Topic: Need some help with a rear master cylinder

[cfree]

I am restoring a 1978 CB750F. I replaced the frozen solid rear master cylinder with one from a '77. I rebuilt it with a '77- '78 rebuild kit. The rebuild kit parts were identical. It is the type with the seperate reservoir connected by hose. I installed it but fluid will not move from the reservoir to the line. Took it apart, checked for blockage, everything is clear. The inlet from the reservoir into the cylinder comes in right between the seal on the piston and the seal on the spring. From what I can see the seal on the spring can not move far enough in the cylinder to clear the inlet and allow fluid to fill the line, so fluid just sits in the piston area but cannot be pumped down the line, no matter how much I pump. I am sure I am just missing something, but I am stumped. Has anyone experienced this? Can anyone advise?

[chewbacca5000]

An Ultrasonic cleaner does wonders for stuck master cylinder. You may also want to try boiling up a pot of water turn the heat off and drop in for 5 to 10 minutes.  I would avoid forcing so as not to score the inside of the master cylinder.  Break fluid can crystalline and get rock hard the ultrasonic cleaner heats it up and breaks up the crystals.  Hope this helps!

[CoachDoc]

I agree with Chew and I love those ultrasonic cleaners. Use hot water in the ultrasonic, and in no more than 15 minutes you will have the cleanest mc around.

[w1sa]

I am restoring a 1978 CB750F. I replaced the frozen solid rear master cylinder with one from a '77. I rebuilt it with a '77- '78 rebuild kit. The rebuild kit parts were identical. It is the type with the seperate reservoir connected by hose. I installed it but fluid will not move from the reservoir to the line. Took it apart, checked for blockage, everything is clear. The inlet from the reservoir into the cylinder comes in right between the seal on the piston and the seal on the spring. From what I can see the seal on the spring can not move far enough in the cylinder to clear the inlet and allow fluid to fill the line, so fluid just sits in the piston area but cannot be pumped down the line, no matter how much I pump. I am sure I am just missing something, but I am stumped. Has anyone experienced this? Can anyone advise?

It sounds like you aren't drawing/evacuating enough air thru the line to allow the fluid in....you need to create a lower pressure in the line so that the fluid moves beyond the mc/piston to the line and calipers......normally achieved by the pump and bleed process but usually much more successful if you apply some suction/vacuum at the bleed nipple before closing it....then release the lever for a more effective priming stroke.

[wedoo2]

By the way, welcome.  This is a good site as you will find out.  Are you using a MityVac?

[cfree]

Thank you for the welcome and the replies. Guess I will have to invest in a MityVac, installed the rebuilt front MC yesterday, same issue, moves freely but fluid will not pass the seal on the spring.
 I have never encountered this before, rebuilt the MC on my  CB400, actually an '80 CB250N with a '78 400 Hawk engine, and fluid flows easily into the lines. Thanks  for the vacuum pump idea, sounds like that should do it.

[Hondawggie]

I just rebuilt a master cylinder on a different bike and thought I'd damaged the m/c bore because when I bled the line, I was getting zero fluid out of the bleed nipple using the 'pump the brake lever and hold/open the bleed nipple briefly' method.  Zero brake fluid came out of the bleed nipple regardless of how much 'pump-and-hold' I did on the lever.

So I decided to show the gott-dang master cylinder who the boss was and got out my speed bleeder heres a picture of a 2nd one I bought still in the package this sucker works I accidentally bought a 2nd one when I thought I lost the 1st one



this device is 100% incredible
and costs very little $$ at Harbor Freight

I hook up the end of the hose to the bleed nipple and pump it up to 20 to 25psi suction and IF the brake lines and master cylinder are correct I get the system bled very fast


So I hooked up this speed bleeder to the Vulcan speed bleeder and still it would not bleed, so like you said, when you buy and try your speed bleeder and if it fails to work, then you might have the same problem I had

check the parts diagram I had one part installed wrong in the master cylinder, I did not feel bad as it was the 1st time I rebuilt that type of m/c good luck sir and welcome

[chewbacca5000]

+1 on the Mighty Vac.  One tip with the vac is to pump it up then hit the little brass relief valve, but don't let pressure drop to zero.  Then pump it back up again.  Works pretty good with a little practice.  I once spent an entire day squirting brake fluid all over the place till I got one of those mighty vac hand pumps and was done in 15 minutes.  Let us know know you make out!

[cfree]

Sorry for taking so long to reply. The company I wage slave for was crazy busy last 2 weeks, it's slowed back down now, good for giving me time to work on my project, bad for my paycheck.
I vacuum pumped the rear brake, worked like magic, quickly filled the line and caliper with fluid and have a real solid brake pedal.
The fronts are a different story. After half a dozen sessions of vacuum applied to both caliper bleed nipples, fluid has flown through but is slow, and the master builds probably only 10% pressure. I suspect there is an air block in the manifold ( where the master cyl. line connects to the caliper lines) and I thought I would try removing the master cyl. and manifold and keeping them below caliper level, and try to vacuum bleed.
Thanks again for the tip on brake bleeder. 

[chewbacca5000]

Hi cfree,

Can you give us a little more info on when the bike was last on the road?  It may be in your best interest to rebuild your calipers depending on how long it's been off the road.  Either way make sure you bring a wrench with to crack the bleeder valve on a few small test rides.  Brake fluid has a bad habit of crystallizing and glazing up the inside of the caliper cylinder.  This can cause the piston to stick and your brakes to drag.  So bring a wrench on the first few rides is always a good idea.  Heck at close to 40 years it is always a good idea to bring a few common size wrenches with you.  I never leave home without a 10MM.

Glad to hear they let you loose to run around in the yard.  Got the feelin were are turning into wage slaves.  Let us know how things progress.

[fatmatt650]

I like to verify that everything is open and clear by blowing compressed air through. On my 77F front I ended up replacing one of my hoses because no matter what I did, I could not get fluid or air to pump through properly. Also, when rebuilding the system I pour brake fluid into the calipers before I insert the pistons. This pushes fluid up into the lines and eliminates a lot of the air bleeding headaches. I did a 76F rear recently and it barely needed bleeding after assembly. Only got a bubble or two out of it.

[BeSeeingYou]

If you have the original flexible lines chances are there is a blockage in the line especially if it has been sitting a long time.  Best to get new lines but you can try clearing them by poking a piece of 1/16 flexible stranded wire through the hose.