Losing front brake lever over time?

[bruce_launceston]

Some time after restoring my LS 12 months ago I found that the front brake lever was nearly coming in to the handlebar. About 2 months ago I bled the brakes and it came good.
I now see that it is coming close to the bar again. There doesn't seem to be any leaks from the calipers and the fluid level in the m/c is still good.
Could the fluid be leaking past the rubbers in the m/c and going back to the reservoir?
Anyone else had a similar problem and know the cause and/or remedy?

The master cylinder has been rebuild twice but was never badly corroded, only lots of crap to clean out, the Brembo calipers have never been dissassembled in 90,000km. I swapped the bleed screws over with the brake hoses when I fitted new S.S. brake hoses (to better clear the front guard) last year but can't see that that would have any adverse effect.

Cheers Bruce

[nhmaf]

It may be time to pull those calipers and check conditions of the pads, and also examine the caliper pistons to see if they are fully retracting, or may be 'caught up' on a ridge of "gunk" or rust ?

IF no fluid is indeed leaking out, then there are (2) other possibilities that come to mind:
1.  The system's volume is increased (either with stuck/extended piston) and/or from pad wear - but you would see at least some drop in m/c fluid level, I think.... but a little goes a long way in that regard...
2.   Somehow air is leaking into the system - this would most likely be coming from the m/c reservoir cap, but could theoretically be coming in almost anywhere - I would first suspect the top cap if this were the case because a leak there would not show any fluid coming out.

I'd try bleeding the brakes and topping off the fluid, after unbolting the calipers - or at least taking off the covers - to check the pad thickness and piston condition.

[bruce_launceston]

Thanks for the quick response.
The pads were new 12 months ago as I also fitted new brake disks.

Bleeding the brakes 2 months ago did seem to fix the problem, at the time I assumed that there must have been some air in the system and that it had gradually moved to where it caused a problem.
I originally had a god-awful time bleeding the brakes after fitting new hoses. I had to buy a vacuum bleeder which worked fine.
I will bleed them again and try to see if there is much air in the hoses.

If the cap is leaking how would this cause a problem, there is always air in the top of the reservoir anyway?

I agree that it is probably time to overhaul the calipers, maybe the air is getting sucked in from the calipers, could this happen?

Thanks again.
Bruce

[nhmaf]

I suppose that if there were some particles or something  that sort of created a "check valve" type of  operation, in which fluid pumped down toward the piston didn't fully return, that could result in a soft, squishy brake lever, with no outward sign of fluid leaking but I'd also expect to feel dragging brakes along the way.

Does the lever come back fully to the front position after you've let loose of it?   I've had levers that stayed "floppy" and didn't come back forward due to blockage/restriction of the TINY return fluid hole in the m/c - it doesn't take much for this to get blocked up enough to cause problems - it must be cleared out carefully with a tiny strand of copper wire so as not to gouge or enlarge the hole in the soft alloy.

Yes, there is air at the top of the reservoir in the master cylinder, but it isn't supposed to be fully open to the atmosphere to let in more air when the fluid level in the m/c drop from when you are applying the brakes.  It isn't a very likely scenario to occur, but I think it could happen that under heavy braking it might be possible for it to suck in air this way.    It isn't easy bleeding the air out of the brakes with the rubber bellows and top cap firmly secured in place, correct ?  At least I can't.   this is probably a really long shot on the list of possibilities.

Maybe you might be getting leakage back 'round the seal on the piston but not past the , but anytime I've observed leaking piston seals, there was also fluid coming out the end where the lever pushes in on it.  MAybe your new bleed screws are letting air in if they aren't fully seated ?   Again, I'd expect to at least see/hear some little spit of fluid round them if this were the case, but a gas (Air) can get through places that are still rather water/liquid tight.

[bruce_launceston]

I will bleed the brakes tonight after checking the fluid return hole.
The brakes don't drag and the lever returns normally. Definately feels like air in the lines. I will have another good look for any signs of a leak.
I am not keen on stripping the calipers just now as I am using the bike daily and I will have to wait for parts and after 90,000km and 24 years it's not worth not replacing the rubbers when I pull the calipers apart.
From memory the kits are not cheap!

Cheers Bruce

[drewboid]

From an engineering viewpoint:
If the volume of fluid is not dropping (you are not loosing fluid) but the lever is moving more then the fluid must be going somewhere in the system. A few ideas:
1. There is a leak around the seal on hte piston in the MC and it is allowing the fluid to pass back under pressure. - if you keep pressure on the brake lever does it slowly move further toward the grip?
2. The new hoses are getting soft and expanding under pressure - increasing the volume of the system and requiring the MC to put out more fluid for the same pressure. - did you install SS hoses?
3. for some reason the caliper pistons are moving further away from the disk requiring more fluid to move them back into braking position. - is your disk warped?

That's all I can think of at the moment - good luck in your search for a solution.

[bruce_launceston]

Hi Drew
I have had a closer look, at the moment the lever comes in to within 1/8" of the bar. If I hold firm pressure on it it doesn't seem to pull in any closer.
The new hoses are braided S.S., the brakes dont have a spongy feel, the lever feels firm but just comes back too close to the handlebar.
I have twin semi floating disks, on close inspection one of them is not 100% flat, if I spin the wheel it sqeaks past the brake pad on every rotation.
The disks are bolted to the original disk holders which I did paint before assembly, maybe I need to dissasseble the rotors from the disk carriers and get them skimmed to check that they are dead flat and reassemble them without any paint on the mating surfaces.

I am still leaning toward blocked fluid return ports in the mastercylinder so I will also try and clear them.

[drewboid]

Bruce,
I don't see how a blocked return port could cause your problem but I may be a little dense.
Can you confirm that the MC is 15mm and not the 12mm for a single disk?
Other than what I have said before I don't have any answer. Perhaps it is a warped disk - be aware there is not a lot of extra to grind away.

[Mike V]

Bruce,
I wanted to chime in here and hopefully not complicate things. First of all, I experienced the same problem you describe. During my restoration I replaced the brake lines to steel braided, completely rebuilt the dual ATE twin piston calipers and replaced the original Master cylinder (16mm) to a new 15mm master cylinder. The original was badly pitted and basically irreparable. I experienced great difficulty in getting the entire system bled and went through nearly 2 pints of fluid before getting all the air out of the system. I bled the system by hand with the aid of speed-bleeders and turned the bars to the left to get the master cylinder at the highest point possible and let set overnight and even slightly (& carefully) tapped both calipers with a rubber mallet. Eventually I was able to get a good solid handle and force all the air out of the system.
I think it would be helpful if you could report some details for a better diagnosis; look at the bottom of your master cylinder - there should be a stamping with the piston size for the cylinder. Next, it would be helpful to know the diameter and number of pistons in your caliper. If you can determine this look up your brake ratio on the chart from the attached link. Depending on your brake ratio you may find some interesting information based on your application. Based on the chart a higher ratio will result in a softer lever and a lower ratio will result in a firmer or wooden feel. I changed my master cylinder piston size from a 16mm (22.56:1) to a 15mm (25.67:1) to create a better feel. I think the chart and contained information is very useful and I refer to it often. Vintage Brake is a very valuable resource and Mike Morse will always answer the phone when you call.  But, for your issue I would first tackle the simple issues...if you report no leaks I would look to air in your system or the master cylinder rebuild piston cup and components. Then...brake ratio. Good Luck!
http://www.vintagebrake.com/