OK, Scooter, Vespa GT200. 2003 model
Had mushy brakes when I bought it, new pads and a bleed cured it. (Back brake is in question here, front is fine)
Recently, over about 2-3 months the rear went slowly mushy again. Bleeding didn’t cure it. New pads didn’t cure it. So there’s only 3 parts left – Master cylinder, calliper, brake hose.
Calliper not leaking
Brake hose is S/S braided
Couple of guys on Modern Vespa forum suggested that the M/Cyl might be leaking past it’s seals internally

So I bought a brand new one, just fitted it, no different, still mushy to exactly the same degree….

Now, I’m not sure how old my brake fluid is. It might be 1-2 years old, it might be older. It’s always been kept with the cap on, but could it have degraded and be causing mushy brakes? Like REALLY mushy, a huge difference, not just a bit? I knw fluid takes on water/air naturally, but I’ve never had this before and I’ve used some pretty old fluid in the past…..

Ideas please, otherwise…?

(Off out now, I’ll check back later)

do they not say the fluid is to be treated as used as soon as you undo the lid irrespective of cap being on or not.Probably the cheapest way to start.

Dunno, do they?

Weapy seals at the caliper is my bet ….

Shimano caliper style

And yes if its dot once its open thats it , you cant store it for a couple of years in theory in practice will it give you mushy brakes – will it fack

Glycol fluids have a nasty characteristic of being “Hygroscopic” in that they attract and absorb water. This means that over the years the fluid in your vehicle system will degrade by the absorption of water. It is not uncommon to find glycol fluids containing up to 13 or 14% water which of course corrodes internal steel parts in your brake system and will promote early brake fade under heavy brake use. It also means that glycol fluids should be used only form a sealed container which once opened starts the degradation process immediately.

I had it on an old fiesta once, I had no idea how long it had been since it was last done (if ever)

double post

Unless of course it mineral oil

I use brake fluid from an opened container all the time – It might be less resistant to heat but I can’t see how it would make the brake mushy – after all you can run a hydraulic system on water. Leaking calliper seals or stuck piston is my bet

Leaky caliper seals? Nope. They’re perfect. I know this because I checked when I replaced the pads. Besides, if it were that a) the fluid level would drop and it hasn’t. b) the brakes would bleed up fine and be firm, then go mushy again fairly quicky, which isn’t the case

Stuck piston? Nope. Cleaned and checked that too.

I’ve bought some new fluid but I’m pretty certain it won’t work.

Even though it’s a braided hose, that’s next for replacement at this rate!

TJ me too. Done it many times.

air trapped in the system? Try pushing the pistons right back into the calliper then bleeding? Try removing the calliper and putting it higher than the master cylinder then bleeding?

I am clutching at straws really

Teej, I’m thinking there may be an air lock of some sort, yes. Problem is that taking the calliper off the back of a scooter involves removing the rear wheel, which involves stripping off bodywork, exhaust, half the swing arm and a rear shock. And I’ve just done that to put the new pads in a couple of weeks ago, and I can’t be arsed again! Also, to get the calliper higher than the lever I’d have to remove ALL the bodywork to remove the brake line from the innards of the bike. Which also isn’t happening!
I think I did a bleed with the calliper off and the pistons pushed right in too, when I did the pads.

Currently my thinking is:
It’s not pads.
It’s not M/Cyl
I can’t see any leaks or sticky pistons
Disc looks OK, but if it was worn and too thin, putting new pads in would have compensated for this and it would have gone mushy a LOT slower than 2-3 months of lightish use.
I’m going to try new fluid, and bleed, bleed, bleed and bleed until I’m bored to see if I can flush out any air lock there may be.

Next I’ll replace the hose, but that’ll be a right ball ache of a job………. then the calliper, but that’s £70-odd I don’t want to spend TBH…..

Reverse bleed?

Sort of tries it already but I couldn’t force any fluid back up. Will try again. Doing it now!

Fluid can’t be compressed, so if you have a mushy lever, it must mean that either something is moving or there is something compressible in the system (ie air). Moisture in the fluid shouldn’t give a problem when cold, only when heated, when it turns into steam.

Go on then Dibbs, have a read of all the above and make a suggestion!

For the record, new fluid now in, reverse bleed, normal bleed, not a jot of difference. Bear in mind I’ve bled this brake before and it’s been fine for the best part of 2 years since, until recently, before you suggest I can’t bleed brakes! 🙂

Next I’m thinking of replacing the hose, which is what I originally thought it was before I replaced the M/cyl. Can braided hoses degrade over time like rubber ones, and will this process make a brake go mushy within 3-4 months max? I doubt that, TBH……

EDIT
Nowt wrong with the brake disc either, there’s virtually no detectable wear at all.

