Sticking Brakes

[jimmyg]

A little more info.

I do need to get the hand lever cable adjuster as mentioned previously. But another observation when working on it again over the weekend, is that the right side caliper was moving as I applied the brakes. I deduced that the inside pad wasn't quite perfect and therefore, when pressure was applied, the caliper was shifting to attempt to align the inner pad, as the outside pad was forcing it to do so.

So I spent another 1/2 hour or so aligning the inner pad and at least the caliper isn't shifting around anymore when applying pressure.

Another thing I tried was to try and pull the bubbles out from the M/C. To do this, I went to the local hardware and got a 1 1/2" rubber stopper that would fit inside the M/C opening. Then I drilled a small hole in the center and with my Mity-Vac hardware kit, inserted one of the long tapered fittings. This fitting would then hook up to the Mity-Vac pump where I could create vacuum at the M/C, in hopes of pulling the air from the caliper all the way up to the reservoir.

I'll see how this works tonight as I created about 15" of vacuum and will let it sit over-nite.

thanks everyone. The saga continues............................

jimmyg

[George Ryals]

Putting a vacuum on the m/c will not pull air out of the system. Take a minute and look over the hydraulic part of the system. Is there any place where a bubble could be that it would have to go down to get to the m/c? With the R90S under tank m/c, air accumulates in the front end of the casting where the two brake lines connect.
Unhook the m/c from the frame backbone and lift the back of the m/c so any air trapped in the front part of the m/c will float uphill into the m/c.

[barryh]

I'll wait with interest to hear what the outcome of the vacuum experiment is.

[jagarra]

Having tried all sorts of experiments when i was bleeding my system, I found the best final step was to pump up the lever till I had a really strong brake and then strap the lever to the handlebar and leave it overnight. Last time I did that I left it for 4 days as i didn't have a chance to get back to the garage, it ended up with a really solid feel.

[George Ryals]

Allowing time for the air to be absorbed into the brake fluid works to a certain extent as long as there is only a tiny amount of air in the system. However, this also lowers the boiling point of the brake fluid.

[Ken in Oklahoma]

A little more info.

I do need to get the hand lever cable adjuster as mentioned previously. But another observation when working on it again over the weekend, is that the right side caliper was moving as I applied the brakes. I deduced that the inside pad wasn't quite perfect and therefore, when pressure was applied, the caliper was shifting to attempt to align the inner pad, as the outside pad was forcing it to do so.

So I spent another 1/2 hour or so aligning the inner pad and at least the caliper isn't shifting around anymore when applying pressure. . . .

You have a R90S which means you have the so-called swinging ATE calipers. I developed a technique for fast and faultless alignment. As you know the alignment of the brake pads to the disk is done with an eccentric bolt at the bottom of the sliders. The manual tells you to align the fixed brake pad to the disk by marking lines on the disk and then seeing which lines get wiped off. You then make adjustments, turning that eccentric pivot bolt, until the lines are uniformly wiped. It is a repetitive process which theoretically tells you when the brake caliper is squarely aligned with the disk.

Well, I never could master the technique. In an eureka moment I figured that squeezing the brake lever would automatically do the alignment. Before tightening things down I put some brake line pressure on the caliper by squeezing the lever and using an inner tube rubber band to maintain the pressure. I then turned the eccentric bolt until I felt where the eccentric 'wanted to be'. And that's all it took to do the best alignment. With the rubber band maintaining pressure on the brake lever I would then tighten everything down that needs tightening.

I never had to repeat the process because I hadn't done well enough the first time. (Incidentally I don't claim to be the first or the best person to implement this procedure.)

Ken

[barryh]

Allowing time for the air to be absorbed into the brake fluid works to a certain extent as long as there is only a tiny amount of air in the system. However, this also lowers the boiling point of the brake fluid.

I have become totally convinced that this is the mechanism that is happening when we tie back the lever. It is even listed in the SAE standards under the heading, "Air Solubility." with a typical level of 5% ±2% by volume of dissolved air for glycol ether based type fluids. Although it isn't explicitly stated I believe the 5% figure is when the fluid is not under pressure so with the lever tied back it will be higher. All of this suggests that brake fluid has enough capacity to absorb the small amount of trapped air that produces a soft brake lever. By definition the volume of air that produces a soft lever must be less than the volume of the master cylinder piston stroke otherwise the lever wouldn't be soft, it would be non existent !

So not a lot of air then to absorb, perhaps in the order of 1ml.

The physics of gas dissolution in a liquid state that the volume of dissolved gas must be proportional to pressure. So the air dissolved during tie back can't remain in the fluid when the pressure is released. The only mystery left is how quickly the air out gasses again. I suggest it's not near instant like a bottle of soda but slow like a diver experiencing the bends typically an hour or more after surfacing. Because the dissolved air is distributed throughout the system when it does eventually out gas the majority of it is able to rise up to the master cylinder. A little may out gas in a location where it can't rise which might explain why repeated applications of the tie back over a period of days may sometimes improve the lever feel.

[Ken in Oklahoma]

Your explanation makes a lot of sense to me. I like it.

Ken

[jimmyg]

A little more info.

I do need to get the hand lever cable adjuster as mentioned previously. But another observation when working on it again over the weekend, is that the right side caliper was moving as I applied the brakes. I deduced that the inside pad wasn't quite perfect and therefore, when pressure was applied, the caliper was shifting to attempt to align the inner pad, as the outside pad was forcing it to do so.

So I spent another 1/2 hour or so aligning the inner pad and at least the caliper isn't shifting around anymore when applying pressure. . . .

You have a R90S which means you have the so-called swinging ATE calipers. I developed a technique for fast and faultless alignment. As you know the alignment of the brake pads to the disk is done with an eccentric bolt at the bottom of the sliders. The manual tells you to align the fixed brake pad to the disk by marking lines on the disk and then seeing which lines get wiped off. You then make adjustments, turning that eccentric pivot bolt, until the lines are uniformly wiped. It is a repetitive process which theoretically tells you when the brake caliper is squarely aligned with the disk.

Well, I never could master the technique. In an eureka moment I figured that squeezing the brake lever would automatically do the alignment. Before tightening things down I put some brake line pressure on the caliper by squeezing the lever and using an inner tube rubber band to maintain the pressure. I then turned the eccentric bolt until I felt where the eccentric 'wanted to be'. And that's all it took to do the best alignment. With the rubber band maintaining pressure on the brake lever I would then tighten everything down that needs tightening.

I never had to repeat the process because I hadn't done well enough the first time. (Incidentally I don't claim to be the first or the best person to implement this procedure.)

Ken

Good explanation Ken. I like that approach.

jimmyg