GSXR250 brake issues

[imdying]

Didnt seem to make any difference

Right, then either the mcyl is rooted (probably a main cup failure) or you still have air in it between where you clamped and the mcyl outlet. Out of interest, when you pull the lever in with the lid off, can you see a wee jet of fluid squirt up inside the (supposed to be unpressurized) reservoir?

[motorbyclist]

and the only way for air to leak in is for fluid to first leak out.....




removing the master cylinder assembly isn't hard, so do so

[NinjaNanna]

Out of interest, when you pull the lever in with the lid off, can you see a wee jet of fluid squirt up inside the (supposed to be unpressurized) reservoir?

if one was to see this what would it mean?

[Pumba]

Right, then either the mcyl is rooted (probably a main cup failure) or you still have air in it between where you clamped and the mcyl outlet. Out of interest, when you pull the lever in with the lid off, can you see a wee jet of fluid squirt up inside the (supposed to be unpressurized) reservoir?

I have observed this when I bleed the system, but only when the lever was pulled in a small amount, when the lever was pulled anywere past half travel there was no sigh of this occuring.

[Pumba]

and the only way for air to leak in is for fluid to first leak out.....




removing the master cylinder assembly isn't hard, so do so

Your right, put I didnt want to do this today as I am going camping tomorrow morning, and I forget how shit goes back togeather

[imdying]

if one was to see this what would it mean?

On a typically unsophisticated motorcycle mcyl it could mean that the height of the fluted front cup is too shallow for the unit, or that that cup is failing and rather than keeping the pressure down the line where it belongs, it's bleeding a little back past the cup, against the rear cup, and then up through the filler hole (which is the first thing the main cup blocks, to stop it squirting out there).

Doing it on a slight stroke, well it's hard to say what's causing this without seeing it. It's possible that the main cup has insufficient wall tension left (i.e. it's old and fucked), and it letting fluid past when the outlet side is at a relatively low pressure (because it has insufficient pressure to push the cup against the walls fully). It's also possible that for some reason the piston is returning too far, although unlikely unless someone has modified it for some reason. It's also possible the cup is too shallow because it's been replaced with something unsuitable. Most likely is it's old and rooted

[JMemonic]

Air is only ever going to be 'leaking in' at 14.7psi, compared to up to 800 odd psi of fluid coming out. If they leaked, you'd know about it.

Experience dictates otherwise these are a cheap expendable component, not a big issue to replace them, or if copper re anneal them.

and the only way for air to leak in is for fluid to first leak out.....

If that is correct why is it recommended to replace brake fluid every two years, if the system was perfectly sealed then this would not be an issue and it would not be possible to have water invade the fluid causing degradation of it boiling point.

[imdying]

Experience dictates otherwise these are a cheap expendable component, not a big issue to replace them, or if copper re anneal them.

Well if basic physics doesn't convince you, I've 10 years experience in the automotive brakes industry if that gives you piece of mind? I think you'd be suprised how little it takes to seal something like that.

As an aside, it's very rare to see a mechanic replace the copper (or sometimes aluminum) washers when replacing a hose. Given that the two most popular cars on the road (323 and corolla) both have between 4 and 8 washers a car, that's a lot of hoses being replaced with the original washers, so it's pretty safe bet that ninety nine times out of a hundred reusing them gives no trouble.

If that is correct why is it recommended to replace brake fluid every two years, if the system was perfectly sealed then this would not be an issue and it would not be possible to have water invade the fluid causing degradation of it boiling point.

The system isn't perfectly sealed... you're only thinking on a basic 'I can see this' level, think a little closer to the molecular level and soon you'll be seeing the holes. That's only for a start, there's a number of ways that moisture can enter the system; brake fluid being hygroscopic, of course, exacerbates the problem.

[JMemonic]

Well if basic physics doesn't convince you, I've 10 years experience in the automotive brakes industry if that gives you piece of mind? I think you'd be suprised how little it takes to seal something like that.

As an aside, it's very rare to see a mechanic replace the copper (or sometimes aluminum) washers when replacing a hose. Given that the two most popular cars on the road (323 and corolla) both have between 4 and 8 washers a car, that's a lot of hoses being replaced with the original washers, so it's pretty safe bet that ninety nine times out of a hundred reusing them gives no trouble.

Ok you win you 10 years in brakes beats my 25+ working on vehicles of various shapes and sizes, although I did say "Experience dictates otherwise", musta been that one out of 100 hey.

The system isn't perfectly sealed... you're only thinking on a basic 'I can see this' level, think a little closer to the molecular level and soon you'll be seeing the holes. That's only for a start, there's a number of ways that moisture can enter the system; brake fluid being hygroscopic, of course, exacerbates the problem.

Actually no I wasn't.

[motorbyclist]

well so far that's all you've been saying (although the molecular talk does apply at the washers anyway)

there are plenty of other ways for contaminants to enter brake fluid, namely the rubber seals at both ends and the reservoir - and then of course every time somebody opens up the system to add more fluid or perform maintenance

yes the washers/gaskets have the potential for leaks, but in MY experience, you see noticable amount of brake fluid spurting out whenever the lever is pulled

[imdying]

yes the washers/gaskets have the potential for leaks, but in MY experience, you see noticable amount of brake fluid spurting out whenever the lever is pulled

I like JM so I've really little desire to ruffle anyones feathers. But I will add it is worth inspecting each joint after you're done, as often you won't see any fluid spurting out.... theses joints are far more likely to have a very subtle weep, than spurting.

No need to fear brakes, just be thorough and relatively meticulous.

[motorbyclist]

right, fair enough

i've just made sure that is mentioned in my mammoth brake thread

[imdying]

There's a number of errors in that thread, one of which could lead to a fucked master cylinder... are you sure you're qualified to write it?

[motorbyclist]

well then get correcting - i'm open to any suggestion

and in one of the first paragraphs i DID say i'm not an expert

[Max Preload]

and the only way for air to leak in is for fluid to first leak out.....

Not at all. I've seen air drawn in through the master cylinder cup when the lever is released. Pistons are spring return and with a tired cup air can make it's way past as the pressure in the master cylinder is, at the time of release of the lever, LESS than atmospheric courtesy of the spring. Think of a syringe.