GSXR250 brake issues [Archive]

Pumba

7th January 2009, 10:08

Ok I have run out of ideas on whats going on so someone please help.

I have an 89 GSXR250 front end on the bucket, which has the appopriate brake system, twin four pot calipers and a master cyclinder.

In the procces of trying to solve a soft lever problem I have changed the calipers with another second hand set I picked up, and put on braided brake lines but that hasent seemed to make any difference.

When I bleed up the system, especially when I put a cable tie on the lever and leave it over night, the system feels great, but within half a day I am back to a soft lever :argh:

Any one got any ideas? The only thing I havent changed in the system is the master cylinder, does this need a rebuild and could be causing my problems?

NinjaNanna

7th January 2009, 10:38

If the lever comes right after leaving it tied over night then it must be air leaking back into the system.

Have you looked very closely for fluid leak after compressing the system over night?

If there's no sign of fluid leak then I'd suspect that one of your seals must be deforming when the pressure is released.

I'd say the most likely culprit would be the master cyclinder seals as they'd have significantly more travel then the pistons.

One test that's probably a bit subjective would be to see just how long you really need to leave the lever tied for before it comes right.

My understanding is that by leaving it tied the air bubbles have enough time to work their way to the top - that being so then surely it would take a lot longer for air leaking past the pistons seals to clear than it would for air leaking in directly at the master cyclinder.

They'd be my thoughts any way.

motorbyclist

7th January 2009, 10:42

The only thing I havent changed in the system is the master cylinder, does this need a rebuild and could be causing my problems?

would sound about right

my little brother had the exact same similar problem and sure enough when we pulled out the master cylinder assembly there was a big chunk missing from the main seal

if i was you i'd replace the seals top and bottom... just did it myself;)

i'm actually about to post up a big how-to on brake rebuilds seeing as i took photos of the whole process

SixPackBack

7th January 2009, 10:44

Its the master cylinder.
Similar issues on my K6.......re sealed the master cylinder and it helped but still ended up bleeding the system every month, recently that became once a week. I swapped the master cylinder for a ZX10 unit and the problem has disappeared. Note when the original master cylinder was disassembled [again] very little evidence of wear was discovered.

westie

7th January 2009, 10:50

You may need to re-set the brake pots.

Ie, push them apart and with the front wheel in the air, spin it as you squeeze the lever so as to let them contact evenly.

Sometimes with discs they bend or move, especially floating discs, when the pads contact unevenly. ie one side before the other. Then they feel really squidgey.

But a new rubber cup and hone in the master cylinder will help big time too.
Once did it with a drill and some sand paper wrapped around it. Pretty easy to fuck it up tho

The Stranger

7th January 2009, 11:55

I have a nissin radial master cylinder here if you want to try that.
It is surplus, but belongs to Two Smoker - you could deal direct with him re cost.

Pumba

7th January 2009, 12:27

Awsome, everyone else confirmed what I was thinking. Thanks guys.

Westie the brake pads were already set that way, thats some thing I was taught by the old man years ago when I did the seals on a old honda we had in the shed.

I have a nissin radial master cylinder here if you want to try that.
It is surplus, but belongs to Two Smoker - you could deal direct with him re cost.

That would be great if I could try it to see if it solves the isuue, I could pick it today if you are around home.

R6_kid

7th January 2009, 12:55

imdying

7th January 2009, 13:03

If you clamp the line off when the lever is soggy, does that make it firm?

Pumba

7th January 2009, 15:03

that master cylinder of noels is going to give you a completely different feel - i'd say try it before you buy it.

I've got an OLD rs125 master cylinder on the RX125 atm and it has almost no feel - but it gets the brakes on really fast!

Yea I know, but that rx had sweet FA feel with the rx master cylinder so sounds like you havent made it any worse.

If you clamp the line off when the lever is soggy, does that make it firm?

Didnt seem to make any difference

Mrs Busa Pete

7th January 2009, 15:12

the rx 125 master cylander improved the fxrs front end 10 fold r6 has one for sale

Pumba

7th January 2009, 15:31

the rx 125 master cylander improved the fxrs front end 10 fold r6 has one for sale

What the stuffed one that the fluid dissapeared from that used to be on my bike??

