Topic: Dual Disc Conversion for 1975 Honda CB750F fine tuning Issues

[kaptainkid1]

I'm having problems with my Dual Disc Conversion on my 1975 Honda CB750F mod. The bike has originally had a stock set up with a single front disc with no problems. The front braking with stock parts functioned very well. No spongy feel and caliper work freely. 
So let me start off by saying I'm using used parts maybe not in the New condition but in great condition. So I've been troubleshooting a few problems and I need help on fine tuning the right side brake caliper set up and possible brake line issues with a spongy feel and brake pad alignment problem.
1st I bought new SS brake line with 10mm end caps. So after attaching all the new parts, I noticed the following problems. The brake lines have been bleed and all the new brake parts are attached regardless if they are working properly. I noticed right away the brake handle felt very spongy after both calipers were bleed properly.  I attached the GL1000 Splitter and 2x new ss brake line with 10 mm end caps. Will the spongy feel come from not using a hard lower line attached to brake calipers? Would using lower hard line fix the spongy problem?
Here is the part list:
2x New SS brake line with 10mm end caps from the splitter to the brake caliper.
1x 1975 Honda GL1000 Brake Line Hose splitter
1x SS brake line from Front Brake Master Cylinder to a splitter
Used the original 1975 Honda CB750 master cylinder. I was told it held enough brake fluid run the system.
 

One of the Calipers on the right side is hanging up. It seems the brakes lock up and engaged due the fact the calipers are not aligned properly. Also, the alignment of the caliper is not lining straight with the right disc set up. It lock up once the calipers are lock tight. I was told I need to used shims to align the calipers and wanted to know from the 3 attachment points to the fork where the shims should be placed? Even using the fine tuning screw didn’t push or align the right calipers properly.  I’m guessing the caliper bracket can be offset but I’m not sure.

Any suggestions on the shims placement and replacing the SS 10mm lines for the lower hard line set up.
Is ok to use the Red tacky Grease for the Brake caliper? I'm guessing it's ok. I also believe one the caliper:s is hanging up and needs to be rebuilt since it doesn't free up so easily.

[seanbarney41]

Hang tight... don't have time to type this all out on my phone...I got pics and help for you later

[seanbarney41]

OK, gonna give it a shot.  This is tricky to explain and hopefully my pictures won't confuse you more as I realized I don't actually have pics of what I finally ended up with.  Also, I am unsure whether every bike can be done exactly the same as I have found nothing but conflicting information even with reports on the same year bike, parts, etc,..

So the goal is to shim the pivot arm so that it is parallel with your disk both front to back and top to bottom AND so that the the opening of the caliper fits properly over the disk.

When I first tried to set up my dual calipers I mistakenly thought I could use calipers from a 71-76k.  You can't.  But that is what I have pictures of.

First attempt...got the arm parallel but I could not get the halves of the caliper centered on the disk without moving it into the spokes.  I used stainless washers for shims. 
KIMG0123 by Sean Barney, on Flickr  This is from the back.  You can see I had 2 washers on the larger bottom bolt and about 6 washers on the top 2 bolts.  I wrapped the washer stack in electrical tape to keep them together so I could get the bolt through.  View from the front...
KIMG0122 by Sean Barney, on Flickr

So when I finally figured out I had to have the correct caliper for an F, I ended up with 2 or 3 washers on the 2 top smaller bolts and 0 washers on the smaller bottom bolt.  I think that should get you real close.  Like I said before, I have not found any 2 reports on this to find the same spacing method.  If you can understand the relationship between the disc and the caliper on the lh side, just mirror image that relationship to the rh side.  Some folks even report having to trim down one or more of the bolt bosses on the fork slider


I don't think you will be able to bleed your brakes properly until you get the rh caliper spaced properly.

[scottly]

The top two spacers should be about .120" (3mm) IIRC.

