Brake binding

When I bought my '75 MK III a month or so ago the front brake was essentially non-existent, so I did the madass resleeve upgrade to the master cylinder, and sent the rotor off for grinding and drilling. When I reassembled it all I thought for a while it wasn't going to work, because I wasn't having any luck getting any pressure at the brake lever. But I kept at it and finally got it bled, and it felt pretty good. I also installed new Ferodo Platinum pads, and when I had the caliper off it appeared that the pistons in the caliper were new.

But now it seems that I keep having less and less free play at the brake lever, and at the end of my ride today I had very little play in the brake lever, and it seems that the brake is beginning to bind without even applying the lever. Any ideas what the cause of that might be? Maybe an issue with the caliper? Or could it be possible that I overfilled the reservoir?

One other issue that I wonder about is that when I was bleeding the system, I had to do it by pumping the piston by hand with the brake lever off, because with the lever in place the piston seemed to not be able to extend far enough to get any fluid movement in the reservoir. In other words, when I pumped it by hand I would see the little jet of fluid in the reservoir, but with the lever on the internal piston spring appeared unable to extend the piston far enough to make that happen. I thought that was odd, but forgot about when I finally got the brake working well. Now I'm wondering if the brake lever isn't allowing the brake pressure to release fully, and it is incrementally not allowing the brake pads to retract when the lever is released?

oediehl said:

One other issue that I wonder about is that when I was bleeding the system, I had to do it by pumping the piston by hand with the brake lever off, because with the lever in place the piston seemed to not be able to extend far enough to get any fluid movement in the reservoir. In other words, when I pumped it by hand I would see the little jet of fluid in the reservoir, but with the lever on the internal piston spring appeared unable to extend the piston far enough to make that happen. Now I'm wondering if the brake lever isn't allowing the brake pressure to release fully, and it is incrementally not allowing the brake pads to retract when the lever is released?

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That could be the cause of the problem as the master cylinder piston must retract far enough after each brake application for the piston seal (B) to uncover the vent port (C) otherwise pressure can become trapped in the system.

have you fitted any of the shims (which fit over the end of the mastercylinder piston) were supplied with the resleeve kit?

I'm not using any of the piston shims.

I just went back to the garage, a few hours later, and the play in the brake lever has increased just a bit, and it now feels OK (if still immediate). I jacked the front end up, and the wheel will now spin freely (though it takes just a slight touch on the brake lever to activate the brakes). Per L.A.B.s diagram I suspect, given my difficult experience in trying to bleed the brake, that the port labeled C in his diagram is not being cleared by the piston when the brake lever is installed (I was only able to bleed the brake by manually thumb-pumping the piston, with the brake lever off).

FWIW, I've also noticed that, even though I cranked the lid on the master cylinder down as tight as I could without using a lever tool, that it is slightly seeping fluid as I ride.

Today, why would the brake pressure have been released, just enough, after a couple of hours? Perhaps the dissipation of heat, or more likely the imperfect sealing of the caliper pistons, allowed the pressure to ebb and the brake pads to back off slightly? I'm not at all sure.

What I'm thinking is that perhaps I should remove the brake lever and take it to my grinder. The brake lever lobe that presses in the master cylinder piston is perhaps just a bit too long, and so if I were to grind that down a bit, when the brake lever was released the piston could extend further, allowing the C port to be exposed, allowing the release of pressure.

How's that sound?

one of the reasons the shims are supplied is because of the different brake levers available and being fitted, often thet are not needed.
Removing a little from the foot of the lever cant hurt, you can always shim it back up. I'd try that.

I really think you should look elsewise before grinding on your lever. Remember you mentioned the difficulty in bleeding the system in the first place because the lever did not seem to bare on the piston enough to activate the piston. If you go grinding it down, that problem will be worse. I'd look closely at the resleeved master, especially the tiny return drilling in the res. Also, how old is the rubber master cylinder hose. Old ones can suffer from internal delamination of the lining causing in some cases exactly what you are experiencing. Like little check valves that don't allow pressure to relieve well after brake application.

