Topic: Rear Master Break Cylinder for a '78 CB750F

[Crimson750]

Hello all,

This has been a headache of mine for awhile, not to mention a safety problem seeing how I had the bike slide out from undernieth me when I locked up the front breaks on a wet road surface. Whenever I step on the back break pressure builds in the caliber and it doesn't release. It made for changing the piston in the rear break really easy but its an annoyance during normal use. I don't think there is a problem with the caliber itself seeing how its only a piston and an fluid in hole. The master cyclinder for the rear breaks is equally as simple. But howcome the pressure does not get released when I release the break pedal? I've taken everything apart and rebuilt it with its respective kits and the break pressure does not release. The previous owners of this bike had the same problem but they just dealt with it and didn't use it. What options do I have aside from just buying a hole new master break cylinder? What experiences have others had with this. I found this thread:  http://www.sohc4.us/forums/index.php?topic=2131.0 but I can't download the file for some strange reason. If someone could email it to me that'd be great. Also what solutions have others found to this problem?

-Crim.

[eldar]

I think we have the manual for the 78k & F on file here somewhere. Check that out. I would say that is pressure does not release then there are a couple possible problems.

1: The master needs to be cleaned and that means the WHOLE thing, not just the fluid resevior. Take the master apart and clean the actuating piston and the seals. Also clean BOTH holes, there are 2 holes and if one is plugged it could cause this issue.

2: Completely flush out the old fluid. No point to cleaning since old fluid is what made most of it dirty anyways.

3: Clean all the old fluid out of the caliper, remove piston and use a dremel or fine sandpaper in the CALIPER but not on the piston. DOT3 fluid attracts moisture and it will build up a white crust inside the caliper which will cause the piston to stick no matter how good your piston or seals may be.

This should take care of any problem unless somehow the caliper housing is bent and is stopping the piston but I dont see how that could have happened.

This is why I like the drum brake on my 78K.

[sounddoc]

i'm a little confused - are you saying that the front break locks up when you step on the brake pedal? if so, i think you have some serious brake hose routing issues 

in any case - take apart the offending caliper, and do these things, if they haven't been done already:

pop the piston out, and take some steel wool to it's sides.
get the seal out CAREFULLY, check it's condition, clean it, chek it's groove in the caliper, clean that too CAREFULLY as there can be salt / rust buildup in there, applying needless pressure to the piston, causing it to stick.
before putting the piston back, apply a super-thin coat of grease, so it pops in and moves about easily.
inspect the pins holding the rear brake pads; are they rusty at all? these you'll want to clean, and grease as well.

if these don't help, then i would look into rebuilding the master cylinder of whichever brake is locking up.

and finally - change the brake fluid, *all of it!*

-Pete

[Crimson750]

I'm sorry for some of the confusion.  Its my rear calibers that lock up and don't release...My front breaks work fine, thank god
I have completely rebuilt the rear M/C and caliber. Its been cleaned and looked over. I did some more investigation and I don't  see relief holes, like what is described in other threads that have this similar problem. The piston in the caliber moves fine. It is free moving. I found the 2 holes which are mentioned, what is the best method cleaning the mentioned hole?  I assume the holes are the resevior feed and the outlet to the break caliber. And every time I fuss with the breaks the fuild gets changed...completely. lol.

Thanks,
-Crim

[Crimson750]

Oh, and what type of grease would I use in the rear M/C?

-Crim.

[eldar]

You follow the same procedure as the front brake. DOT3 or 4, some say no synthetic but I use it now and have no problems but use what you are comfortable with.

I guess I have never used grease on any seals. I suppose some people do. If you decide to, use  a grease that does not eat away the rubber seals. I would think a silicon grease would be ok but again, check the container to be sure.

[sounddoc]

i'm not positive on the type of grease, but i use standard stuff from auto zone that comes in a tub that's heat and water resistant....

*ducks*

-Pete

[Bob Wessner]

I can't honestly say what the consequences would be, but the Honda shop manual calls for silicon grease with a specific notation "NOT to use molybdenum grease known as brake grease."

[Crimson750]

I don't think grease is going to help this problem. As a temporary solution I spreyed some penetrating grease into the M/C and the break caliber. So I know everything is lubricated over and above the luberication properties of the break fuild. Now, exactly how much drag is acceptable on rotor? Too much drag is being  created when the  break is not being applied. I have bled the breaks so I know there are no air bubbles in there. What happens that the pads drags and the whole heats up and eventually seizes. Is there a small pilot hole in the M/C that can get clogged? I think there is and I think mine is blocked, I'm not sure. I haven't been able to see the phyiscal layout of the M/C itself w/o any accomanying components. Does anyone have the machinist plans to this damn thing? So everything is well luibericated, there are no air pockets, fresh fuild in the system, and I think that covers it. Its a new hose, new piston in the caliber, the M/C has been rebuilt by me. The only thing I can think of is that the rebuild kit is not the right one. Its either a blocked pilot hole or the piston used in the M/C is not the right one and is too long.
Any ideas?

Thanks,
-Crim

[Bob Wessner]

I've never heard of putting any kind of grease in a master cylinder, fluid only. If it is anything at all like the front master cylinder on my early 750, there is a very small hole at the bottom of what appears to the eye to be in incompletely drilled hole. At the bottom is a v-e-r-y small, almost pinhole. Hope this helps.

