the piston retracted properly again.

[Rosinante]

Thank you!  Much appreciated!  I am likely to replace the hose anyway and perhaps the hard brake line, just because.  I have seen hoses swell closed internally and prevent relief of pressure against the piston.  But yeah, your description surely makes sense.

Still wondering the hydraulic system would not work as well as or better than the grease gun method for pushing the piston from the caliper bore.

I'm probably missing something, but brake caliper rebuilds are usually not rocket surgery:

Remove disk and pads
Pump brakes until the piston falls out
Disassemble the system
Rebuild the caliper and MC on a bench.
Reassemble
Fill and bleed

Is it not this simple?

[willbird]

Thank you!  Much appreciated!  I am likely to replace the hose anyway and perhaps the hard brake line, just because.  I have seen hoses swell closed internally and prevent relief of pressure against the piston.  But yeah, your description surely makes sense.

Still wondering the hydraulic system would not work as well as or better than the grease gun method for pushing the piston from the caliper bore.

I'm probably missing something, but brake caliper rebuilds are usually not rocket surgery:

Remove disk and pads
Pump brakes until the piston falls out
Disassemble the system
Rebuild the caliper and MC on a bench.
Reassemble
Fill and bleed

Is it not this simple?

It is worth a try :-), should work although I have never tried it, worst case it does not work and we learn something LOL. I used a Dewalt battery powered grease gun on mine :-).

[bryanj]

Pumping with master is the best way to do it but for some reason people insist on removing the caliper and struggling.
The master cylinder circlip is a real swine to remove, personally i get the rubber boot out the use a canister type gas torch to heat the old circlip down the hole which removes the crud and, usually, loosens the circlip.
Second the cleaning of piston seal groove thouroughly, a dental pick or similar it ideal for the work.
Also bleeding the master can be a chore, i have described it on the forum before so please search if you really cant find it i will do it again

[BenelliSEI]

That’s the correct hose for the K8. Your brake light switch is a limit switch built into the laver on the master cylinder.

When rebuilding the master, make sure both the holes from the reservoir to the bore area are clear. The rally tiny one is often plugged.

[Rosinante]

Pumping with master is the best way to do it but for some reason people insist on removing the caliper and struggling.
The master cylinder circlip is a real swine to remove, personally i get the rubber boot out the use a canister type gas torch to heat the old circlip down the hole which removes the crud and, usually, loosens the circlip.
Second the cleaning of piston seal groove thouroughly, a dental pick or similar it ideal for the work.
Also bleeding the master can be a chore, i have described it on the forum before so please search if you really cant find it i will do it again

Here is a thread:

http://forums.sohc4.net/index.php/topic,3476.msg25956/topicseen.html#msg25956

And here is your post in that thread.  Is this what you suggested I read?

OK folks I GUARANTEE this works cos i dun it!

(1) Collect several old knackered master cylinders ( its easier to swear at several at once)

(2) obtain an oxy/gas cutting torch

(3) remove the rubber boots to see the offending circlips

(4) With cylinder clamped in vice and circlip upwards  play the  the lit cutting torch (NOT USING CUTTING LEVER) into the hole until the cylinder is just warm.

(5) use two scribers , one in a circlip hole and the other under the clip to wangle the now clean and un corroded clip out.

(6) use a pin punch to knock all the guts out from the other end

(7) Clean up the bore and reassemble with a NEW seal kit

I do not have oxy/acetylene but it sounds like you are simply suggesting to get the cylinder warm, which propane can do.  Makes sense.  Warm cylinder will be bigger than cold cylinder.

I do not understand #5.  Scribers?  I have snap ring pliers, including the one pictured above which I believe was sold for this specific application.

[bryanj]

Yes warm to touch will do, i also use the plumbers type torch that fits to a disposable canister.
I wrote that when circlip pliers that worked did not exist or broke so decent engineering scribers were what i used, like dental picks but not as bendy.

Hope that makes sense, i have been on this forum for many years and things have changed.

I had a batch of 15 master cylinder at one time and stripped them all in less than 30 minutes using that method

[Stev-o]

Yes, using the bike's hydraulic system to remove the piston is the best way [but the air compressor method can be the most fun!]
If you need to roll your bike around, you can just loosen the caliper bolts.

And about the master, it can be a bear! Many of us dont even bother as a re-pop isnt much more that the cost of a rebuild kit.

There really isnt anything to "rebuild" on a caliper, just a very thorough cleaning and a new seal.

A new hose is a great idea, the hard line can be easily cleaned once removed. Good luck....

