Temperature? related hydraulic brake sensor error

[Ilya]

Hello,

I got a problem where it seems like the brake sensor is always triggered in warm weather. The brakes are Promax Lucid (hydraulic). When I start the bike I get a "brake error" that disables engine power (which also occurred previously when I simultaneously happened to press a brake during start). The weird thing is that the error rarely triggers in the morning (which is cold, approx 5C) - or at least it can be corrected easily by pulling the lever completely, while a simple press of the brake in the afternoon (approx 15C) irreversibly, for the remaining part of the day, seems to trigger the sensor.

I don't remember how it got that way, but I did notice that lever gradually lost pressure over a couple of weeks, until there was hardly any resistance left (and no proper brake function). I then proceeded with replacing the mineral oil (pushing from the wheel side up) which with some force applied to it corrected the lever and added good resistance to it. However, once I pressed it fully with the line clamped, it didn't flex back (no apparent leaks). I repeated the process until I gave up and had an initial loose 20 deg of the lever, but at least a working brake. While riding, it corrected itself and provided excellent resistance (which it does in the mornings). In the afternoon I can still press it, but the initial 20 degrees are completely loose until resistance and brake power is felt, then I can't seem to de-trigger the sensor, even if I pull the lever back fully.

I don't know how this specific sensor works, but the problem seems pressure related since the 20-deg loose lever coincides with the error. I have no idea why it seems to be temperature related. Expansion of the plastic is greater than that of the liquid, leaving less pressure? Air bubbles? Sensor readjustment? Any ideas?

 

[amberwolf]

Strange that a *Ioose* lever (low pressure) would cause the brake to engage, if the sensor is hydraulic?

Electrical faults can be temperature related--most of the time intermittents fail when cold, and work when hot, so if it shorts when heated up but doesn't when chilled, that would engage the brake input when heated but not cold.

 

[Ilya]

The loose lever engages the brake sensor, but not the brake per se. If it were (maybe it is) a magnet sensor I could understand that there could be a lever misalignment with the 0-degree sensor off position in a loose configuration. But it seems like there are other internal factors, other than the position of the lever itself. It's as if I would have to pull the lever beyond 0 degrees, to disengage the sensor, beyond the mechanical range of motion.

The loose lever and the sensor issue appear simultaneously (seemingly in warmer temperatures). I will try another morning tour shortly.

 

[AussieRider]

Could be air in the brake lines. Brake fluid doesn't expand with heat, air does. My guess is that as the lines warm up, any air bubble warms up, expands & increases the line pressure to where the sensor trips. Bleed the lines & let us know mate.
AussieRider

 

[Ilya]

I have now bled the lines, pushed through loads of mineral oil, syringes bottom and up.

If I push fluid in from below the brake lever straightens to a position where it should be, and the brake sensor reacts properly. If I then push the lever it does not bounce back as it should and the brake sensor reports an issue.

When I push fluid it's initially very difficult, I have to apply a lot of force, after that there's good easy flow between up and down. Maybe an internal issue with the line that acts as a valve?

I also removed the line lever side, I expected the lever to drip down lots of fluid from the reservoir, but just a drop or two dripped down. Can there be air here that I can't seem to flush away?

If I disconnect the brake sensor it works, but wouldn't want the other brake sensor to malfunction. Main thing is that the brake works and that the engine starts, in any case.

At least it primarily doesnt seem to be an electrical problem.

 

[Ilya]

It is extremely temperature dependent. Worked perfectly outside. Then I took it inside close to a radiator and the brake stopped working. I think it might be due to the pistons getting stuck. Maybe dirt that blocks them. Maybe will have to take them apart to clean.