Mrs Busa Pete

7th January 2009, 15:33

What the stuffed one that the fluid dissapeared from that used to be on my bike??

No gravety it all ran out the bottom.

JMemonic

7th January 2009, 15:50

Ok how come no one has pointed out the "washers" in the system ie at the interface between the master cylinder and the brake line are actually gaskets, single use gaskets, (ok you can get away with a couple of tightens) but the can leak air, itsd easier to leak air into the system than fluid out, so along with all the other suggestions made thus far try replacing these, you may be pleasantly surprised.

imdying

7th January 2009, 16:09

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.

imdying

7th January 2009, 16:11

Didnt seem to make any differenceRight, 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

7th January 2009, 16:13

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

7th January 2009, 16:15

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

7th January 2009, 16:48

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

7th January 2009, 16:49

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:blink:

imdying

7th January 2009, 18:21

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 :lol:

JMemonic

7th January 2009, 21:45

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

7th January 2009, 22:01

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

7th January 2009, 22:16

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

8th January 2009, 02:28

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

8th January 2009, 08:37

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 pulledI 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

8th January 2009, 11:09

right, fair enough

i've just made sure that is mentioned in my mammoth brake thread (http://www.kiwibiker.co.nz/forums/showthread.php?t=89784)

imdying

8th January 2009, 11:19

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

8th January 2009, 12:08

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

8th January 2009, 14:29

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.

JMemonic

8th January 2009, 16:48

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.

I apologise if my words were harsh, I do appreciate where you are coming from, but I wanted to point out often its not always the most costly or technical thing that causes minor irritating to find faults, and I have see a couple of situations where with no visible leaking of fluid out yet positive air in, one was the gasket/washers the other was a damaged resleeve job on an alloy master cylinder, PBR cylinder off an old 70's valiant I may still have here somewhere, not sure if it was moved or thrown.

imdying

8th January 2009, 17:12

Ohhh yeah, seen odd shit with resleeve jobs... very rare now if done by one of the major sleevers... weird stuff like fluid squirting out of the sleeve to body interface etc.

erik

8th January 2009, 18:48

sounds like similar problems to what I've got with my bandit. I think they're the same callipers. I think assembling the callipers with rubber grease helped a bit, I think the pistons were retracting too far. But they're still mushy, so I either need to bleed them again or something else is wrong. I don't have much luck with brakes, I rebuilt the sv's front callipers and the lever is noticeably more mushy now than it was before... Maybe I shouldn't admit to that...

imdying

9th January 2009, 11:17

The purpose of the square section oring is for it to distort (like a parallelogram) when the piston comes out, and drag it back in as the oring rights itself to a square shape. Some times you need to bleed it up a little to get some pressure inside the system, then use short strokes to move the pistons only a little at a time to let the pistons move out, rather than just out and in like the full stroke gives you (when the pistons are all the way back, say after a pad chage), then completing the bleeding as per normal. Normally this isn't a problem as with new pads a caliper is generally a tight fit to the disc, but worn discs or refitting a caliper with the pistons all the way back but with old pads, you can experience this.

Erik, if the lever is noticeably more mushy than when you started, give the pads a few hundred kms to bed in to the disc (to take any 'fresh pad to worn disc mating tolerance' out of the equation) and rebleed it.

Pumba

17th January 2009, 21:04

Rebuilt the master cylinder today, was a little supprised how easy it was (hope that means I haven cocked somthing up). Has made a difference to the lever feel, defently an improvement on what it was, but it still fells like it could be better (or maybe I have just goten used to the feel of the Speed Triples brakes). Will really know more once I get it to the track in a few weeks.

Any way the piston and seals I pulled out had no noticable missing parts (should have taken a photo) but there was deffently a difference between the fit of the seal on the old one vs the new one. The old one also had a significant amount black crud built up within the spirals of the piston. Not sure if this means any thing but surely it would have been effecting preformace, one way or another.