Precis wrote:
Do not use Dot 5 brake fluid without first changing ALL lines, pistons and seals to materials suited to it - AFAIK Dot 5 is silicone-based and will therefore eat up the seals intended for use with mineral oil-based fluids.
Exactly. When I looked into the DOT 5 issue more closely I came to the conclusion that it's just not worth it.
Precis wrote:
Brake fluid is hygroscopic - it readily absorbs moisture from the atmosphere; the oil doesn't mix with the (oil-based) fluid, but it does boil at a much lower temperature (100 deg C); Dot 4 boils at (from memory) about 130 and Dot 5 boils at about 160.
The brake fluid absorbs a lot of the heat generated during braking - and when those droplets of water vapourise is when you lose braking pressure - remember from High School physics that a liquid connot be compressed but vapour (in this case, steam) can be.
Exactly. This describes what happened to me when I was installing 'new' DOT3 that had actually been around for a while and absorbed some moisture. The brakes were great until I really nailed them repeatedly and then, clearly, the moisture boiled and brake performance went downhill. I used to do some exotic MVAC work (automotive air conditioning) and it's nice to see someone who understands hy
groscopic
Precis wrote:
Stainless lines: I habitually replace all the hydralic lines on bikes we intend keeping with braided/stainless - and will soon do so for the CBR125.
Assuming that only the lines were changed on the crashed grey-import and no changes were made to master-cylinder or caliper sizes, the whole and sole cause of the crash was operator error: the brake lines are only pipes and cannot alter the ratio between pressure applied at the brake lever and subsequently transmitted to the caliper/pads.
I'd have to try this to be sure. One thing that comes to mind though, wouldn't the difference in break performance be increased pressure at the piston, made more rapidly, due to the fact that the rubber lines are not swelling and eating some of that loss? I would expect that breaking with steel lines would be sharper and faster as compared to rubber lines.
Precis wrote:
What I found was that when the brakes were applied, the original rubber lines (which had apparently delaminated inside) were absorbing pressure (instead of transmitting it), swelling up - and gradually resuming their normal shape over several minutes - thereby effectively still applying pressure to the calipers all the while.
This is something that comes up in the automotive industry a fair bit. The liner inside the hose tears in one spot and creates a 'flap'. When the brake fluid is pushed down past the 'flap' it's not an issue. When the pressure is released and the fluid tries to rise back up past the tear it opens and creates the 'flap'. At that point it becomes an obstruction and pressure is held at the caliper creating brake drag - as you mentioned.
Precis wrote:
FWIW, fitting stainless lines is pretty-much the first 'modification' made to CBR125 and 150 race-bikes here.
It's worth a lot! On the track I would certainly go stainless. But I'd have to try it up here to get real-world experience with it to see if your experience proves out. I'm still thinking that because there is no swelling of the rubber lines the brakes would apply faster and harder because there is much less pressure loss due to swelling.