[Rosinante]

The snap ring was indeed challenging to remove but I got it.  I have a new question.  Someone suggested to be sure to clean and open both of the small passages in this MC.  See the two passages at the bottom of the fluid reservoir in the attached pic.  The one on the right is open, and it leads into the MC piston bore.  The one on the right appears clogged.  Question:  Should this passage also lead into the MC piston bore?  If so, then I suppose I could drill it out.  If not, then unplugging that passage becomes more challenging.

Also:  There is a teeny weenie cotter pin holding the brake pad to the caliper half at the far end (the non-piston side).  The smallest replacement I have found has an 'eye' that it too big to fit into the tiny hole in the caliper half.  If I use this, then the 'eye' of that cotter pin will be visible on the outer surface of that caliper half.  I don't care about appearance, but just wonder if I should move forward, look harder for the exact correct replacement cotter pin, or just reinstall the old pin.

[Rosinante]

I think that plugged hole, on the left, does NOT lead into the MC piston bore.  That being the case, the only other place it might go is to connect to the other hole which is not plugged.  If so, then this passage would be impossible, or near impossible to clear.

[denward17]

Try brake cleaner and compressed air for the small hole in the caliper, do not drill as it could affect hydraulic pressure and release of piston.

[Rosinante]

My air hose has a rubber tip, making it possible to test whether this small hole goes anywhere.  No air passes into it.

I took a small drill bit and just ran it a few revs lightly and the shiny surface that drill bit cleaned suggests the bottom of this hole is the aluminum of the MC casting.  I'll try to plug the two ends of the bore and run air into the cleared hole in case it can push an obstruction, connecting to the plugged hole, out.  Otherwise, I'm not sure there is anything further I can do.  I am prepping these parts for a shot of paint and I might have to just assemble and reinstall, and see what happens.

I'm not sure this hole goes anywhere and if it does, then perhaps someone can explain its function.

[bryanj]

I can assure all that that tiny hole DOES go into the master cylinder bore and is so tiny 2 stands of twisted fine muticore copper are a maximum fit DO NOT DRILL IT ! Use a small torch shining into base of reservoir over the hole and look for light down the bore

[calj737]

My air hose has a rubber tip, making it possible to test whether this small hole goes anywhere.  No air passes into it.

I took a small drill bit and just ran it a few revs lightly and the shiny surface that drill bit cleaned suggests the bottom of this hole is the aluminum of the MC casting.  I'll try to plug the two ends of the bore and run air into the cleared hole in case it can push an obstruction, connecting to the plugged hole, out.  Otherwise, I'm not sure there is anything further I can do.  I am prepping these parts for a shot of paint and I might have to just assemble and reinstall, and see what happens.

I'm not sure this hole goes anywhere and if it does, then perhaps someone can explain its function.

It is well worth the price to get a set of these: https://www.amazon.com/Hobart-770086-Welding-Oxy-Acetylene-Cleaner/dp/B0017Z1SXA/ref=asc_df_B0017Z1SXA/?tag=hyprod-20&linkCode=df0&hvadid=693071814451&hvpos=&hvnetw=g&hvrand=8409742207197485896&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9008162&hvtargid=pla-571772983143&psc=1&mcid=13948a208713314fa426c22ad1b5e3d0&gad_source=1

They are invaluable to have in the shop and there is most certainly one a,one them that will fit into that bore and allow you to hand turn it to clean that bore hole.

[Rosinante]

Okay.  I have a little sharp bent poky tool that fits into the bore and can 'feel' the surface of that bore for holes.  It feels the other hole but not this one.  Perhaps it is too angled or not sufficiently sharp to detect such a small hole. 

Thank you for that.  I will do as instructed. When the pain dries.  The painting has begun.

[bryanj]

All the picks, little bent sharp pockey things, i have are way to big for that tiny hole, somebody mentioned a guitar string once but no idea which one.
Single strand from a wire brush MIGHT do it not sure.

Yes its a pain
Yes its super tiny
Yes most people dont believe its there

BUT it is and it need to be open or your brake will drag

[Rosinante]

Hm.  I do have guitar strings.  Great idea.  If this works, then you and Dennis are both my new best friends. 

[M 750K6]

I keep a top E string in my garage. So far only use has been carbs. Just avoid scratching the hole., I use WD40 with it, I don't know if it helps prevent damage, but so far, so good.
Edit: on a brake m/c I guess you could use a light silicon lube, like in the spray cans.

[newday777]

All the picks, little bent sharp pockey things, i have are way to big for that tiny hole, somebody mentioned a guitar string once but no idea which one.
Single strand from a wire brush MIGHT do it not sure.

