NC35 - Trouble with Front Brake

[andrelopes]

Hello all,

This year has seen me having loads of fun with the NC35 doing trackdays. Other than proper suspension setup (that I'll need to figure out after a bit more time on the bike), I've only one problem with the bike on the track.

My front wheel keeps braking after I've let go the lever.
This is not that much noticeable on the track, but then I think I'm getting away with it because I only ever apply feather-like trail-brake. In fact, for most corner entries, I dive into corner without any brake applied - out of fear of overdoing it because of the brake self-applying.
Putting the bike on a stand and lifting the front wheel, there is noticeable effort in turning the wheel by hand. It turns with one hand, but it is clearly hard to turn. (this has got to be robbing me of some 10mph down the finish straight?!)

My brake setup is all standard (discs, calipers, MC), except for EBC HH pads and braided lines.
My mechanic was suggesting that this self-braking could be due to the brake lines that are 2 straight lines, one to each caliper, instead of the original 1-into-2 that had a T somewhere between the MC and calipers. His reasoning is that the MC is not moving enough fluid on return after I let go of the brakes.

Calipers were refurbed and pistons move smoothly, fluid replaced and obsessively bleed, no improvement.

Any ideas guys? Do I need to go back to a 1-into-2 brake line setup, or would I fix the problem all the same with an MC upgrade? I do fancy the idea of working with less lever pressure, so was considering a Brembo radial MC.

Thanks for any help, Cheers!

[CMSMJ1]

NOthing wrong with 2 lines - they are compulsory for racers..so it is not that!

Have you got too much fluid in the reservoir? Should not really be filled to the "top" marker. If itis, then when the fluid gets hot it can apply the brakes....which sounds like what is happening to you?

[andrelopes]

NOthing wrong with 2 lines - they are compulsory for racers..so it is not that!

Have you got too much fluid in the reservoir? Should not really be filled to the "top" marker. If itis, then when the fluid gets hot it can apply the brakes....which sounds like what is happening to you?

Cheers CMSMJ1,

I had no idea too much fluid could have that impact... Anywho, I failed to mention I have a tiny reservoir like those that Jap4 sell (mini brake pot). I don't remember it having a level marker, will have to check once I'm around the bike.

Most importantly though - the brakes are clearly self-applying when cold. I did the inspecting before and after fluid replacement and caliper refurb, and during all of this the bike was idle on a stand. Then the next day after being at the track, same thing (cold checked).

[CMSMJ1]

I rekcon the small res has something to do with it.

get a lot of fluid out of it...see how it works.

If you push the pistons in the brakes back by hand and then apply the brakes to a spinning wheel. Does it still drag once you have applied them the once?

Small res pots are not cool IMO - once the fluid is hot you can easily have brakes that are applying and even start to lock on. it is a vicious circle.

[abrmoto]

Can see the logic in the above, if new pads fitted when rec cap is in place would cause pressure in they system.
I have used Tyga mini res on my race bikes for 5-6 years and never had a problem. Are seals on calipers OK?

[vr46]

Is your brake lever Honda original or aftermarket copy? The symptoms described relate to a common fault with aftermarket levers.

[Neosophist]

I've seen the same problems Mark has described, especially with after-market resevoirs that have a good seal.

If you push the pistons back you pressurise the sealed resevoir.

Compound this with hot fluid expanding and the brakes tend to put themselves on.

Drain some of the fluid from the res, maybe try it 1/2 full and see how you get on.. cheap and easy exercise to see if it fixes the fault

[andrelopes]

Thanks for the input everyone!

I rekcon the small res has something to do with it.

get a lot of fluid out of it...see how it works.

If you push the pistons in the brakes back by hand and then apply the brakes to a spinning wheel. Does it still drag once you have applied them the once?

Small res pots are not cool IMO - once the fluid is hot you can easily have brakes that are applying and even start to lock on. it is a vicious circle.

Pushing the brakes back by hand and then applying the brake to a spinning wheel, even just once, is enough to see them dragging again. The small pot is definitely a suspect though, not on it's own, but perhaps coupled with too much fluid?

Can see the logic in the above, if new pads fitted when rec cap is in place would cause pressure in they system.
I have used Tyga mini res on my race bikes for 5-6 years and never had a problem. Are seals on calipers OK?

Seals looked damn fine. When cleaning up the calipers, I found that the previous owner must have been a caring one, as everything was spot on - no dust to be found anywhere, no scratches or any odd movement from the pistons when backing up by hand, seals looking good.

The reservoir cap was possibly on when I first mounted the new pads, but the calipers were cleaned up since, new fluid, OCD bleed. Again, might be using too much fluid - bike's at the mechanic (who turns out to have become a friend since, and another trackday nut!), so I'll have to check it out later/tomorrow.

Is your brake lever Honda original or aftermarket copy? The symptoms described relate to a common fault with aftermarket levers.

Good point! This had been the 1st suspect - as I have some aftermarket shorties. I did try the original one when the problem was 1st spotted, and no change!

I've seen the same problems Mark has described, especially with after-market resevoirs that have a good seal.

If you push the pistons back you pressurise the sealed resevoir.

Compound this with hot fluid expanding and the brakes tend to put themselves on.

Drain some of the fluid from the res, maybe try it 1/2 full and see how you get on.. cheap and easy exercise to see if it fixes the fault

Think the best try for now is to have a go at lowering the fluid a bit. I'll start off with 1/5 full as suggested. I'll report back on how this works out - possibly just early next week.

Cheers mates!

[Lane]

Sounds like the master cylinder piston is not returning properly and failing to vent pressure through the relief.

[arsey30]

I thought master cylinder too, has it been dismantled recently?
If so, check the rear piston rubber to make sure the inner lip is not turned inside out.

The volume of fluid in the system will have no bearing on lever travel, the caliper piston travel and master cylinder piston free play controls that.

There should be no pressure in the reservoir, air must enter to take up the space as fluid level drops [pad wear]
The original Honda reservoir allows air to enter above the bellows through 2 slots in the underside of the cap, across 2 slots in the top side of the white plastic retainer and then through a hole in the centre of it.
The bellows move down as the fluid level drops.