Disc brake help

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Disc brake help

Post  Jase_MK on 2008-05-08, 02:33

Hi folks, got a bit of a worrying problem. I'm running a Hope Mono M4 (I think) front brake which is fairly new, I've only done maybe 10 rides on it. Up till now it's performed brilliantly.

The bike went into Evans last week to have the rear hose shortened and bled but they also said they'd 'adjust' the front if it needed doing, not sure if they touched it or not.

Anyway, whilst giving the bike a clean and mini service yesterday I noticed that whilst riding up and down my road, if I really gently apply pressure to the front lever, it would gradually get closer and closer to the bar until it was pressed against my grip (no brakes!). If I release the lever and immediately re-apply, the pressure came back fine and the brake was rock solid.

Braking significantly wouldn't cause a problem, it only seemed to happen if I pull the lever gently enough such that I could continue riding. Only happened at the front, back was fine.

Anyway, after half an hour of head scratching, I tried again and the problem has gone away and I can't recreate it. I'm slightly concerned though as I'm doing a 5 hour race on Saturday and could do without the extra worry of brakes that might fail on me!

My thinking is that if it was air in the system, they would be consistently spongy. However, a quick grab of the brakes gives a solid feel. Could fluid be leaking back up into the resevoir? I'm not sure exactly how the res/piston arrangement works in the lever.

Any tips gratefully received! Thanks!

p.s. will come join you guys for a ride as soon as I can!
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Re: Disc brake help

Post  Tomosa on 2008-05-08, 04:00

1st problem is taking it into Evans!!

It sounds as though you have air in the system or there is not enough fluid in the resevoir. check the resevoir first.

How do you store your bike? if its not on both wheels I.E hanging or upside down it can play havoc with hope brakes.....
I have a hope bleed kit if you need to borrow it..

Thanks Tom

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Re: Disc brake help

Post  Jase_MK on 2008-05-08, 04:47

Cheers. I thought about air in the system but then would that not give me a spongy feel? There is no sponginess, they feel pretty solid, it was just that with light pressure you could pull the lever all the way to the bar.

Having just read a few bits on the web and looked at some exploded diagrams of the Mini lever, I have a theory that perhaps the rear piston seal in the master cyl may not be making a good seal with the resevoir. Pulling the lever should shut off this opening to the res but with a small amount of lever movement, the seal is not properly made and fluid leaks back up to the res. Pulling the lever more closes the hole properly, hence the problem not happening when you grab a handful of brake.

Sound plausible?

As for not going to Evans... I took it in simply to have the rear hose shortened. All my bikes have been built up from scratch and stripped down reasonably regularly so I'm pretty confident in my mechanical skills. Unfortunately though I didn't have the right bleeding bits to sort the hose out and was pushed for time, hence taking it there.

The bike came back two days later with a puncture, a buckled rear wheel and a hose that rubs on the tyre. I shan't be going back, trust me! I'd complain, but it was quicker and less painless to sort the probs out myself.

Thanks again for the info. It has never occurred to me about the orientation of the bike, something worth thinking about. I've only recently moved to discs...
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Re: Disc brake help

Post  Tomosa on 2008-05-09, 08:25

It could be what you describe yes,
I would top up fluid if required, if no luck bleed the brakes (properly!) I bought a hope bleed kit and it is awesome perfect brakes in seconds!!!(easy too!)

If still no luck get in touch with Hope their aftersales is brilliant. they will sort you out with a replacement or repair.....

Oh and if you have air in the system it wont necessarily mean you will get a spongy feel all the time.(usually you do) but it can be one big air bubble or similar which means the brakes can be brilliant at one time and terrible the next!

All this is from past experience.

Thanks Tom.

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Re: Disc brake help

Post  Jase_MK on 2008-05-12, 04:08

Thanks again for the info. The bike survived a gruelling 5hr race on Sat with only a couple of brake probs at very low speeds, thankfully. Now I have a bit more time on my hands, I'll get a bleeding kit, top up the fluid and spend a bit time testing out a few things.

Cheers!
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Re: Disc brake help

Post  Jase_MK on 2008-06-02, 04:15

Just thought I'd update this thread for the sake of completness.

Finally found time to look at my brakes again over the weekend. A full bleed didn't help unfortunately, but I can see what's happening. Taking the caps off both lever resevoirs reveals the problem (I think). The rear brake (which works fine) can be operated without the level of the fluid changing at all. When I pull the front brake lever, the level in the resevoir rises up. Letting go of the lever drops it back down again. I'm pretty sure the seals on the master cylinder piston aren't sealing properly, so the fluid is coming back into the res instead of going to the caliper.

Ordered two new seals online yesterday for a couple quid and will atempt to dissasemble and rebuild the lever one day this week.
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Re: Disc brake help

Post  phil on 2008-06-02, 11:41

Arrow good luck.
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