Yes its a pain
Yes its super tiny
Yes most people dont believe its there

BUT it is and it need to be open or your brake will drag

I learned years ago that a single strand of a wire brush is the correct size for the return port hole in the Honda master cylinder. Hold the wire strand with a pair of needle nose pliers.
Brake cleaner alone will not clean out the hardened gunk. You have to poke it out first.

[Rosinante]

Last night in the back of my mind I worried I had destroyed this MC with my little drill bit trick, but this morning I saw the teeny tiny hole with my eyes and even verified it was still plugged.  I also discovered that only the small E-string in a set of Super Slinky guitar strings would fit.  The slightly bigger B string does not fit.  This means you really need to use something VERY TINY.  I cleared the hole and assembled the MC.

Here is my new question:  Which way does this caliper piston go?  Which end goes into the caliper, and which end faces outward toward the pad?

[Rosinante]

I'm pretty sure the square end with the notch (on the left) faces the pad, and the rounder end (on the right) is inserted into the caliper.

[bryanj]

That looks like the wrong piston to me, Goldwings were the same diameter and had that step whilst i think all 750 were parallel sides, not done much on the last 750 sohc so may be wrong

[pekingduck]

That's correct, the pivoting calipers were a single diameter, the Gold Wing and DOHC models had an extra lip to hold a dust seal, but that piston will still work fine. The hollow end of the piston always faces out.

If rebuilding master cylinders, I've found that if you take the lever out and put the bolt back in, it serves as a fulcrum point when using a screwdriver to push the piston in fully for easier access to the circlip. 

[Medyo Bastos]

Last night in the back of my mind I worried I had destroyed this MC with my little drill bit trick, but this morning I saw the teeny tiny hole with my eyes and even verified it was still plugged.  I also discovered that only the small E-string in a set of Super Slinky guitar strings would fit.  The slightly bigger B string does not fit.  This means you really need to use something VERY TINY.  I cleared the hole and assembled the MC.

Here is my new question:  Which way does this caliper piston go?  Which end goes into the caliper, and which end faces outward toward the pad?

Hollow point towards brake pads otherwise you will be filling piston with fluid, no? I don’t remember pistons having that notch for sohc cb750


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[Rosinante]

I rode the heck out of a '75 back in about '77-'80 and I was very impressed with its braking.  The front brake worked notably better than the rear, and with both brakes at once that '75 would stop NOW.  In my memory.

Report:  I rebuilt this caliper and MC because I was not happy with braking, and I also installed new shoes in back.    I am still NOT impressed with the braking on this '78.  On this current bike the front brake is not better than the rear.  Is this normal?  It's a little better than before and I will do some more brake bleeding to see if I can squeeze a little more performance out but I may be looking for a better solution.  Suggestions?  Dual disks?  Can I grind the front disk for additional friction?

I gotta say....I am very grateful for the advice.  Thank you!  FWIW, I think the little plugged passage in the MC was likely the reason for brake dragging.  More so than the seal-flex thing.  I could give a long explanation but the short version is that I think hydraulic pressure against the pad/disk interface is needed for brake dragging.  I don't think pistons move when braking. 

[newday777]

If you still have the rubber hoses get some stainless steel brake hoses for full pressure to the pads.
Yes you can thin the rotor, and drill them too for better stopping. Godffery's Garage (Jeff is a member here) does them correctly. Less weight and better stopping.

http://forums.sohc4.net/index.php?action=profile;u=7132

[Rosinante]

I believe you.

Keep it coming.  I am waiting to hear about a MC cylinder with smaller bore.  Everybody talks about big bore MCs and those would actually provide less braking.  Smaller bore would provide more braking power, but also more travel.  Which SS lines could address.

[calj737]

The front brake worked notably better than the rear, and with both brakes at once that '75 would stop NOW.  In my memory.

The front brake on any motorcycle always provides more stopping power than the rear. Under aggressive braking, weight is transferred to the front, unloading the rear. There is a substantial increase in friction of the front tire during this moment. The rear brake is nearly useless during aggressive braking as the suspension has already rebounded to react to the front loading.

Adding rear brake near the end of the braking event can help improve stopping distance, but should not be activated during the initial event. And absent of a blown front tire, gravel, dodgy surface, the rear brake is not recommended as a primary source for stopping. There are ample uses for the rear brake but poor technique uses of it contribute to more crashes than avoidances.

[Rosinante]

Thank you.  Front brake performance on this bike sucks.  It does not have more stopping power than the rear.  Or at least, if it performs better than the rear, the difference is difficult to perceive.  I started a new thread for this specific topic/question and asking for tips and solution alternatives.