Topic: First Ride on the cb750 - brake question

[Ericcb750]

Hey guys,   SO after all the work I've put into my first CB750 since last August, I FINALLY GOT TO RIP IT UP AND DOWN THE ROAD!!!!  I wish I could have gone further as it was f-ing AWESOME, but it's not registered yet. I cannot tell you what a feeling that was to finally get to actually ride it after all the issues I've had with my first build. It was so smooth and had some real good power. I instantly fell in love with that bike and can't see myself getting rid of it ever. That few minutes made me understand why these bikes have such a following.  I do just have a question if anyone can help. My front brake was a bit spongy and after opening the cap on the master cyl I see that the brake fluid that is in there is absolutely disgusting and needs to be changed. It says DOT 3 but I was at cycle gear this weekend and they told me that DOT 3 and DOT 4 can be mixed and either can be used. I know DOT 5 is an entirely different animal as I use it on my Harley and it will eat seals/o-rings.  Is it true that I Can use DOT 4 on the CB750 ? Or should I just use the DOT 3 ? They also had a bleeding kit that wasn't too expensive, are these worth it ? I did the brakes on my Harley without one but it was messy and a pain. If anyone had any advice , please let me know. I really just wondering about the fluid. Thanks Guys, take it easy.

[70CB750]

DOT3 or DOT4 will do.  DOT5 has to go into completely pristine system and IMHO is not worth the trouble.

Bleeding - there is a trick to tie the handle half way in and leave it overnight with the lid off and let the bubbles come up top.  I used a bleeding kit too, sometimes it takes a bit of effort to get brake lines completely bled, but it is worth it.

I know you are excited since you just got it running, but this would be good time to take caliper off, clean it up and change the rubber ring in there and also clean the master cylinder.  You will thank me later

[Ericcb750]

Oh alright, that actually is a great idea. It was pretty nasty so you are probably right. I will take off the caliper, clean it out and everything. Where is the Ring that you mentioned? Is there a specific place to buy a new ring that exact types/size? I will feel much better going down the road knowing the brakes are in good shape. I did buy new front pads for it a little while ago. However I didn't put them on yet because the pads that are on there have a lot of life left. Is it the plastic ring that comes with the Brake pad kits??

[David B]

While you're at it, it's not a bad idea to rebuild the master cylinder.  Kits are available from several sources.  These guys offer both: 

http://www.vintagecb750.com/products/2/chassis-brakes/42/brake-caliper-rebuild-kits-brake-caliper-pistons

One other thing to consider, especially since you'll have it all apart, is to replace the old brake lines.  I replaced mine due to their age and it was cheap peace of mind.  You can get OEM rubber ones or a common upgrade is the braided stainless steel ones.  I think I got mine from Slingshot Cycles:  http://slingshot-cycles.com/

Aren't we great at spending your money?   

[70CB750]

What calj says ^^^

 This square profile rubber ring and the return hole in MC are responsible for brake pads release when you let of the brake handle and together are the number one reason for locked front wheel brake.

The grove the ring sits in has to be cleaned well before assembly.

To lubricate the piston and the #8 rubber ring the Dow Corning Vacuum grease is recommended by experienced people - you can get it on eebay in small jar.

Before you take the caliper off, use the brake to pump the piston out, it will save you some headaches later.

[bjbuchanan]

To add to all the good suggestions

Behind that square o-ring in the caliper is the groove it sits in. Clean it well, then clean it again. That is one of the big culprits for a sticking brake. I take a brass dremel wheel and make it shiny

[Ericcb750]

Thank you all for the help on this , I appreciate you guys taking the time to reply. Is this the seal that you're referring to? I just want to make sure i'm getting the right one. The brake is not stuck it does squeez and retract but i'm postitive it could be cleaned out to work better so I don't mind doing that at all. I'll probably just do this for now and maybe get one of the re-build kits and do that over the winter? What do you guys think? Has anyone used a re-build kit ? Is that only for when the brakes are not working at all? Thanks

http://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-chopper-custom-motorcycle-brake-parts-honda-cb750-front-caliper-piston-seal-45215-533-004.html

[Ericcb750]

Thanks Calj,  I will clean it up and bleed/add new fluid for now. Depending on what it looks like when I clean it , I will either rebuild it then or over the winter.  So it can take with DOT 3 or DOT 4 ?? It doesn't matter which one?  I can't wait to actually take this thing out for a long ride, It was so awesome but i do want to make sure I'm able to stop well. That will be # 1 priority right now.  On the re-build kits what exactly is being re-built? is it all the parts in the caliper? Is it best to put in a new piston if the current one is in good shape?

[70CB750]

If the piston is in good shape, than the only thing to replace in the caliper is the rubber ring.

Rebuilding master cylinder is when you need the kit - it replaces bunch of things:



Edit:  yes, use DOT3 or DOT4, mix them together, all is good.

