Topic: Need CB400F Master Cylinder Help

[Ernest T]

The PO rebuilt the master cylinder, but couldn't get any pressure.  I bled the system with a vacuum bleeder and it didn't appear to be leaking anywhere like the caliper or the joint below the headlight, but there was still no pressure.  I took the MC apart and it looked like it was installed correctly according to the picture in the clymer manual.

Here's a pic of the way it came apart



Also, I've never seen one of these before, what is the purpose of the nub on the top?  It doesn't seem to move.




Finally, what is the purpose of the flower looking piece of metal on the end of the piston?

[MoMo]

There are many  threads here on mc rebuilding but it sounds as though the little hole in the reservoir is blocked.  There are two indentations looking down into the reservoir, one hole is quite noticeable but the other is so small it can barely be seen.  If blocked you will not be able to get pressure...Larry

[Ernest T]

Yeah I did a search and found lots of threads about replacing the MC with a modern one, but not very many about rebuilding them. 

both holes are clear--when I shine a light through the bore I see light through both holes, one very small and one quite large.

[camelman]

These masters can be tough to bleed.  I've had luck just wiggling the lever back and forth quickly, and then slowly squeezing it.  After a few attempts, it should start to produce bubbles.  Tapping along the lines and the switch housing can also dislodge air bubbles.  If that doesn't work, then try slowly squeezing the lever, and slowly releasing it.  I've also heard of people having success just letting it sit overnight.

When I'm bleeding difficult masters, I leave the cap off the reservoir so I can see if my efforts are making any progress.  Just make sure not to squeeze very quickly, or you'll shoot fluid all over... and possibly into your face.

Camelman

[Ernest T]

A related question, what is the function of number 12 and 13? 

[faux fiddy]

These masters can be tough to bleed.  Camelman

I'm not to this point yet, but I heard a tip for thouroughly bleeding the brakes on other bikes, and it involves taking the whole caliper assembly (and I guess the light switch on this one) off the bike where you you can manipulate the places that catch bubbles to the top where they may exit up the line.

I have heard of pulling the lever back and tying it back overnight, too.

[Lars]

As Photolar points out, there are two holes in the bottom of the master cylinder. One is fairly large and causes no problem. The other one is so small, that you will not spot it, if it is clogged, and you really have to look carefully. I use a hair from my steel brush to make it free all the way trough. This is normaly the most common problem when it comes to bleeding, it is not possible to bleed unless this is in order.

The piceses in your photo is displayed in the correct order. The cup in the end goes with the dome into the spring. Inside the dome there is rubber, so when you press the brake handle, it lets the fluid pass the rubber in the dom, but stops it from flushing back.

When you are doing your brakes, do it all the way trough. Take the caliper off, rinse it, clean out the groove for the square piston seal properly, install a new piston seal and check the piston itself. Chipped off chrome on the piston wall will damage the seal and fluid will eventually seep out. Clean pipe the connects to the caliper, the hoses and the joint in between. I use compressed air and sometimes a piece of stee wire comes in handy. Old rubber hoses get soft and will expand when you brake and you will have poor brakes.

Something to read here:

http://www.hondahobby.no/website.aspx?displayid=1610

[Bodi]

nub on top is a dust seal. It doesn't move.
hat thing is a check valve.
the pin and plate are to prevent the lever holding bolt from loosening with the lever action. If the nut falls off, the pivot bolt could be knocked out by a bump and then you would have no lever and thus no front brake available.
The piston can only pull fluid from the reservoir under vacuum. It will n ot work at all with the caliper port open. Squeeze the lever in tight and cover the caliper port with a finger, tightly. Release the lever, and some fluid will be sucked from the reservoir. Repeat until you can hold your finger firmly on the caliper port while you squeeze the lever, forcing out fluid past the finger when squeezing and sucking in more from the reservoir on release. It's pretty much full and primed then. Any air remaining in the MC after hookup can be bubbled out to the reservoir by jiggling the lever through about 1/4 of its travel.
Backfilling the lines by filling the caliper with the piston at its outmost position then forcing the piston in to flow fluid into the lines from bottom up, will reduce the amount of bleeding needed.

[Ernest T]

nub on top is a dust seal. It doesn't move.
hat thing is a check valve.
the pin and plate are to prevent the lever holding bolt from loosening with the lever action. If the nut falls off, the pivot bolt could be knocked out by a bump and then you would have no lever and thus no front brake available.
The piston can only pull fluid from the reservoir under vacuum. It will n ot work at all with the caliper port open. Squeeze the lever in tight and cover the caliper port with a finger, tightly. Release the lever, and some fluid will be sucked from the reservoir. Repeat until you can hold your finger firmly on the caliper port while you squeeze the lever, forcing out fluid past the finger when squeezing and sucking in more from the reservoir on release. It's pretty much full and primed then. Any air remaining in the MC after hookup can be bubbled out to the reservoir by jiggling the lever through about 1/4 of its travel.
Backfilling the lines by filling the caliper with the piston at its outmost position then forcing the piston in to flow fluid into the lines from bottom up, will reduce the amount of bleeding needed.

thanks for the information.  What is the caliper port?  Also when using a vacuum bleeder shouldn't it suck all the air out through the caliper?

[MoMo]

I think Bodi means you can bleed the mc without the hose connected by placing your finger over the outlet.. I'm sure he'll get back on that, and the vacuum bleed in theory should bleed all the air.

 I usually start  at the master cylinder banjo bolt and then each banjo until I get to the bleed valve on the caliper. I gently pump the lever, release it, open the banjo and then squeeze the lever until fluid comes out. Do this at every junction-usually takes no more than 10 minutes to bleed a system...Larry

[Bodi]

Missed the vacuum bleed bit. This should work, but you can still have a bubble in the MC. Try the jiggling of the lever, if you see bubbles come up from the tiny hole in the bottom then there's air in the MC cylinder.

[Ernest T]

Missed the vacuum bleed bit. This should work, but you can still have a bubble in the MC. Try the jiggling of the lever, if you see bubbles come up from the tiny hole in the bottom then there's air in the MC cylinder.

Okay, I saw bubbles coming up when I tried the lever.  I'll give it another shot.