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Got Shimano Devore brakes and the back brake develops sponginess quite quickly after a bleed - it can be perfect following the bleed but after a few weeks it'll need 'pumped up' before a ride. Usually this brings it back, but the other week I ended up with no back brake on a ride as it wouldn't stay pumped up.
I think there are 2 possible causes: (1) classic Shimano leaking calliper or (2) the bike is stored upside down. I'm 95% certain it's a leaking calliper as the front brake has never had an issue, but want to make sure that I'm not missing something obvious with the storage before I buy new brakes. Can't really see why being upside down should be an issue as it's a closed system, but open to the possibility I'm wrong! It's the calliper though, right?
If there's **any** air left in the system after a bleed, then if the bike is stored upside down then even a tiny amount of air will likely make its way up to the caliper.
You need to be very meticulous on a bleed to not have any air in at all, so I'd say the likelihood is it's from storage. Have you tried storing for a week or two the right way up and see if you have the same issue?
Try closing the lever with a rubber band before storage. That will help you figure out if it's air in the system or air in the reservoir that's causing problems.
Wash and dry the brake thoroughly, then dust with talc to check for leaks.
By the way, it's an open system, not a closed one.
It's more difficult to properly bleed the rear, more chance of not doing it fully.
If the bike is upside down DON'T close the lever as all air will work up the the highest point - the caliper...
Can it be stored on its back wheel with front against the wall? If so, clamp lever, the air will go to the reservoir and after releasing the lever it will remain firm - you could also top up the reservoir to replace the air with fluid.
Have you tried storing for a week or two the right way up and see if you have the same issue?
Not really got space to do that unless I put it in the shed, which is a definite no-go in Manchester!
If there's **any** air left in the system after a bleed, then if the bike is stored upside down then even a tiny amount of air will likely make its way up to the caliper.
That's what I'd assumed it was initially, but surely that should lead to a consistent issue rather than one that gets worse over time? Can more air get into the system unless there is a leak somewhere? What convinces me that it's a leak rather than storage is that the front brake has never had this problem in 3 years of being stored like this. It needs an initial pump up before a ride, but it maintains a consistent feel and has needed bled perhaps once or twice over 3 years. In contrast, I seem to bleed the back brake once every 2 months. I don't think it's bleed technique either as I've used syringes and bled from the bottom up when it's been particularly bad.
Can it be stored on its back wheel with front against the wall?
Probably not, it lives in a small cupboard under the stairs in a state of semi-permanent filth. Might try a bit of Jenga with it and the other stuff to see if it works though!
If the bike is upside down DON'T close the lever as all air will work up the the highest point - the caliper...
Not if the air is in the reservoir. Pulling the brake closes off the reservoir and will stop the air getting into the hose. If the air is already in the hose it makes no difference if the lever is open or closed if storing upside down.
I'm 95% certain it's a leaking calliper...
You've fluid on the caliper or lever? No fluid, no leak.
I'm pretty sure the Shimano is an open system, like most modern brakes, so leaving the bike upside down (rear in the air) would probably mean any air from the lever migrating to the caliper, as has already been said, though a properly fitted diaphragm shouldn't have any air under it. I'd have the caps off, refill the system, bleed and try again. I'd try storing with an elastic band on the lever, too.