Topic: CB350F Master Cylinder Question

[RockinCB350F]

No matter how hard I try, this stock master cylinder will just not push fluid (I get air but no fluid).

So, in my frustration, I'm wondering if anyone knows if this would work for a replacement master cylinder:

http://www.ebay.com/itm/Front-Brake-Master-Cylinder-for-Honda-CB400-92-93-94-95-96-97-98-CHROME-Fit-7-8-/280861558659?pt=Motorcycles_Parts_Accessories&hash=item4164a74783&vxp=mtr

[bender01]

Probably . Did you get the little hole in your master cylinder cleaned out? Those are hard to clean.

[flybox1]

what do you mean you get air?
where?
bubbles from the holes in the MC, or at the bleed screw?

[RockinCB350F]

Yeah, I cleaned out the little hole.  Used a fine sewing needle my Wife kindly said she didn't want back

When I squeeze the brake lever, if I am holding the master cylinder I can hear air come from the outlet.  If I connect to the brake hose I can get air being pushed out through the bleeder at the caliper.

However, when the reservoir is filled with brake fluid,  no fluid moves out of the master cylinder...just air.

[flybox1]

there are 2 little holes.  Get both of them clear?

[flybox1]

i'm picturing you have the MC full of fluid, either off or on your bars, but no brake line attached to it? right?
if so, squeeze the brake lever, put your finger over the banjo bolt hole, and then release the lever. over a bowl, squeeze it again.  does fluid come out?

[RockinCB350F]

Yep, cleared both holes. 

And, with the method you just described, no fluid.  but, It does build air pressure.

[flybox1]

if it builds pressure, the seals may still be good in there, and it may push fluid.
But, fluid is not getting into the piston. 
please check the holes again...
if that still doesnt work, a rebuild kit is about $30 and easy to replace.

[RockinCB350F]

Cleaned the holes out with carb cleaner and the pin.  Every once in a while fluid comes UP from the smaller of the 2 holes.  but, nothing goes out the proper direction.

[RockinCB350F]

I'll pull it off the bike and try cleaning out the holes again.  Would be fantastic if that's really all it was.

[flybox1]

if its DOT3, its water soluble(and horrible for your liver, wear gloves), so a good overnight soak might be a good thing. or boil it outside for a few hours.
blow clear w compressed air.

i just did a MC rebuild on my 750 i bought a while back. 
there was what looked like crystalized DOT 3 caking the piston....

[RockinCB350F]

Ok, going to soak it.  Best liquid to soak in? 

Or..Boil it?  Really?  That sounds like too much fun not to try.  But, why a couple of hours?

[flybox1]

WATER, if you're not going to take it apart and rebuild it....just to make sure any of the crud in there dissolves.  but the only way to be sure is to rebuild it and make sure the bore is shiny clean.
once its dry, its a good time to polish it if the outside looks shi##y.

[Psychonaut]

Bleeding small bore bike MC's is a pita. I had to resort to tapping on the brake line and fittings with a screw driver handle lightly to get the stuck bubbles to move. Tapping and squeezing the lever 1000 times. They move up and out of the MC. Once I can build a little pressure I bleed them like normal. If you have too much air in there it is pretty hard to get it to come out the bleeder.

[eastyork1977]

I have rebuilt a couple 550 master cylinders and I have this to say.

1. Buy a M/C rebuild kit and strictly follow order of installation. A special set of pinching pliers is needed.

2. Coca-cola will do WONDERS for cleaning the inside of the piston-side caliper. I have pics if you want to see before and after. Will eat any rust or crud stuck near the piston. Takes about 16 hours or so for the coke to do its trick. I did two 8 hour soaks, then scrubbed with a strong brush.

Cheers

Jay

[RockinCB350F]

Well, I would like to rebuild it, however there is one problem I am running in to...  I can get the piston out, but how do you get the rest out past that?  It seems to be the same width as the lip the snap rings were on.

[camelman]

There are two snap rings. One to hold the rubber boot and one to stop the piston.

Are you sure you need to rebuild?  You are supposed to get a small jet of fluid out of one of the holes, so that isn't a problem. It sounds to me like you just need to work the system a little longer. Try wiggling the brake lever. That and half strokes of the brake lever always seem to do the trick for me.

Canelman

[RockinCB350F]

Strange, I had 2 to stop the piston.  One spring clip and one snap ring.  So, there is another that holds the boot and spring in?  I'll have to dig deeper....

Oh I have certainly been working the system.  Been squeezing the lever for hours now it seems.  I get a puff of air still, but no fluid.  I've boiled it and taken it down to the boot, but still no luck.  After boiling it I did get lots of air from both holes.  But, still no fluid movement.

Will be breaking it down further once I get off of work.  Hopefully the area past the boot is just full of crud or something.

--Rob

[ADW]

Definitely do take the whole thing apart to clean it. I had the same problem you did after rebuilding my 350F master cylinder with not getting the MC to push fluid, just air. What I ended up doing was "reverse priming" my MC by using a syringe and squirting some brake fluid into the cylinder from the end where the brake line goes on. I then did the "put your finger over the hole" trick someone's already mentioned. Eventually it did start pumping fluid after I reverse primed it. That trick may help you out as well.

Also, there is a Honda service bulletin out (posted somewhere on this site, I downloaded it from here) that states not to pull the lever any more than 15mm away from the handgrip, that over-stroking the lever will force the guts of the MC to a position where it may leak fluid out the lever end of the MC. So, heads up on that...I just take a nut with about 15mm thickness and tape it to the grip while I'm bleeding so I don't oversqueeze the lever.

Keep at it, you'll get it. And as a side note, once you do get everything bled, don't be surprised if you end up with a sorta spongy lever. My 40-year-old brake lines were swelling enough to not get a firm lever even though the system was completely bled. Ended up having to get new ones from a local brake line place that had universal ones that we sorted out to fit. Now the lever is hard like it should be.

[flybox1]

Hey Rob, once you get the boot and snap ring out, the piston just pulls out.  take a pic of the components all laid out in the order you removed them....get some 0000 steel wool down in the bore and get it nice and smooth.  shoot it really clean with some brake cleaner.
even though this is for a CB750, its a good vid for steps for a rebuild.
Sully's Garage Episode 3

[rboe]

Also, there is a Honda service bulletin out (posted somewhere on this site, I downloaded it from here) that states not to pull the lever any more than 15mm away from the handgrip, that over-stroking the lever will force the guts of the MC to a position where it may leak fluid out the lever end of the MC. So, heads up on that...I just take a nut with about 15mm thickness and tape it to the grip while I'm bleeding so I don't oversqueeze the lever.

Thank you!! I missed that. Should save me from rebuilding it AGAIN. The priming tip sounds like a winner too. Double thanks!

[ADW]

Glad to help! Be sure to let us know if you get everything worked out.

PS: Here's a link to the bulletin about not "over-squeezing" the lever when bleeding: http://forums.sohc4.net/index.php?topic=102388.msg1145064#msg1145064