Front Brake bleed issues

[Sit]

Trying to get the front brake to bleed on a 90 K75 hack that has an earles fork front end.  The calipers on on the lower part of the rotor about the 4 o'clock position.  I can't get any pressure to build in the system.  I have rebuilt the master cylinder, I have rebuilt both calipers, installed new brake lines, new reservoir, cap etc. basically everything is new except the body of the calipers and the body of the master.  I have tried to bleed the calipers off the bike so the bleed screw is the highest position, I have turned the handle bards to the left and let sit, I have jacked  the side car up to lean the bike to the left, I have removed master with lines hooked up and rotated it slowly to try to free any air bubbles in it.  When calipers are off the bike, if i work the lever, the pads move and close till they touch. 

I have once or twice gotten pressure to build and hold, but as soon as the bike moves, pressure is gone.  And if it matters, I am running two lines from the master to the calipers.

What am i missing?

[schrocketeer]

How freely do the pistons move when the calipers are not contacting the rotors?
As an experiment, you could put a thin piece of wood between the pistons, squeeze the brake lever, and see how much the pistons compress, with just a little resistance.

[Martin]

You could try vacuum bleeding or pressure bleeding. A mates early Brick still had air even when thoroughly bled. By tying the lever back and turning the bars hard left and on the side stand after leaving it overnight the last of the air came out.
Regards Martin.

[Laitch]

Following up schrocketeer's and Martin's suggestions:

• Empty the master cylinder reservoir and verify that neither of the two ports in the piston is clogged. They can be unclogged gently using fine wire.
• Observe if the piston moves smoothly and fully through its stroke.
• If those conditions have been met, try this.

[Chaos]

+1 on vacuum bleeder, I got a mighty vac and love it.  I had once getting the lever to firm up and it magically became solid after letting the bike sit a while facing uphill.

[Sit]

Thanks all.  I can't lean the bike to the left due to the sidecar, that is why I jacked it up and let it sit that way with the brake tied back, helps a bit, but does not solve problem.  The pistons move freely, both calipers have been cleaned and new seals etc, they move freely.  Master cylinder has been rebuilt (twice) and both holes are clean and clear and piston moves freely.  Vacuum, bleeder, have it, tried it, wasted fluid even faster this way.

[Bon Jon Bovi]

I had a heck of a time getting the front brakes air-free on the new-to-me K75S. I don't know your full setup, but I have OEM Showa forks with Brembo OEM calipers and ABS-WON reinstalled. The master cylinder pumps fluid to the left-side pressure modulator by the rear wheeel, then back to the right front caliper, with a steel crossover line to the left front caliper. Only the left side has a bleed screw, as does the modulator.

All the rubber lines were replaced with Spieglers, and I could not get any pressure to build even using the Mighty Vac. I was getting only air-free fluid at the modulator so I concentrated on the calipers. I even took the calipers off and turned them to different positions, thinking air was trapped in the steel crossover line, but still no pressure.

I noticed using the Mighty Vac that I kept getting air bubbles when I shouldn't so tried these two things: I trimmed the end of the plastic hose to get a tighter fit on the bleed screw, and I removed the screw and put teflon tape on the threads and reinstalled it. That got me to where I was only pulling fluid at the calipers.

I had to go back to the modulator at this point but didn't use the Vac, just the old pump-and-crack open method. And instead of a slow pump I pumped like mad. More air suddenly burped out and the lever started to firm up. I did this till five more pump cycles produced no air bubbles, and now the lever is very firm.

If you don't have ABS then some of this is moot, obv. And in the link Laitch provided is an admonition to slowly pump the brake lever and watch for bubbles coming from the bottom of the master cylinder while tapping here and there to free up any trapped air. I think that may be all-important to getting air farther down the system moving toward the bleeders.

Hope this helps, my curse jar had to be emptied and started over on this one.
 

[Laitch]

Thanks all.  I can't . . .

Have these front calipers on this rig ever worked correctly during extended usage or is this a used rig you recently purchased, haven't run and are in the process of rehabilitating?

[Sit]

This is a hack I am trying to rehabilitate.  When  I first got it brakes worked but pulsed bad.  I put on new rotors and new pad and that is when the brakes went away.  When I put on the new lines, I did pump slowly until no air bubbles, then bled like normal, but no joy.  I have replaced the bleed screws with new BMW and now those have been replaced with Spiegler speed bleeders.  As soon as quarantine lifts, I plan on trying reverse bleed process, but until then, was looking for ideas to try with what I have in garage............

[Laitch]

When  I first got it brakes worked but pulsed bad.  I put on new rotors and new pad and that is when the brakes went away.  When I put on the new lines . . .

All you did was install new pads and rotors without disconnecting any brake line or opening the reservoir cap then after installation of the pads and rotors there was no brake pressure?

