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My wife's bike has the old Hope Mono M4 brakes. (Slightly over-braked I know- we bought the bike with them fitted.)
She doesn't get as much chance to ride as we'd like, so the bike gets infrequent light family (trailer-towing) use. Over the last 12 months, on a couple of occasions the brakes have jacked fully on so the lever cannot be moved. Bleeding a little fluid from the bleed nipple corrects the fault.
Although I'd like to think I got a mechanical knowledge, I'd have thought that the more likely fault that the lever would become soft if there was a leak of fluid or air in the system (I've noticed no leaks).
The only explaination I can think of is that there is some moisture in the system, and as Dot 5.1 fluid can absorb moisture, it has in some way got moisture in.
Does anyone have another explaination?
I've never bled the brakes. I've bought some silicone tubing and a syringe. It appears fairly straightforward*. DO you think a bleed will solve the problem or could they require seals/pistions? I think if that was the case I'd replace them with some lighter brakes more suited to her riding style!
*Will any Dot 5.1 synthetic automotive fluid do the trick or does it have to be Hope?
If she's getting them mega hot then it could be air in the system expanded. Doesn't sound like she's doing alpine descents on them tho...
Best advice is to bleed them, if that doesn't work lube the pistons, if that doesn't work strip them and rebuild - you can buy *all* the parts (just about the only brake you can do) off Hope / CRC / wherever. Any 5.1 fluid will do - 5.1 is a standard, so it's non brand specific. Just don't use mineral oil.
Thanks for the reply Ewan. You guessed correctly, The brakes haven't been steaming after the braking required on the High Peak Trail! The problem actually occurs when the bike has been sat for a while.
I'll bleed them and see how thinks look after that.
Sounds like the pistons are seizing. Probably got the aluminium ones. Change for phenolic and new seal should sort it, mine have been much better since.
Thanks for the advice Autoelec. I would have thought that if it was the pistons sticking, the problem would be notable during a ride, with the pad not releasing properly. I would also expect 'slack' in the lever as the piston had not returned. This is not the case: the problem is at it's worst when the bike has been sat unused for any length of time and the lever is rock-solid open rather than loose.
I would also expect 'slack' in the lever as the piston had not returned.
That's not how the brake works. The lever is fixed to a hydraulic system and, assuming there's no air in the line, has no free movement - whether the pistons are seized or not.
My first thought was that it's a seal-related problem, and it's not going to do you any harm to investigate. The seals/pistons need maintenance and if you haven't done any in the 12 months that you've had the brake it not unlikely that they're going to start causing trouble. Check out the first paragraph of [url= http://steveukmtb.wordpress.com/hope-caliper-overhaul-minitrialsm4v2m6/ ]this guide[/url] for how to clean the pistons.
If there's still a problem after (or during) cleaning the pistons, then the next step would be a complete fluid flush. Air in the system isn't going to jam the brake on - the exact opposite, in fact. You can get an idea of the condition of the fluid simply by removing the MC cap. Take note of the diaphragm's condition, too: is it swollen or deformed in any way?
You may be presented with darkened fluid, specks of stuff that looks like it shouldn't be there, or even something that looks more like jelly than brake fluid. The condition of the brake fluid in the MC will either be the same as or better than the condition of the fluid around the piston seals.
Get your hands dirty and let us know how you get on...
Lube the seals for a first step - A bleed is very simple and won't harm - dot fluid brakes should be bled thru every year.
Thanks chaps. The brakes were on the bike when we bought it second-hand about 5 years ago. I've not touched them since. Although the bike has had pretty light use away from any extreme weather or mud/grit, I'm guessing it's overdue a service!
If I do need to do a seal/piston job, will I need a specific Hope tool for the gold bore caps?
If it looks like a major overhaul, I might look at the option of some second hand hope Mono Minis or Shimano discs. They'd be a little lighter, more in keeping the the riding style of my wife, and it would simplify the spare pad situatiion, as I have these on my other bikes
There is a tool for the caps.
But apart from that, a complete strip down and replace of seals wasn't particularly difficult...just make sure you have a suitable pick such as a cocktail stick or tiny screwdriver to hook out the seals.
My old Mini's had aluminium pistons that were corroding and sticking in the caliper, but I think all Mono Minis/M4's already come with Phenolic pistons. They are black rather than silver.