Master-cylinder 750 SECA

Its the old problem XJ750 SECA Master Cylinder headache. You know, the one buried behind the headlight and a mess of wires with the hidden reservoir.?

Here is the background: rebuilt MC a few months ago with parts from XJ4ever. Reinstalled with new stainless braided lines and bled fine. Good pressure, worked fine; except my calipers were not releasing, so I decided to pull them off too and rebuild.

Now after reinstalling the calipers, I can't get the system bled! I've tried all the recommended tactics; vibratory sander, letting it sit for a day with the lever tied close and a bleed screw open. Taken apart everything at the junction up to MC, tried vacuum bleeding but couldn't pull fluid through. The MC will create pressure, but doesn't seem to pull any fluid from the reservoir. Pulled MC off again to inspect. Disassembled MC; everything looks fine, All passages are clear, inside bore is clean and smooth, rubber gaskets appear fine. it should be good, right? Only been 2-1/2 months since I rebuilt and hasn't been used. The piston pushes in, but seems to take forever to spring back. What is going on? It's like one of the rubber parts are too tight?

 

Ok, I got the master cylinder piston to return now. It springs right back. I still cannot the MC to take fluid from the reservoir. ??

Tried the vacuum again, and it just collapses the plastic tubing, even after the bleeder is opened; nothing comes through, not even air except what leaks around the bleeder and the attachment.

 

Is it possible that internal spring is in backwards or the lip on the piston has curled back when installed?
Did you hone the mc bore before installing new parts?
Is cable too tight or stiff not letting piston return all the way?
Or the little vent hole in mc blocked?
Bleeder screw has crap in it ( drill bit by hand is best)
You have probably checked all of the above and it's making you NUTS!

 

Pull the bleeder fitting off of the caliper. Clean the gunk out of it, or replace it with a new one.

Brake fluid won't flow (much) if the lines are full of air, and you can't get the air out if the bleeder is clogged.

 

The thing is, it worked prior and I didn't touch it since until the brake calipers rebuild. It does return now.
I didn't hone the bore, but did polish it with some Emory cloth and it was clean and smooth, almost shiny, no pitting. Checked spring orientation.
I can feel pressure on the outlet port when the piston compresses. Confirmed vent hole is clear; saw brake cleaner spray through it (yes, the teeny tiny one too).
The bleeder valve is same result on all four bleeder points (both sides, calipers and anti dive.) So I think it unlikely that all four bleeder valves are full of gunk, besides, I cleaned those too when I had the calipers apart.

YES, it is driving me absolutely NUTS, since I know it was working before I disassembled the calipers! The only thing I did to the MC is the lines emptied as they hung there unattached to anything.
I even disconnected the brake line on one side of the junction; no fluid, but I can feel air pressure when I hold my finger over it and squeeze the lever, yet the reservoir remains full.

@k-moe , respectfully, I followed your procedure(s) exactly and many times over with slight variations, even the sander. Left bleeders open with lever tied for 2 consecutive nights and also during day while at work twice. Been fighting with this for the last 4 evenings now. At one point, I did have fluid going through the system, but could never get firm pressure to squeeze the calipers. I have now taken the MC off and reinstalled at least 4 times this evening.

 

Some questions that might help me clarify my thinking:

Did you tear down, thoroughly clean, and rebuild the calipers?

Did you thoroughly flush the distribution block that is on the forks?

I will tell you that if the bleeder wasn't clogged the vacuum line to your vacuum bleeder pump would not collapse. There are a few nooks and crannies in the claiper that are difficult to get spotless. Perhaps a bit of crud loosened up and blocked the bleeder.


I recommend walking away from the problem for a day or two. Fresh eyes, and a fresh brain, often exposes solutions that were hidden before.

 

Yes, with new seals too.
At this point, the entire brake system has been gone through. There isn't a trace of old fluid or gunk left.

If you mean flush it with brake cleaner at the time of new line install, yes. I can't even get fluid to flow through the bent metal tube between the junction and the MC; i disconnect the tube prior to the distibution block and nothing is there. It is like the MC isnt taking any fluid in.

Probably a good idea. This is the last thing I needed to do before taking it out on the street, my first ride, so I am a little anxious to get it done. Also taking advantage of time while my wife is away out of town.

I really appreciate the help, at this point, I am just stumped.

 

I'd suggest when you're ready to come back, take a look at the system in sequence. Bench test the MC. If that works, connect the next component and test, and so on. This should narrow down where the problem is.

 

Dont know if this is helpful or not, but I think it confirms the bleeder passages are ok.
I connected a tubing sourced to brake fluid into the anti-dive bleeder then attached a vacuum to the caliper on same side and opened both bleeders. I was able to draw fluid between the two and vice-versa.
Then I connected the vacuum to the left anti-dive and was able to draw fluid all the way through; that is right anti-dive, through right caliper, up the line to the distributive junction, down the left line, through left caliper, and out left anti-dive. Did the same thing with both calipers as well; so we know fluid is flowing through from the junction on down. Next I will remove the bent metal tube between junction and MC; suspect it is OK, but will confirm. Will also inspect the inlet into the junction. After that is the MC itself, which I have already taken apart and inspected multiple times. Still not able to pull fluid with vacuum from MC to any of the four bleeder screws.

