Topic: CB750 Can't get the front brake to bleed at all

[edwardmorris]

So at this point, I've tried everything except fully tearing down the front brake assembly. I have a vacuum pump which helps quickly get the fluid at the caliper. All banjos are torqued down to 15ft-lbs with new crush washers. Caliper fully rebuilt with new piston and seal after a thorough cleaning. Master cylinder got a brand new rebuild kit, everything installed correctly and was dunked in the same brake fluid being used when assembled. This is what I did:

Filled up the reservoir and used the vacuum pump at the bleeder to get the fluid to come through. Refilled as needed, and let some of the fluid "bleed through" to the pump's collector to ensure no air gaps were left.

Handlebars turned left so the MC is at the highest point, saw only very few bubbles come out on pumping the lever.

Started bleeding per instructions in manual and seeing a very strange thing happen. When I did this two years ago when I first bought the bike, when I pump, I saw the fluid squirt through with good pressure out one of the two small holes in the reservoir. When I do it now, the fluid is staying stationary doing nothing except for the occasional micro bubble popping up. However at the bleeder screw, I see fluid gushing with each pump, but the level in the reservoir stays the same almost stationary.

When I use the vacuum pump at the bleeder, the level goes down

WHAT THE FCUK IS HAPPENING?

[robvangulik]

 Probably the smallest of the two holes in the mc is blocked, take sewing needle and, carefully, unblock it.

[oldhatt45]

Rafi,

When I bled my system, after a while all I got was tiny bubbles.
I did get "gushes" out of the bleeder. 
At that point where I got "gushes" from the bleeder screw I closed the bleeder up and continued to get tiny bubbles coming up.
I tied back the brake handle half way put the master cylinder cap on loosely to keep moisture out.

Let it sit over night with handlebar to the left.
The next day I put maybe 1/2 an eye dropper of DOT3 brake fluid in, closed up the cap with diaphragm in place and worked the brake lever.
The brake worked fine.

Questions:
Is the fluid level good now in the master cylinder?
Have you locked up the bleeder screw?
If the bleeder screw is tightened up, have you waited for the final tiny bubbles?
Is your brake working at this point?

Last thing.  Is the caliper piston in correctly?  (Yeah, I know, stupid question, but had to ask)  With the solid face pointing in toward the caliper body.

If the brake works, then it might just be that you are over thinking things.   
PS:  When you said "gushing" how much fluid is gushing??  (It may not really be a lot of fluid.  A typical cut on your head looks like a lot of blood but in reality is probably not enough to fill a thimble). 

Charlie

[edwardmorris]

@Rob, no, they're both clear, bubbles come out of both when they do.

@Charlie Answers:

Yes
Yes
Yes
NO

Yes

Brake doesn't work, lever has no resistance other than that of the MC's spring.
Gushing as in jumping outta there with pressure as opposed to dripping, which seems right as you need that for the pad to engage.

[oldhatt45]

Rafi,

From the answers, to me, it keeps pointing to something wrong with the master cylinder.
I'm at a total loss why your brake isn't working but if this was happening to me, I would be looking at the master cylinder.
Something isn't right in there.  I don't know what, but it's something not right in there.

I am attaching a picture of my OLD master cylinder internal parts. 
They are laid out in exactly the order they came out. 
From Left to right (with the parts to the right going in the master cylinder bore first) that is how I assembled my new mc parts.
The Rubber grommet in the middle of the piston is directional (in my mc) and goes over a metal tab.

I hope this gives you some frame of reference and helps at least rule out something.

Charlie

[Powderman]

Rafi,

When I bled my system, after a while all I got was tiny bubbles.
I did get "gushes" out of the bleeder. 
At that point where I got "gushes" from the bleeder screw I closed the bleeder up and continued to get tiny bubbles coming up.
I tied back the brake handle half way put the master cylinder cap on loosely to keep moisture out.

Let it sit over night with handlebar to the left.
The next day I put maybe 1/2 an eye dropper of DOT3 brake fluid in, closed up the cap with diaphragm in place and worked the brake lever.
The brake worked fine.

Questions:
Is the fluid level good now in the master cylinder?
Have you locked up the bleeder screw?
If the bleeder screw is tightened up, have you waited for the final tiny bubbles?
Is your brake working at this point?

Last thing.  Is the caliper piston in correctly?  (Yeah, I know, stupid question, but had to ask)  With the solid face pointing in toward the caliper body.

