Bleeding ** brakes

[Rob]

OMG Charlie, you'll have me throwing it over my shoulder next. I will give it a whirl.

Thanks

Well, that IS the best place for it - in favor for a handlebar mounted one. ($)

[George Ryals]

Could someone please explain how tying back the lever overnight makes air in the system move differently than when the lever is not tied back.

[jagarra]

I believe the theory is that the constant pressure forces any residual air out of the system. You may have noticed if you look into the reservoir and apply the brake, sometimes a small air bubble rises.
Since I heard of this process it has worked well on my motorcycles and autos as the final step in bleeding a system.

[barryh]

Without question one thing that happens when you tie back the lever is the air bubbles get compressed and are much smaller. Does that make them rise up to the reservoir easier ? I'm not so sure, the relative difference in density between the bubble and the fluid just got less so the bubble is now less buoyant. Brake fluid believe it or not can absorb a small amount of air so when you tie back the lever any air bubbles could be compressed and dissolved into the brake fluid. What happens when the pressure is released and the air comes out of solution is less easy to explain. The dissolved air would presumably be distributed throughout the system so some of it at least would come out of solution in a way that allowed the air to rise into the master reservoir. So tying back the lever effectively moves the bubble from a place where it can't rise to a place where it can.

Just as well a brake system can't get the 'bends'

[tsa]

+1 on Airbears advice to loosen the under tank MC fixing clamp and raise the rear of the MC.

I tend to favour reverse bleeding, by using a 50-60 ml syringe connected to the bleed screws to allow the brake fluid to flow in the same direction as the bubbles want to go, not in the opposite direction.

[jagarra]

Right now the MC is raised in the back with a wedge holding it up, lever tied back under pressure. Fingers crossed.

I have used that reverse method before, suspending cans with fluid in them and a hose from each bleeder on all 4 wheels of a car. Worked pretty good, allowed air to bubble out when pedal depressed and when the pedal released sucked fluid back into the line.

[jagarra]

After 24 hours with the M/C rear end up and tied back, the brakes are no better than they were before. Bit the bullet and pulled the M/C apart, seals were installed correctly as I thought they would be. So that leaves the only issue I see when I look down the cylinder, the pitting , which I thought would miss the seal , is not. Thought about spacing the seal piston out, (putting a small spacer between it and the spring) but that may affect the lever operation.
Ideally sleeving the bore to 15mm would give me an optimum ratio to the 40 mm calipers, but the 16 is within the ratio specs and the smallest listed for the 90S.

[Chuey]

On the under tank master cylinder, I was told a long time ago that each part of the system needed to be filled before you could bleed them.

I have used the method tsa mentions and got my best results from that. That is, to use a syringe to push fluid up through and into the master cylinder. Also, I have used the bungee cord around the lever overnight trick and found it to work very well.

I am rebuilding the left caliper on my RS and will be in "bleed land" in a couple days. We'll soon see if I should be trying to answer questions or will need to be asking them.

Chuey

[grant81rs]

Gidday,

Just this Sunday morning I have been bleeding the brakes up on the K1100.

I ordered and installed a new front MC piston after the old one dropped a few very small dots of brake fluid on the inside fairing while in transit. I came home pulled it to bits, cleaned and inspected everything and a little corrosion had started to creep about the housing through lack of maintenance. I had flushed the brakes and ABS a couple times since owning the bike but never pulled the front brake housing completely apart..

Anyways I ordered a new one a couple of weeks back, I gave them the VIN and model number etc, and this morning the brake would not bleed up probably. I tired the reverse bleed etc, pulled the piston out a few times for a closer look and nothing would work.

So I put the old brass piston back in and the they bled up straight away.

On closer inspection the new piston is slightly smaller by a hair width, so a phone call will be made Monday morning to sort it out for a replacement or refund..

[jagarra]

Grant, that can be really frustrating to have new parts that are out of specifications. One would think a couple thousandths smaller would be handled by the seal. Did they also make that a bit thinner too?

My brakes are waiting for the M/C to come back from the re-sleeving folks, figure it won't be back for another 10 days at least. Snowed last night, my bike isn't going anywhere except staying in the garage for awhile.