A few things:

1) your brake cap is leaking because you overtightened it. The rubber gasket has a lip that is compressed by the cap against the edge of the reservior. If you tighten the cap too much it pinches the lip of the gasket and pushes it to the inside of the reservoir and allows brake fluid to leak out around the cap.... Ask me how many times I fekked up my original roadster tank's paint with brake fluid dripped onto it before I realised why my master cylinder leaked...

2) As you said, your pressure did eventually back off, so I suspect that the plunger is probably right at the edge of the "C" hole when it rebounds which allows the fluid pressure to slowly unload over time... The problem might be the travel of the lever is restricting the piston's rebound to reach the "C" hole. You can test this by removing the lever from the bracket and actuate the plunger with a drift to engage the plunger to pressurize the caliper then release the drift from the plunger and see if the brake pressure releases quickly like it's suposed to. IF it does, you could deduce that the lever is limiting the return of the plunger, and grind down the heel of the lever to allow the plunger to rebound further. (if grinding the lever fixes the brake, I'd carefully test the brake with a few starts and stops in a safe place, before I went out riding in traffic)

Firstly, I think you should listen to what Madass is saying.
Basically to test if the relief hole in the reservoir is working correctly, you need the watch the fluid in thee reservoir when you pull in the brake lever. You should see evidence of the movement of the fluid in the reservoir which tells you seal is behind the hole. If not, you need to make adjustments to enable that relief hole to be uncovered. The diagram picture does explain it, but if you don't know what to look for it is hard to see in that diagram.
My own set up requires the fitting of shims to get the correct clearance. In my own case I had to remove metal to get the correct clearance but i did it in a way that alowed the lever to move away, which does allow for that tiny bit of extra travel as to make up for the lost travel created by the smaller piston.

Secondly. Listen to Madass.


Dereck

If you are getting enough pressure in the MC to force fluid out the cover seal I would think the relief hole is exposed for too long before the line pressure is going up. I would try one of Don's shims before grinding the lever. If the problem gets worst then you are going the wrong direction. I'm probably using the wrong terms to describe the parts are but I hope your get what I'm saying.

I also recall another member having a similar problem and it turned out being the wrong lever installed on the bike, but if it worked before the change it should be the right part.


Pete

oediehl said:

Today, why would the brake pressure have been released, just enough, after a couple of hours? Perhaps the dissipation of heat.....

Click to expand...

Partly.

oediehl said:

......or more likely the imperfect sealing of the caliper pistons, allowed the pressure to ebb and the brake pads to back off slightly? I'm not at all sure.

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More likely any residual pressure will slowly get past the master cylinder cup seal rather than the caliper seals.

An easy way to check if/when it happens again is to rapidly open and close the caliper bleed nipple (preferably with a rag over it) as that will release the pressure. The brake should then return to normal operation (until the next time!).

oediehl said:

FWIW, I've also noticed that, even though I cranked the lid on the master cylinder down as tight as I could without using a lever tool, that it is slightly seeping fluid as I ride.

Click to expand...

As mentioned by o0norton0o, cranking down the cap as hard as you can may be causing the leakage.

Did you 'fold' the diaphragm? If the diaphragm and rim of the reservoir are completely 'dry' of brake fluid it can help to stop the rim of the diaphragm from being squeezed out of position when the cap is tightened (moderately).
There should also be a 'paper' washer (60-4328) between the (Mk3 plastic) reservoir cap and the diaphragm.

Biscuit said:

I really think you should look elsewise before grinding on your lever. Remember you mentioned the difficulty in bleeding the system in the first place because the lever did not seem to bare on the piston enough to activate the piston.

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That doesn't seem to be what oediehl means.

He said the piston wasn't extending, as in 'not returning' sufficiently with the lever in place, as he said; "but with the lever on the internal piston spring appeared unable to extend the piston far enough to make that happen"

L.A.B. said:

Biscuit said:

I really think you should look elsewise before grinding on your lever. Remember you mentioned the difficulty in bleeding the system in the first place because the lever did not seem to bare on the piston enough to activate the piston.