Enough for tonight. Need to sleep on it. Besides, I'm out of brake fluid.

 

Could that metal tube be kinked/collapsed?

 

Nope. Pulled it, blew air through it, sprayed brake cleaner through it. Open passage.

 

Yea out of brake fluid kind of does you in for tonight... NO bad dreams about not being able to stop

 

Allright. At this point I think that your work shows there is a problem with the master cylinder (or you have made the mistake of feeding you mogwai after midnight). . The trouble is that I can't ever say that I've had a pronblem with one after a rebuild, so I'm having to work from the same point that you are; inexperience. While you're on a break from solving this, I'll do some pondering. I wonder if maybe one of the piston seals is not seated correctly, or had a bit of schmoo on it that is now blocking the outlet.

 

OOO OOOO Mr. Kotter!!! OOO...

I know it's a royal PITA to check on this MC, but maybe the problem is a plugged return port. When bleeding do you have the fill cap closed? That combination could prevent fluid from being able to be moved.

 

when your bleeder line collapses, crack open the line going into the wheel cylinder. this will prove the bleeder is open.
i find that without teflon tape the bleeders leak past the threads enough not to let the line collapse.
do these MC's need primed?

 

Got some more brake fluid.
Unattached the steel tube from the MC. Attached a tubing to that submerged into brake fluid.
Used a vacuum at the caliper and anti-dive bleeders and got a steady stream of fluid flow through; no bubbles.
Did the same on the left side, got a steady stream of fluid through, but mixed with steady stream of bubbles. Hmm.?

Could it be I don't have everything sealed up on the left side somewhere? Possibly in the brake cylinder itself? I thought of this before, but didn't see any fluid at any of the connections. Maybe when the MC piston is returning, it is pulling air backwards in through the leak instead of from the reservoir?

@k-moe I tried both with the filler cap in place and off. First off, thinking it would allow air to replace the fluid in the reservoir as it is depleted.

 

Disassembled MC, AGAIN!
Just to verify, this is the orientation and order of the MC parts. Correct?



Close up of the rubber gaskets. Look to be in good shape. They were new when I rebuilt MC about 10 weeks ago, and has been sitting idle since. Orientation of the skirt shaped seal on the plunger is correct?


Interior of MC bore, shining light in through the return port to the reservoir. You can see the pin-point of ligh also, so those passages are clear.


Could it be that the brake cylinders are inserted too far, blocking off proper flow from the brake line? Even the slightest gap should allow fluid through, I mean, look at the bleeder screws, those openings are small.

I also know the passage from my reservoir is clear; it empties out onto my garage floor every time I separate it from the MC to remove it. Gawd, I must have lost at least a quart of that stuff by now!

 

So, now I'm wondering if it might have something to do with the diaphragm in the reservoir and not allowing atmospheric pressure to replace fluid that may be pumped out.?
When I cleaned out the reservoir, the gasket was very stretched out. It has a permament impression of the fluid level float in it from it being pulled? or pushed down for a very long time. Does anyone know what the diaphragm is supposed to look like?

Unfortunately location on this machine is a beast to deal with, it is difficult (if not nearly impossible) to test operation with the reservoir cover off without removing handle bars first, or removing reservoir to take cover off, then install on MC (not easy), then remove again to replace cover and reinstall again again after testing. But then again, leaving the filler cap off while bleeding should serve the same purpose.?

I was about to consider replacing the seals on the MC again, but the $90 for a rebuild kit is too much for a "lets see if this works." approach. I think by this point, I would be able to replace seals with MC in situ. It would be a great mystery if it needed replaced already, but this whole situation is a mystery.

I attached a clear tubing to the end of the metal tube and had a syringe at the end of the tubing held upright. When I activate the MC, a little geyser of fluid fillied up into the syringe barrel. Also disconnected lines from calipers and am able to pump fluid through, so I am fairly confident ?? that the seals are OK.

 

The diaphram on the reservoir helps seals the outside air from getting to the brake fluid. The cover has a vent hole in it (I can't remember where) that allows the diaphram to move as the brake fluid level in the master cylinder drops. If you're not getting fluid to move with the fill cap off, then the diaphram is not the problem since the system is open at that point.

Pull her off of the bike (PITA I know), and see what she does when set up on the bench without the reservoir cover in placce.

 

Just a question when I did brakes on my Seca I bleed the left anti-dive, then left caliper , then go to the right anti-dive , right caliper , I use a one man bleeder. To make this all easier I pull the headlight bucket bolts pull headlight out of bracket , remove MC cover use a long funnel and add brake fluid as required. It took a few times to pull fluid thru , but finally got a firm feel . Is your cable to MC adjusted properly ?