[Crimson750]

There is the inlet hole for the resevior and what looks like a pinhole above it, but I don't have a clue though if iis a functional hole or not. As for lube...we'll see. I may end up buy a new M/C seeing how the local bike junk yard is rather lite in 78 750Fs.

[sounddoc]

i can't say i've had much experience with blocked reservoirs. to clarify - i didn't mean put grease in the reservoir! just a thin layer on the side of the piston to assist in it's sliding.

one thing to check if you're getting drag while the brake isn't being applied - the swingarm (the arm that holds the caliper) can be adjusted by the screw / spring that attaches to the fork. with the front wheel off the ground, loosen the 10mm nut on the end of the screw and adjust the screw with a flathead until the wheel just about spins freely. this is of course not going to fix it if both pads are hard on the rotor. if when putting the caliper back together you cranked this screw on, the pads would be applying pressure to the rotor. after this, i'm all out of ideas unless i actually saw the bike. good luck!!!

-pete

[Crimson750]

I took a look at my rear break assembly but I don't see anything that you described, Pete.  I really don't have a clue as to why this is happening. My last choice is to buy a new M/C housing and use the parts inside the one I have now and see if its that M/C itself. If this doesn't work out then I don't know. I don't think there are any pilot holes in the break caliber to get clogged.

[Mr. Grimm]

 I think eldar's got the right fix. Since brake fluid doesn't want to be compressed, it will naturally relax after you let of the pedal. But the piston in the caliper can stick in it's housing and refuse to relax back in. Try taking the caliper off the rotor, but not off the bike. Don't un hook the  line, but place a piece of wood between the pads that would mimick the rotor thickness. Now, only with the wood in place press the pedal slowly and watch the piston move outwards. Is it returning to it's original position? Does your master cylinder rod return as well when you release the pedal? It is also possible you have a warped rotor and it continues to rub on the pads slowly building heat. Does the rear brake seem to get grabbing after a certain time of driving? Does it get more noticable farther down you travel? You could swap one of the front calipers to the rear and see if it gets better, but be mindfull that if the bad caliper is on the front, you could have an even more interesting ride. Good luck. Let us know how it goes.

[Crimson750]

Hey, sorry for the long delay. I haven't been able to attack the breaks on my bike again. I sort of took apart a spare engine for my bike to rebuild it. I'm not sure, if I could swap one of the front calibers with the back caliber. But I'll let you guys know how I make out. Thanks for the help.

-Crim.

[eldar]

Did you ever powder coat your caliper? If you did you probably warped the housing. Thats why they are painted.
if you look int the area where your fluid is stored on your front master, you wil see 2 holes. There are 2 holes on your rear brake aswell.
If you do not see these 2 holes then I would say the masters need serious cleaning.
I did my front master on my 78K and it was seriously gunked up.

[eldar]

Did you find the pdf file for the 78 K & F? If not, send me you email addy and I can send you some pics from the honda shop manual.
The pics show the expoded views and assembled views and the service limit measurements.

[dusterdude]

i havent read all the replies,so has anyone mentioned the rubber brake hose back there.it will swell when pressed then syphon shut and wont let the fluid back into the mc.cars do it all the time.

[eldar]

If this is happening, it is time to replace the hose. If you dont you are looking to get hurt when the brake fails. I suppose this is always a possible thing. I have never seen or heard of it but I can see how it happens.

[Crimson750]

Hello all!

Sorry for the slow response time, I started a second job and my time and energy have floored. Ok, eldar. I looked at the front mc/resevior and I see the two holes. I'm guessing  its a similiar setup with the rear MC. if this is the case. Then that little hole I found is blocked and I have nothing around the house to clean it out with. I have not powder coated any of the parts on my bike. I don't possess the skill or cash to get them coated.  What would be the best way to clean out the 2ndary passage way on the rear MC when the hole is almost smaller than the brissels on a course wire brush? I can tell that the rear MC is actually 2 pieces fused somehow together and that there is probably a small diameter passage machined into it. I'm at a loss though on how to get it cleared...Suggestions?

Much thanks,
Crim.

[TwoTired]

Have you tried that canned brake cleaner in aerosol?  They come with a nozzle and tube to direct spray and pressure into small spaces.

What about a nylon bristle from a tooth brush held in a needle nose pliers?

Cheers,

[Crimson750]

I don't think a nylon brissel from a tooth brush would work. These breaks have been abused tramendously, by the previous owner. The breaks have locked up a couple of times and have heated up to the point where the plastic window on the caliber was literally burnt off. So who knows what fused in that small passage way.

-Crim

[eldar]

do you have a carb cleaning tool? The wires on that would poke through the crust. You will need to disassemble the brake though to clean it properly. I dont know about the back but the front brake has a rubber boot that seals the part the gets pushed in by the brake handle. The rear is probably the same. There is a c-clip that should be removable. This clip holds everything in. Remove it and note how everything goes together. Clean good maybe rebuild if things are scary bad.

[Crimson750]

Yeah, taking apart that cylinder is no problem, I've done it a bunch of times. I would like to see the actual manufacturing diagrams of the MC to see if what I'm poking at is actually a passage or not. I should post pictures...The next time I feel like playing in break fluid again I'll take some.

-Crim.

[Bob Wessner]

..and note how everything goes together

I can vouch for this one.  Lay the parts out on something as you remove them, and if you can, take a picture, particularly if there is any amount of time between disassembly and reassembly.