Topic: Master Cylinder Rebuild

[cvanscott]

So, I am doing my first master cylinder rebuild on my cb350f.  I got it apart, cleaned it and installed all the parts from my rebuild kit that I bought and put it all back together.  It seems that I can pump and pump and no fluid gets pushed down the lines.  Did I not put it together correctly? 

[aperry]

Is the caliper bleeder open when you're pumping?  I think you want it open when you pull the lever down, then close the bleeder when you release the lever.  Not sure about your bike, but on the 750 you want to pump and release very slowly, and you don't want to pull the lever all the way to the handlebar, only a couple of inches.

Hope this helps,
Aaron

[cvanscott]

Is the caliper bleeder open when you're pumping?  I think you want it open when you pull the lever down, then close the bleeder when you release the lever.  Not sure about your bike, but on the 750 you want to pump and release very slowly, and you don't want to pull the lever all the way to the handlebar, only a couple of inches.

Hope this helps,
Aaron

I should probably mention that I replaced the bleeder valve with a speed bleeder.  That being said, I have tried pumping it with it open and closed.  There is 100% air in the lines.   It's like the fluid isn't even being pushed into the line.  I can pull the lever all the way to the handlebar with zero resistance.

[aperry]

Speed Bleeder should make it easy.  I have on of those too.  When pumping, make sure the speed bleeder is opened about half of a turn.  It takes time (maybe 5-10 minutes with a Speed Bleeder?), and you won't feel any pressure in the lever until the line is full and the bleeder is closed.  You need to pump slowly.  I've heard people talk about "priming" the MC to get things started, which I think means disconnecting the MC, filling the bore (and putting your thumb over the bore hole to keep the fluid in) and then reassembling.  I didn't have to do this, but maybe worth a try if you continue having issues.

Regarding the rebuild, the things that I think people can sometimes get wrong is installing the MC piston seal backwards (so the seal is facing wrong direction), and not cleaning the two reservoir holes well enough.  The return hole is very small and often gets clogged shut.  If that's clogged then I would imagine it'll be hard to fill the lines (and your brake won't work properly), especially if you have the reservoir cap on.  Try with cap off, to relieve any vacuum pressure you may have, but make sure to pump slowly or you'll get fluid jetting out.

Hope this helps,

Aaron

[apehanger 550]

 Yes, the priming with thumb does work. With the master cyl off the bike and reservoir full,point the bore up and fill it with fluid. Pump the lever,and brake fluid should build pressure and fluid will expel past your thumb. When that happens,take your thumb off,and fluid will suck backwards into the reservoir. You may notice that the fluid inside turns cloudy. That is caused by grundge inside the 'little hole' and the good news is that it is now cleaning itself out.
  Once it does prime up,be very generous with pumping plenty of fluid through the system, as the brake lines and wheel cylinder may have stale brake fluid in them as well. Hard to describe,but I hope it makes sense to you.

[Nikkisixx]

I got one of these vacuum bleeders from HF after struggling with the same thing - even with a speed bleeder.   I've had good results with it:

http://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-92474.html

[cvanscott]

Well I am having NO luck with this thing.  I primed the master cylinder like was discussed and it seemed to be working the way described.  I reassembled the thing, cracked the bleeder open and started pumping.  Fluid started moving through the tube I had connected to the bleeder, although EXTREMELY SLOW!  Should it be moving MUCH more fluid or am I just being impatient?  I pumped it for awhile and the reservoir never seemed to drop and the fluid would advance every once in awhile, but for the most part, nothing happened.

[Johnie]

Never used a speed bleeder, but on all the masters I have rebuilt I always had issues with getting the fluid down. That has been solved 100% by using my MightVac to pull the fluid down. Let us know how you come out.

[derelicte]

Read in the diy some people have filled from the bottom up to ease the bleeding process.  Could probably be done by mouth if you buy a long enough section of tubing - like 10c a foot at home depot.

[DedHed]

Read in the diy some people have filled from the bottom up to ease the bleeding process.  Could probably be done by mouth if you buy a long enough section of tubing - like 10c a foot at home depot.

