SRAM Guide RS Brakes - Lever slow to return

99_FGT

Likes Bikes and Dirt
I'm keen on these too, but have one concern over this.
Is the plastic piston a fail safe, albeit a flawed one?
Is there a way that, in the event of say a crash, it would flex to allow some bypass of the piston. If this piston were ally, would this be creating another problem.
(Mine are going off to get rebuilt today, as they are only a few months old)
 

Nambra

Definitely should have gone to specsavers
I'm keen on these too, but have one concern over this.
Is the plastic piston a fail safe, albeit a flawed one?
Is there a way that, in the event of say a crash, it would flex to allow some bypass of the piston. If this piston were ally, would this be creating another problem.
(Mine are going off to get rebuilt today, as they are only a few months old)
I would have thought that the o-ring seals would give before the plastic piston, and the reason it's plastic is purely from a cost/weight POV - a simple injection moulded part vs a more expensive machined metal one, and plastic is less milligrams of overall weight for marketing purposes (although the difference is probably negligible in reality).

I wonder if the issue as widely reported is related to deformation of the plastic piston under braking pressure. The type of plastic used is perhaps easily squeezed out of shape due to the hydraulic pressure, and a more rigid type of plastic would have not caused the problem encountered?
 

link1896

Mr Greenfield
The seal isn't an O-Ring but a forward facing seal ( no idea what to call it?) they look pretty ordinary.

Piston is plastic for cost reasons alone I think.


Piston is not going to be a failure point during a crash, the brake line is going to rip out of the fitting or the lever is going to snap first.
 

Flow-Rider

Burner
I would expect the brake handle would bend or flex before you would blow or by-pass one of those seals, they are the same type of seal as in a master cylinder of a car foot brake.

I think the main culprit of the piston jamming would be heat expansion aided by the mushrooming of the piston making the piston to bore clearance tighter and then you have that pissy little spring that loses tension over time. From what I have read somewhere else the new piston is .7mm smaller in dia than the original.
 

Nambra

Definitely should have gone to specsavers
Sounds like the OEM fix is in play then, F-R... although I'm still keen to see whether the home brew aluminium piston is a viable alternative.

Edit: I was just looking at Jesterarts pictures, and I see that the modification to the piston is at the lever end, so maybe it is the plunger pushing against the plastic piston that slowly 'flares' it out and jams it in the bore? If so, does the modified piston simply delay this process further but not actually solve it?

rs internals.jpg
 
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link1896

Mr Greenfield
Sounds like the OEM fix is in play then, F-R... although I'm still keen to see whether the home brew aluminium piston is a viable alternative.

Edit: I was just looking at Jesterarts pictures, and I see that the modification to the piston is at the lever end, so maybe it is the plunger pushing against the plastic piston that slowly 'flares' it out and jams it in the bore? If so, does the modified piston simply delay this process further but not actually solve it?

View attachment 332566
My plastic pistons are even thicker further along, at one of the narrow sections, but your suggestion is still valid. The pistons hole for the plunger is tapered, it retains the ball of the plunger somewhat.
 

link1896

Mr Greenfield
I would expect the brake handle would bend or flex before you would blow or by-pass one of those seals, they are the same type of seal as in a master cylinder of a car foot brake.

I think the main culprit of the piston jamming would be heat expansion aided by the mushrooming of the piston making the piston to bore clearance tighter and then you have that pissy little spring that loses tension over time. From what I have read somewhere else the new piston is .7mm smaller in dia than the original.
Still don't know what to call the seal, you're right, they're like 95% of automotive master cylinder seals, a directional seal. I'm sure I've had a master cylinder apart that had quad rings in it.

I have considered finding a better spring, or testing the piston stroke and adding a preload spacer down the bore if the spring isn't compressed to binding at full lever stroke.
 

bowtajzane

Likes Dirt
good job link.....
does anyone know what the piston is made of...
teflon or some other teflon based product ?
 

Nambra

Definitely should have gone to specsavers
good job link.....
does anyone know what the piston is made of...
teflon or some other teflon based product ?
reckon you'd be on the money, although there are a lot of PTFE variants - you'd have to think it would be a low cost version that is easily moulded, rather than something selected as best fit for purpose.

link, I'm wondering if an aluminium piston is going to cause increased wear at the plunger end - an aluminium on aluminium interface as opposed to aluminium on plastic?
 

Jesterarts

Likes Dirt
I would expect the brake handle would bend or flex before you would blow or by-pass one of those seals, they are the same type of seal as in a master cylinder of a car foot brake.

I think the main culprit of the piston jamming would be heat expansion aided by the mushrooming of the piston making the piston to bore clearance tighter and then you have that pissy little spring that loses tension over time. From what I have read somewhere else the new piston is .7mm smaller in dia than the original.
0.7mm or 0.07mm?? Just a decrease of 0.7 is huge in the case of any piston/bore set up. And in the case of the brake bore, it would now be the equivalent of throwing a sausage down a hallway.
 

Jesterarts

Likes Dirt
Sounds like the OEM fix is in play then, F-R... although I'm still keen to see whether the home brew aluminium piston is a viable alternative.

Edit: I was just looking at Jesterarts pictures, and I see that the modification to the piston is at the lever end, so maybe it is the plunger pushing against the plastic piston that slowly 'flares' it out and jams it in the bore? If so, does the modified piston simply delay this process further but not actually solve it?

View attachment 332566
To your point, i suspect the new piston is designed to eliminate the most serious issue. I still have money on plastic expansion.

And the modification of the design is addressing the spot where there was the most surface area causing the issue. Remove this and while the "issue" remains, there is significantly less surface area available to cause the friction required to overcome the return spring.

When I get my pistons out, I will get the macro lense out as see if I can get some detailed pictures to see what is going on with mine.
 

Flow-Rider

Burner
0.7mm or 0.07mm?? Just a decrease of 0.7 is huge in the case of any piston/bore set up. And in the case of the brake bore, it would now be the equivalent of throwing a sausage down a hallway.
Maybe they measured the new recessed area ? Someone should measure a new and old piston with a micrometer and it should reveal what's happening.
 

bowtajzane

Likes Dirt
rather than the hole piston out of aluminium could you just make some type of cap/end plug out of aluminium so the risk of scoring the bore is reduced.....
 

link1896

Mr Greenfield
Maybe they measured the new recessed area ? Someone should measure a new and old piston with a micrometer and it should reveal what's happening.
Jesterarts, are you rebuilding this weekend? Do you have a decent set of vernier calipers or a micrometer?
 

Jesterarts

Likes Dirt
Jesterarts, are you rebuilding this weekend? Do you have a decent set of vernier calipers or a micrometer?
90% most likely rebuilding this weekend. Still waiting on the DOT fluid.

Negative on vernier calipers and/or micrometer. :(

I might check with the father-in-law. He usually had every tool I could ever need.... though his garage is not kept in quite the same order as mine.... so finding the tool you need it the biggest challenge.
 

Nambra

Definitely should have gone to specsavers
Just take your brake parts down to bunnings or supercheap - they sell verniers... not that you need to actually buy one :D
 

Boom King

downloaded a pic of moorey's bruised arse
Jesterarts, are you rebuilding this weekend? Do you have a decent set of vernier calipers or a micrometer?
I have a set of vernier calipers you van borrow. They work but not sure if they qualify as decent? They weren't expensive.
 

Jesterarts

Likes Dirt
Got a caliper thing today and measured up the new piston roughly. Hope this helps. I didn't get the OD for where the seals sit as they where in there tight and I didn't want to fuck about with them. All figures are in mm obviously. :)

I will measure up the old one once I get a chance to take the brakes apart.
 

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