If there is a steering stem pipe, did you flush it?

[Sit]

Did none of that and no pipe, two brand new lines from reservoir to calipers.

[Laitch]

. . . two brand new lines from reservoir to calipers.

Where on the line is the junction for the left and right pipes.? Can you post a photo of the setup?

[Sit]

No junction.  The old lines had a manifold about halfway to fender.  Current set up is two lines directly from master. 

[Laitch]

It seems likely that your modification is interfering with brake function. It probably seems unlikely to you, but the coincidence seems compelling to me. I still can't picture it though so a photo would be helpful.

[Sit]

The change to two line was the last major change.  This issue existed prior to the line change or any modification other than new pads and it remained after the calipers and master were rebuilt.  New lines were the last thing to be changed. Not at home currently, both lines from a dual banjo bolt at master. 

[johnny]

greetings...

me and bobby trippe call that a dueling banjo fitting...

j o

[Skunky]

greetings...

me and bobby trippe call that a dueling banjo fitting...

j o

Strange how Something with 4 strings is better played with six fingers. 

[Skunky]

The change to two line was the last major change.  This issue existed prior to the line change or any modification other than new pads and it remained after the calipers and master were rebuilt.  New lines were the last thing to be changed. Not at home currently, both lines from a dual banjo bolt at master.

I presume you now bleed both caliper separately. I don't think that splitting the lines right at the top with a dual banjo should make too much difference to the pressure so I would look at one or two things. First of all make sure that the Master is clean and not full of crap. Look at the holes when you pull the lever in. Even a small amount of sludge can cause all sorts of issues. the other problem you may have is when you are bleeding one caliper you may actually be putting air into the other.

I would first of all complete a re-bleed of both calipers and then on the last pump hold and tie back the brake lever and leave it for 12 -24hrs this should leave the master cylinder chamber open allow any air in the system to come back into the reservoir. 

The reverse bleed has the advantage of filling the calipers first and then pushing the air up rather than down and should cause less bubbles in the system. Having said that I have never felt the need to try it so far.

Good Luck

[volador]

Is it possible you have reached the maximum amount of force and displacement capable of that master cylinder ?

[Skunky]

Is it possible you have reached the maximum amount of force and displacement capable of that master cylinder ?

Whilst the K brake's are not the sharpest in the world the standard Master cylinder should be more than capable of doing the job. I wouldn't have thought that the extra foot of brake pipe would have made that much difference to the displacement. However to be sure he could run a single line to one caliper and then use the second to loop across to the other caliper with a single bleed valve for both then the pressure wont be divided at the top. My LT was done like this with a solid metal brake line run under the front mudguard.

[volador]

Whilst the K brake's are not the sharpest in the world the standard Master cylinder should be more than capable of doing the job. I wouldn't have thought that the extra foot of brake pipe would have made that much difference to the displacement. However to be sure he could run a single line to one caliper and then use the second to loop across to the other caliper with a single bleed valve for both then the pressure wont be divided at the top. My LT was done like this with a solid metal brake line run under the front mudguard.

My thoughts as well. Sounds like the OP has done everything aside from the reverse procedure.
Not knowing the diameter of the new lines its seemed that the OE 13mm piston reached its limit.

[Sit]

AN UPDATE.  Thank you everyone that offered advice.  As stated, everything has been rebuilt or replaced.  I have even replaced the master cylinder body.  I am confident that the system is bled and there is no air in it. 

Now the weird part.  I can mount the left caliper and torque it down and keep pressure at the lever.  I can mount the right caliper, but anything beyond finger tight on the bolts, I lose pressure at the lever, in that is can travel to the grip.  If I leave the bolts loose a bit, then I get a good firm lever, but finger tight, nope.  ???

[Laitch]

AN UPDATE.  Thank you everyone that offered advice.  As stated, everything has been rebuilt or replaced.  I have even replaced the master cylinder body.  I am confident that the system is bled and there is no air in it. 

I can mount the left caliper and torque it down and keep pressure at the lever.  I can mount the right caliper, but anything beyond finger tight on the bolts, I lose pressure at the lever, in that is can travel to the grip.  If I leave the bolts loose a bit, then I get a good firm lever, but finger tight, nope.  ???

Torque the right caliper down. Work the lever a few times. If you don't see fluid leaking then remove the brake pads and look for brake fluid on the backing plates. There might be a hairline crack leading to a fluid passage that opens up when any tension is applied to the bolts.

Did you open these calipers when you rebuilt them?

[Sit]

Thanks, will give that a try.  Yes, both calipers were opened when rebuilt, had the problem caliper open yesterday to make sure both pistons were moving freely, they both were.

[Laitch]

It might be that the surfaces weren't mating up, or were warped, at reassembly. Just disassembling it a reassembling it might cure the problem.