If the brake works, then it might just be that you are over thinking things.   
PS:  When you said "gushing" how much fluid is gushing??  (It may not really be a lot of fluid.  A typical cut on your head looks like a lot of blood but in reality is probably not enough to fill a thimble). 

Charlie

Actually head cuts will easily fill a thimble, They bleed like hell and look much worse than they actually are. Then of course if you're on blood thinners it's even messier. Been there done that, a number of times in my life.

[NobleHops]

So at this point, I've tried everything except fully tearing down the front brake assembly. I have a vacuum pump which helps quickly get the fluid at the caliper. All banjos are torqued down to 15ft-lbs with new crush washers. Caliper fully rebuilt with new piston and seal after a thorough cleaning. Master cylinder got a brand new rebuild kit, everything installed correctly and was dunked in the same brake fluid being used when assembled. This is what I did:

Filled up the reservoir and used the vacuum pump at the bleeder to get the fluid to come through. Refilled as needed, and let some of the fluid "bleed through" to the pump's collector to ensure no air gaps were left.

Handlebars turned left so the MC is at the highest point, saw only very few bubbles come out on pumping the lever.

Started bleeding per instructions in manual and seeing a very strange thing happen. When I did this two years ago when I first bought the bike, when I pump, I saw the fluid squirt through with good pressure out one of the two small holes in the reservoir. When I do it now, the fluid is staying stationary doing nothing except for the occasional micro bubble popping up. However at the bleeder screw, I see fluid gushing with each pump, but the level in the reservoir stays the same almost stationary.

When I use the vacuum pump at the bleeder, the level goes down

WHAT THE FCUK IS HAPPENING?

I think you have a big fat slug of air in or just after the MC. Try fanning the brake lever with your fingers several times, and not gently.

Are you CERTAIN the front seal is installed correctly on the MC plunger?

Failing this, try strapping the lever halfway back to the bar overnight, see if air will rise out, assuming that's it.

[chewbacca5000]

I did this a week ago and it went pretty good.  All I do is fill the caliper with brake fluid and install the piston.  The piston forces the air out of the lines.  A few quick bleeds and I am good to go.

Much easier than trying to suck the air down through the bleeder.  Air bubbles want to rise.  Don't believe me to to a fish aquarium the baubles rise.

[edwardmorris]

@Charlie, thanks for the pic, I have that exact same one in my photo journal too I did have doubts about my MC, so I took it apart, cleaned it and put it back together with a new kit this morning, everything inside is correctly installed.

@Nils, yes, all rubber pieces are correct, and you may be right about that massive air gap thing.

Because it finally moved the pad All it took, was my wife to come out there and help. Did the bleed process as described in the manual and the lever is finally starting to stiffen up. A lot of air came out, but still not seeing enough of that back-pressure when the lever is pulled in suddenly with force. Left it turned at the high point with the lever tied back for now as bleeding wasn't doing much but wasting fluid. Will let it sit for a couple hours and try again, hopefully what remains of the air will be out by then.

Thank you all for chiming in, I appreciate the help!

[oldhatt45]

Rafi,

Good that you have some progress!!!!!!

Would suggest leaving it tied back over night.
Now that you have some back pressure, I would hope that you got the "Big" bubble out and now only have to let the tiny ones make their way out the mc return hole.  Maybe give the front brake switch housing a few little taps, just to see if any more bubbles come out.

Glad for you, that progress was made!!!!!
And happy that my picture confirmed your assembly was correct!!!!!

Charlie

[my79650]

Just a suggestion, but the few times i have done brake bleeding i did 1st the bleed screw, add fluid, make pressure then banjo bolt, worked my way up to the m/c cover and bled at every bolt, cover, etc. Not saying it works on every go or for every setup but this process worked for me. Good luck.

[NobleHops]

Rafi, how did it do overnight with the lever strapped?

[edwardmorris]

Fully taut! First time ever on this bike that thing isn't the least bit spongy! I have a feeling this one is going to be a moody one and will only be nice to my wife

[oldhatt45]

Glad to hear the brake is working!
I guess She wants everything Just So.  Just like a proper Lady. 

Charlie

[NobleHops]

Fully taut! First time ever on this bike that thing isn't the least bit spongy! I have a feeling this one is going to be a moody one and will only be nice to my wife

Strapping the lever is the magic bullet. Incredible difference overnight every time I've done that too. Glad you're set!

Go ride!