I know this chickee named Rosie that could help you out with that....
Sorry, the school boy humor never forgets...
PEACE,
Phil

[cvanscott]

Read in the diy some people have filled from the bottom up to ease the bleeding process.  Could probably be done by mouth if you buy a long enough section of tubing - like 10c a foot at home depot.

How would you do this?  I'm willing to try anything at this point.

[Brantley]

You didn't mention if you did any work on the caliper. I find this sort of thing happens if you bottom the piston in the caliper bore, as the fluid seems to like a small bit of room to "expand" between them. The MightyVac or HF vacuum bleeder would definitely help; a mouthful of dot 3 might ruin some dental work (and Rosie weighs in at 19 stone, if she's the same one I'm thinking of). I got in the frustrated habit of pumping the lever with the bleeder closed with my other palm over the reservoir (keep rags handy...) to build pressure. Now that I'm thinking about it, are the diaphragm and plastic ring in the res. cap in good shape and oriented properly? Check out TwoTired's brake bleeding tutorial in the FAQ. And I hope you got a Honda master cylinder kit. Aftermarket kits don't enjoy a good reputation.

[cvanscott]

You didn't mention if you did any work on the caliper. I find this sort of thing happens if you bottom the piston in the caliper bore, as the fluid seems to like a small bit of room to "expand" between them. The MightyVac or HF vacuum bleeder would definitely help; a mouthful of dot 3 might ruin some dental work (and Rosie weighs in at 19 stone, if she's the same one I'm thinking of). I got in the frustrated habit of pumping the lever with the bleeder closed with my other palm over the reservoir (keep rags handy...) to build pressure. Now that I'm thinking about it, are the diaphragm and plastic ring in the res. cap in good shape and oriented properly? Check out TwoTired's brake bleeding tutorial in the FAQ. And I hope you got a Honda master cylinder kit. Aftermarket kits don't enjoy a good reputation.

I replaced the Caliper's piston seal and cleaned it up nice, but that's about it.  As for the reservoir cap, everything seems to be in good shape but I wouldn't know if they were in the wrong orientation.  Didn't know they could be, I guess.

[cvanscott]

I guess I will get one of these vac's that've been mentioned.  Nothing else seems to be working.  I tried siphoning manually but the fluid seemed to stop coming out once it got about 3 or 4 inches up the tube.  By the way, I did pull the MC apart again and double checked everything.  It matches the one that came out so I am fairly certain I put it in correctly.

[octagon]

don't know why you're having so much trouble. have you tried bleeding upwards, from the caliper? on some cars. like a mb 240d, you have to do this with the clutch slave cylinder.
if you have a good oil can you can use that plus a small diameter hose to connect to the bleeder valve. i've also put the cap from a bottle of gear oil on the bottle of brake fluid, then connected up a hose. better to use hose clamps to keep the hose on.

[cvanscott]

Finally some progress.  I picked up one of those EZ Bleeders (syringe type) off ebay.  Since I have the speedbleeder on my caliper, I couldn't force fluid into the caliper and up through the master cylinder.  I ended up filling up the reservoir and drawing fluid out through the bleeder.  Once I'd drawn out a fair amount of fluid (and of course lots of air) I reconnected my clear tubing and proceeded to bleed the old fashioned way.  Eventually, I started to get nothing but fluid, so I tightened the bleeder back down and voila I've got brake lever!  Although, I must say that it still feels a little spongy.  Any thoughts on this?  Thanks.

[ADW]

Spongy lever is probably indicative of the need for a new front brake line. On my 350F I bled and bled and bled 'til the cows came home such that I was absolutely certain there was no air in there. Still had a spongy lever. Replaced the brake line and bam! Super hard lever, just like it should be. If you're certain you've got no air in the system, you'll need a new front brake line. Look at it this way, too...that line is probably 35-40 years old, and it's the ONLY THING YOU'VE GOT to get that bike stopped in a hurry if you need to. Don't fool around, get a good one. I found a local place that made custom brake lines for cars and they had me up and running again in about 15 min and $75.

[lucky]

Plugged up vent hole in the master cylinder resevoir.