[PeWe]

New master cylinder is not extra $. The aftermarket version from David Silver plus the additional cap in OEM look.
http://www.davidsilverspares.co.uk/CB750K6-FOUR-1976/part_123916/
http://www.davidsilverspares.co.uk/CB750K6-FOUR-1976/part_189670/

[TwoTired]

To lubricate the piston and the #8 rubber ring the Dow Corning Vacuum grease is recommended by experienced people - you can get it on eebay in small jar.

Um... This is incorrect, I'm afraid.  Dow Corning High Vacuum Grease (pure silicone) is never to get inside the brake fluid chamber.  And, its use on the square section seal will interfere with the seal's "grab" on the piston for proper retraction.

The seal is properly lubed with Brake assembly lube, which can be hard to find these days of replace the part rather than rebuild.

The Silicone grease IS used on all bare metal parts that are exposed to the weather, as it will shed water, prevent oxidation, won't bleed, and still stay where placed when it gets hot enough to melt petroleum greases into ooze, (which will foul brake pads and rotor).

There is a FAQ entry for front caliper overhaul, if interested.


DOT3 and 4 can be intermixed, but the result is a fluid that is only as good as the lower capability, DOT3.  So, there is no benefit to mixing them.  They are both Glycol based, FYI.

[Ericcb750]

Alright cool, Thank you all for your help. Looks like I get to dirty my hands again. I'll clean out the master cylinder and caliper this week. Replace the fluid and i'll take a look at the ring, probably just order one anyway, Good piece of mind for $9.00.  Thanks for letting me know that those kits are to re-build the Master Cyl. I wasn't sure. If I Can't find Brake assembly lube for the seal (if needed) is there something else that can be used in it's place ?

[HondaMan]

To lubricate the piston and the #8 rubber ring the Dow Corning Vacuum grease is recommended by experienced people - you can get it on eebay in small jar.

Um... This is incorrect, I'm afraid.  Dow Corning High Vacuum Grease (pure silicone) is never to get inside the brake fluid chamber.  And, its use on the square section seal will interfere with the seal's "grab" on the piston for proper retraction.

The seal is properly lubed with Brake assembly lube, which can be hard to find these days of replace the part rather than rebuild.

The Silicone grease IS used on all bare metal parts that are exposed to the weather, as it will shed water, prevent oxidation, won't bleed, and still stay where placed when it gets hot enough to melt petroleum greases into ooze, (which will foul brake pads and rotor).

There is a FAQ entry for front caliper overhaul, i interested.


DOT3 and 4 can be intermixed, but the result is a fluid that is only as good as the lower capability, DOT3.  So, there is no benefit to mixing them.  They are both Glycol based, FYI.

Thanks, TT! You're on it!

[Ericcb750]

Sweet, Thanks guys. I'll dig into this during the week sometime and let you all know how it turns out. I just can't wait to go cruisin on this bike, it was so smooth compared to the HD. It was a lot of fun. Thanks to all for your help/advice, WIthout this site I would have been very lost during this first one.

[TwoTired]

If I Can't find Brake assembly lube for the seal (if needed) is there something else that can be used in it's place ?

If you can't find a screwdriver or a proper sized wrench, what is properly used in it's place?
I haven't found anything that is a proper substitute for brake assembly lube.  People keep trying something else and declare success after the next ride.  I haven't seen any reports of a successful substitute after 10 years in service.

Pre-soak it in brake fluid.

This IS the usual short term solution.

However, the seal bridges the hydraulic, atmospheric boundary.  Brake fluid on the inside isn't an issue, of course.  But that fluid on the atmospheric side will draw water from the air and promote corrosion.  Kinda bad long term, if you don't keep the bike in a dessicated location.

Brake assembly lube does not draw moisture or promote corrosion, and can serve properly on both sides of the seal.

[Ericcb750]

Thanks TT, I appreciate it. I will find some of the brake assembly lube as that sounds like the best long term option. I'm sure I can get it somewhere. Thanks for the help on this. I'm a rookie to this so It's good to know that kind of info.

[David B]

Thanks for the education TwoTired.  I just ordered some of this: 

http://www.jegs.com/i/Wilwood/950/290-11087/10002/-1

$8 shipped is easy enough. Beats driving around trying to find it and not having to look at the blank stares in the auto parts stores.

[bjatwood]

I use a Mighty Mite vacuum bleeder to change out my fluid every other year. Pulls the old fluid out and as it's going out top off the MC reservoir and your golden!

[Harsh]

Thanks for the education TwoTired.  I just ordered some of this: 

http://www.jegs.com/i/Wilwood/950/290-11087/10002/-1

$8 shipped is easy enough. Beats driving around trying to find it and not having to look at the blank stares in the auto parts stores.

That is the stuff I use.

[TwoTired]

To be clear, the brake assembly lube is used ONLY for the caliper seal (or master cylinder rubber bits).  It is NOT used behind the caliper puck, as it will run/bleed into the friction pads.

For behind the puck and all caliper metal exposed to water ingress, Dow Corning High Vacuum grease (100% silicone) is the proper material to use.