Click to expand...

That doesn't seem to be what oediehl means.

He said the piston wasn't extending, as in 'not returning' sufficiently with the lever in place, as he said; "but with the lever on the internal piston spring appeared unable to extend the piston far enough to make that happen"

Click to expand...

Yeah, I see what you're saying. It's kind of confusing as written. you're probably correct.

I have had problems like that with sleeve jobs and that's why I am reluctant to go there again.

Thanks for all the feedback. I will definitely assess the cap on my master cylinder for being 'too tight'. With regard to the binding brake I need to do a bit more experimentation both with the lever off and on. But what I suspect now is that the problem is that the piston is not able to release far enough to clear the vent hole when the brake lever is released. As I mentioned previously, my efforts to bleed the new system only seemed effective when I had the brake lever removed from its mount, and I could then let the piston extend completely after each inward press with my sore thumb. I suspect it's something of a cumulative pressure buildup after repeated application of the brakes. I'll report back after I've (hopefully) resolved the problem.

with the brake in the off position, you have fluid trapped between the calliper and the master cylinder piston. the piston is not being allowed to move back far enough to allow for a. fresh fluid into the system and b. expansion of the fluid back into the reservoir. So as the fluid expands in the closed system the brake becomes tight, (less play in the lever) until it seizes on. It's all a case of where the piston in the master cylinder is when at rest. It's too far forward into the cylinder at the moment.

If the new master cylinder sleeve is not fully home in the original casting, the port in the sleeve may not be uncovered by the piston. Re-fit the sleeve. that might explain the difficulty in the initial bleeding process.

Be wary of modifying the heel on the lever - look how it operates across the piston end. Also if you have the wavy washer that sits behind cup seal, fit it, as it's designed to collapse the seal when the lever is released, though not all piston and seal arrangements have one.

The part of the lever that pushes directly on the piston is carefully shaped to roll across, and I wd never grind or file it.
All the stock levers I've seen have a small circular pad near the pivot bolt hole, and that's where I put a file to extend the lever's operating range, if necessary.

Well, finally got around to assessing and working on the issue. With the cap off the reservoir, pumping the brake lever resulted in no discernible movement in the pool of brake fluid. But when I took the lever off and pumped the piston by hand, a jet of fluid would spout from the reservoir. So, I took the lever and clamped it in my vise, and with a fairly fine-toothed cross-hatch file began filing down the cam that presses against the piston. I would repeatedly bring the lever back and remount it, watching for movement in the fluid as I pumped. No change for a while, but finally I saw faint stirring in the fluid. So I put everything back together and took a lengthy test ride.

The lever went from zero play to quite a bit of play, with the brakes not being activated until the lever is almost completely squeezed in. But I do get a fair braking force (perhaps the best I can hope for with this dated brake design?), and the lever does not quite close completely to the handgrip. I suppose I should try some more bleeding, to see if that might be part of the problem (if there is a problem). I may also experiment with inserting the thinnest of madass's shims over the piston, to see if that reduces the travel without going back to the prior problem of gradually building up pressure until the brake start dragging.

I'm still having no luck with the leak around the master cylinder cap, and I tried seating it lightly, to a little more, and then screwing it down hard. But as was pointed out previously, there should also be a paper gasket in the cap, and on mine there is not, and so hopefully when I procure or manufacture one of those, my leaking cap issue will go away. Curious that it was not leaking before I took it apart, though.

Thanks for all the input!

to help with bleeding, tie the lever back to the bars and let sit over night, you would be surprised how many times this has fixed bleeding issues.

Ensure that the lever pivot nut and bolt are only lightly nipped up. If anywhere near tight they will nip lever and prevent it returning fully. Then the piston will not return enough to release pressure or breathe in from the reservoir. Ask me how I know....!