Hey all,

My Mrs has a new bike, a Marin Wildcat Trail 5, and very nice it is too! I noticed something odd the other day whilst cleaning the bike, If I apply the front brake, everything appears to work perfectly. The brake lever comes back about half way to through it's travel, then the brake "bites" so to speak and the wheel is held firm. However, if I relax my grip slightly after a few seconds the brake appears to "lose pressure" and slowly the brake lever will travel further back until it's in contact with the handlebar grip, and the wheel is no longer held firm and will roll.
This only happens when the brake lever is under light pressure, if I squeeze hard then it holds firm and doesn't lose pressure, sorry for using that term if it's incorrect, it's just the only way to describe it.

Any theories? Is this a "feature" or is it a fault?

Here's a link to the bike spec: http://www.marinbikes.com/au/bikes/desc ... at-trail-5
The brakes themselves :
Brakes Front: Shimano BR-M315 Hydraulic Disc, 180mm Rotor
Brakes Rear: Shimano BR-M365 Hydraulic Disc 160mm Rotor
Brake Levers: Shimano BL-365 Hydraulic
- See more at: http://www.marinbikes.com/au/bikes/desc ... o6J6t.dpuf

Could be air in the system maybe?

Try cable tying the brake lever on overnight, might fix it

benp1 wrote:Could be air in the system maybe?

Try cable tying the brake lever on overnight, might fix it

Sorry, I'm not with you. You mean use a cable tie to hold the brake on over night? Does it some how bleed the brake hose?

Sorry, I'm not with you. You mean use a cable tie to hold the brake on over night? Does it some how bleed the brake hose?

It allows any trapped air within the system to find its way back up into the reservoir where it can do no harm.

+1 for the above trick. Shimano brakes seem susceptible to odd behaviour and its typically down to air in the system which weirdly still allows a good solid brake at some times and not at others. If the above don't work bleed it as per shimanos instructions.

Roughly how much pressure should the cable the be applying to the brake lever ? As in "Just closing the lever", "what you'd apply when braking" or "trying to snap it off"?

what you'd apply when braking"

That'll do - it's simply a matter of keeping the fluid feed in the cylinder open.

It could be air in the system, or it could be a fault. But brakes that don't work are definitely not a "feature".
Either way, if it's within warranty take the bike back to the shop and have them solve the problem. It's what you've paid for and you risk invalidating the warranty if you have a go yourself (even if you do know what to do).

I bought some Shimano BR-M615 Deore hydraulic brakes in 2015: they worked fine for several months, then suffered inconsistent braking (similar to your brake's symptoms). They also howled ridiculously loudly and woke the neighbours up. Bleeding with the correct kit, following the correct process cured the symptoms only for a couple of rides. I eventually traced the problem to a tiny leak in one of the piston seals. I sent the brakes back to the shop, which replaced both calipers and bled the systems without question.

I didn't refit them, as the mega-cheap (£2 per end!) Shimano mechanical brakes I picked up at a distributor's open day and fitted as a "temporary" measure have been simple, reliable and provide sufficient braking with 180mm rotors.

I have a set of fairly new Shimano SLX M7000 hydraulics brakes that are being odd. I have bled them very carefully (twice!) and put some new pads in - but they just wont bite. They are very spongy to the point I can ride with them on sometimes, then other times they will grab after a few rotations.

I feel there might be a fault, and wondered how you discovered the leak in the piston seal, how could you tell??

btw, my brakes are not working even for a short while after bleeding. Any suggestions what I can try next?

Thanks

simon72 wrote:Any suggestions what I can try next?

Cable discs

I feel there might be a fault, and wondered how you discovered the leak in the piston seal, how could you tell??

Usual sign would be fluid on pads / round pistons. However, I've had Shimano brakes with 'micro leaks' from the seals. It's not enough to see but it is enough that there's contamination of pads / disc. It's not as uncommon as you may think on Shimano brakes but it is usually the lower end models that suffer most.

Edit: sorry, meant to say. Clean pads, calipers and rotors. Put back together and if it's the above and if you've really cleaned everything properly, they should work fine. Leave the bike alone for a few days, try again and if the problem's back it's likely a micro leak.

I'll whisper this so not to embarrass you (you weren't cleaning the bike upside down were you?) Also look at the disc, does it move side to side when you apply the brake?

Interesting information. My Ramin came with those cheapy Shimano brakes and my wife's bike has them as well. My wife's brakes have been fine but I've had to bleed the ones on my Ramin. The front brake has always been weak and doesn't really bite hard which makes me wonder whether I have this micro leak issue (leading to pad contamination) as well. My long term plan is to upgrade them with SLX brakes (based on the superb performance that I've had from the XT brakes on my fat bike).

pistonbroke wrote:I'll whisper this so not to embarrass you (you weren't cleaning the bike upside down were you?)

No but I did repair a puncture by doing that on my Marin - but only once.

Is turning the bike upside down really a "thing"? I thought it was ok, please enlighten me if I am wrong. I am used to being embarrassed..

It can cause trouble Simon - obviously any air within the braking system will rise to the highest point. Air that might be in the master cyclinder (doing no harm) could find its way to the caliper with the bike upside down. However, don't let that stop you turning the bike upside down as it's unlikely* to happen.

* but not impossible / unknown.