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

Confusion on sequence of assembly of parts for rear calipar of CB750F SuperSport

cary911

CB750 Member
Messages
11
Reaction score
0
Points
1
Location
Cary, North Carolina
I am in the process of rebuilding a 76 CB750F Super Sport. I ordered and received a GL1000 master cylinder rebuild kit, have adjusted the length of the adjusting bolt but I am not certain about the order in which to insert the tapered spring and rubber/metal bushings, for the lack of a better word. I know how the O ring goes on the piston but where I am drawing a blank concerns the one rubber cap, the metal cap and the tapered spring with a small metal cap attached on the tapered end. If I look at my drawings that I have it appears, in order of entering the master cylinder, 1. Metal cap, 2. tapered spring with open end of spring first, 3. rubber cap, 4. piston with seal, 5. adjusting bolt then clip then rubber boot. Can anyone clarify? Thank-you.
 
The metal cup if it has a tiny rubber valve tucked inside goes in first, the spring big end fits OVER the cupped part to rest on the outer lip. Then small end of spring (should be capped with metal) goes to the free rubber cap inside the cupped part of it. That free rubber cup then bumps against the side of the piston with the little leaf valve petals on it.

The very first metal cup going in is simply a one way check valve and kinda stupid as it holds residual pressure that can wear out pads quicker. Honda realized that and later left them totally out of many M/Cs to just have the spring go in big end first. I personally never use the check valve as it can cause problems by being there in the way of flow.
 
I'd be real careful adjusting that bolt ahead of time, you really must check it once the whole brake assembly is together. If the bolt as adjusted pushes up ANY WHATSOEVER on the M/C piston to close off that compensation port then you will have serious brake lockup issues once bike is running. Proper adjustment is all linkage put together and the bolt adjusted to have at least maybe .010" clearance before the M/C piston begins to move at all. That guarantees that port is open to relieve pressure when the brake is released. You should also check that port for fully open after your M/C assembly job.
 
I found a youtube video showing the proper instillation exactly the way amc49 describes in his post. I done the assembly, everything is connected and I have filled up the cup with dot3. As I do the procedure of bleeding the brakes I am not getting any travel of fluid from the cup. I believe I just have too much air in the completely new system all the way to the caliper. I've been able to get a few minor air bubbles to come up from the valve hole but that is it. Any suggestions out there?
 
Did you rehone the M/C bore and how much corrosion damage remained after doing it? If it didn't clean up the casting may be junk. Also, if that same port I spoke is not open you won't get fluid in the M/C nearly so easy. If you went ahead and used the check valve then the M/C is a positive displacement pump or it is not working.
 
The inner bore was very clean. I am going to back off the adjusting screw to make sure the compensation port is open. I am having the same issue with the front brakes. I've completely rebuilt the front from master cylinder, new hoses, to brake caliper. It seems that I can't get the fluid to drain from the cup into the master cylinder piston area. I've thought about buying a used cap and making a power bleeder.
'
 
Thanks for your help amc49. I have verified that all m/c parts have been assembled in the correct order however I still cannot get fluid to move past the m/c into the brake fluid lines. They are all new lines and I removed the front m/c from the first banjo bolt and blew air from the caliper up through to the top banjo bolt. Everything was clear so my problem is in the m/c. I am experiencing the same situation with the back m/c. Should I try removing the "first metal cup" that you mentioned in the previous post? The large hole in the cup of the m/c is clear and after draining the brake fluid out of the cup I blew air into the smaller hole and it blew clear. Where does that hole lead to? I know I cleaned these holes out when I did the original assembly so I am really stumped. I have also verified that I put the "o" rings on in the correct direction. Thanks again
 
True o-rings do not care which way they go in, you are referring to the rubber cups which have to face with the outer lips FORWARD. If M/C bore is not clean or has corrosion the M/C pumping action quits, when the cups simply bypass fluid round their sides instead of pumping it forward. Leave the front metal cup in for now as it turns the action into a true one way pump, give that some thought. If you remove it and the cylinder DOES begin to pump then as soon as you let off with the metal cup gone the fluid which pumped in simply drops back since air coming from below in lines has less resistance than the fluid does. Meaning, with the metal cup out you would have only a one way one time pump instead of one way and hold to add up successive pumps. With metal cup you have to hold a finger over the master end hole at every release to force the fluid to stay in place.

You want.............push piston in and pump a small bite of fluid forward, release and and it stays, then piston forward again and adds more then release and more stays, how all the little bites of fluid add up. If they simply go backwards again at release they don't and all you have is the first pumps' worth and then it just goes back and forth doing nothing. It instead must accumulate to fill the system.

I pretty much wet the inside of master and parts with fluid when assembling it, it gets the seals wet and the initial pumping starts up faster. I also bleed to pressurize before the master ever hooks up to brake lines to determine the positive pumping action is already present. If not then the part will never work.
 
Last edited:
Back
Top