[crazypj]

Yep, right fork leg is drilled and tapped but not machined for a dual disc. I'd previously done the washer thing but  found it easier to machine a 'washer' to fit under rotor and use longer Allen head bolts to get spacing right. Anyone who finds stock single disc OK isn't even starting to try, it's crap for anything above 'pottering about'

[Yamahawk]

Hehe... Being my bike is a 'bitsa' mongrel, it has what I believe to be the whole 1978 CB750A front set of forks and wheel installed with the same triple trees from the 1978 CB750A. I am thinking of converting the front ComStar wheel to dual rotors and calipers myself, is there any difference in the older conversion info here, and what is needed to convert my newer front wheel and forks to twin disk front brakes? I know I would need a caliper and caliper mount, my other fork leg is already set up for the rt. side disk and caliper, as the mounting lugs are there.
Charlie

[crazypj]

You'll probably need different wheel spacers.
I have a 'Comstar' hub kicking about somewhere, only ever used it for dimensions as it's surprisingly heavy  compared to stock spoked hub and much more difficult to lighten any meaningful amount.
 I don't remember where I stashed rim, been meaning to do a BMW style  'tubeless tyre' conversion with it

[Yamahawk]

Thanks, brother I appreciate it. As I have the later front end and Comstar wheel, am not sure if the speedometer drive from an early 550 is what I need or if all I have to do is remove the blanking plate from the right side of the ComStar wheel and install a 2nd rotor, and then fit and shim a brake caliper bracket and caliper to that side, with the mods to the adjuster bolt... so, I will acquire the parts, once I figure out what I need.
Charlie

[kaptainkid1]

Thanks for the follow-up and I'll try the spacers on Right caliper to see if I can get a better clearance. Great photos it explains alot. I'm hoping it's the right direction for me. Since I'm also using the Front fender on the right side which gives a spacer already just not as large as your graps.

[pjlogue]

Your pucks should fit in the caliper loosely.  If you have aftermarket pucks they may be too big of a diameter to have enough clearance to pivot when in the caliper.  The F model may be different than the K but I think the pucks should be free to tilt a bit in the caliper.  If they don't they will bind.  The K 750 has the bracket that holds the caliper slightly offset from the rotor.  The ability of the puck to move is what prevents the puck from binding

Did you get all the air out of the system?  I have found the master cylinder traps air that floats up the lines.  There is a tiny hole in the bottom of the reservoir.  Let the bike sit over night with the handlebars turned to the left.  Next day open the reservoir and gently press the brake handle just a little, just enough to start moving the piston and watch for air bubbles coming from the tiny hole.  Don't squeeze the handle fast or hard or you will drive any air back down your lines.  (you will also squirt brake fluid all over) The hole is so small the air gets trapped in the MC.  The slight movement of the piston will drive it out the hole instead of down the brake line.  You may have to repeat this process for several days. 

When going from single caliper to dual don't forget that with the same diameter MC you are now engaging 2 calipers so the amount of fluid needed to move both pucks will double so I would expect the need for greater hand lever movement.

Hope this helps.

-P.

[scottly]

Kid, it's hard to tell from your pics, but it looks like the right side disc is too far to the out-side; normally it's more towards the center of the wheel than the left side. How did you deal with the speedo drive plate? If you tried to fit the disc over the plate, it will space it out; the plate needs to have clearance machined so the rotor fits over it. 

[kaptainkid1]

Ok, I have a followed up post since I didn't find this information on the web or this forum and wanted to pass down some knowledge.
The biggest problem I noticed and was throwing me off was the alignment of the brake caliper. I noticed the arms and brake pads seems to angle inward on the pad towards the engine and opposite end the pad was angle outwards. Also after installing these Dual Disc upgrade, the calipers were hanging up badly and visually this seemed to be the problem without knowing it was factory specs. Wrong!!!!
So going through the Master Cylinder and bleeding the calipers a ton of times I couldn't get the brakes to free up and they were locked up tight after pumping the brakes. I finally decided to open the calipers and clean them by pulling the piston and checking the rubber seal. It seems both the original and new part calipers were freezing up because they needed to be cleaned and rebuild. After the rebuild of the calipers, I notice brake pad offset from behind to front was still off but wasn't causing the hang-up. I used the old 1975 Honda CB750F brake master cylinder and was told it should be enough to handle the second caliper.
That isn't true. I have all new SS lines and I know I shouldn't feel a spongy lever. I later found I need larger master cylinder so I wanted to keep the Original OEM look and went with 1975 Honda GL1000 master.



I also wanted to let people know about dual disc splitter can installed two ways which can save people money and different options for someone's next conversion.

I went with the splitter from the 1975 Honda GL1000 but I saw a cool option on another build and the rider used the original single brake light switch mount with a dual banjo bolt. It's a $15 part and works


So my solution was running the original fender and installing the right side mount without any spacers.
My issue turned out to be two problems dirt old parts and inadequate brake master.

Hopefully, my problems and solutions will help you tackle a dual brake conversion.