There’s only 2 things left to replace now, both of which are neither cheap or easy 🙁

I’d be having another look at the calliper before spending money

(Serious question) Looking for what? I couldn’t see owt wrong with it last time.

£77 for a brand new calliper, FYI…..

TJ is correct, old fluid will only give you mushyness when you overheat it. I suggest you’ve either bled it wrong or your braided hose has a problem. Weepy seals in either caliper or master cyl will be identifiable by a creep test – get a fairly strong elastic to hold the brake on, leave it a day or two and see if it’s at the bar when you get back. If it is you have a leak, but it’s unlikely to make it mushy.

My bet is you have air in there you’re just not shifting.

CK – creep test, lever comes right back to the bar anyway, so that’s no good…

I’ve bled and bled over and over. I know how to bleed brakes, and I’ve bled this actual bike perfectly before.

That said, you may be right, so what’s changed and how do I remove the air?

Does it pump up if you pull the lever repeatedly?

I’m thinking slightly sticky piston in the caliper. I suppose the hose can fail and swell but I have never heard of it happening.YOu could detach the hose at teh caliper end and see if fluid comes out when you pull the lever?

Struggling to think of owt else I have to say

You sound perfectly competent 🙂 It’s just sometimes they’re a pain (like cars with ABS, air gets stuck somewhere and it’s nigh on impossible to clear it. I’m not sure of this particular application but can you remove the caliper and raise it above the bar, or at least raise it significantly, with the bleed nipple still at the top? Is it an open system or a closed system on those – I’m presuming open so you shouldn’t be sucking air back in past seals unless something’s going really odd. I think it depends what the internals of the hose are – high performance hoses will have something like a PTFE liner, but cheaper ones are often braided over rubber and you can get little slivers of rubber breaking away and causing flow problems. Often they cause the brake to lock on like a one way valve, but I guess it’s equally possible to work the other way causing a bouncy blockage, but to have the lever going to the bar I’m happy to rule out the hose unless you’re losing fluid everywhere lol.

I’d strip the caliper off the bike, ensure all the pistons are moving freely and rotate freely (this reduces the likelyhood that the seal is stuck to the piston and so just stretching/deforming) and bleed it while it’s off.

Some brakes are a royal PITA, I don’t envy you.

TJ, no, as already mentioned (I think) it does NOT pump up. Which a brake with air in usually will IME.
Fluid bleeds through fast and clean, so no need to remove the hose

CK – re. Raising the caliper, please read above, that’s already covered and not practical.

I do appreciate your suggestions chaps, I’ve been bleeding my own car and bike brakes for 23 years but this has really stumped me 🙁

nowt else to suggest

Caliper innit. There’s one on eBay for £40 ish.

Not worth arsing about for that price.

keep us updated ..I’d think air somewhere also despite your attempts . you may change the caliper and all will be well cos the air lock clears and you’ll be sure it was the caliper but never really know! ( until the next time it happens ) fingers crossed.
presume you’re bleeding by constantly topping up what’s already there.
how about emptying/draining the whole system then refilling just in case is a real stubborn stuck air pocket?

Probably right, I love the fact that you’ve replaced parts in order of descending price though. 🙂

Kaiser, yeah, I’ll try draining it. Good idea although I dont hold out much hope…..

Update:

Back end is in bits (again. I’m getting fast at ripping it apart now) and the caliper is not siezed as I thought.
With the disc (off the bike) and caliper in my hand, and the disc between the pads, I can see both pistons moving. After I took the caliper off it was as dry as a bone so no leaks.
I bled it, pushed the pistons around a bit to try and dislodge any air that may be in there, tapped the caliper on the floor, bled it again and again. Still zero improvement.
I’m going back to my origional diagnosis (before I posted this on Modern Vespa forum) of the brake hose. Even though it’s a braided hose, I reckon it’s flexing/damaged somewhere under the body work where I can’t see

I’ll keep you posted

Another update if anyone cares!

I stripped the rear hose out last night. God that was a pain in the ass! The amount of bodywork I had to remove to get to it….!
Anyway, the hose looks perfect. It’s fully braided. I was expecting to see something wrong with it TBH, but no.
So I took it off, emptied the fluid out, refitted and bled again as I said I would. Still no change. This is boring now.

But, with tha caliper on the disc in my hand I can feel slight flexing in the caliper itself. It’s a 2 piece caliper, held together with 2 bolts, so, clutching at straws, I think there might be a problem with play or flex in the caliper body or the bolts. It’s not much, but I can feel it.
So I shall replace the caliper and update again soon!

is the lever an assembly with the master cylinder or separate? Could it be flex in the lever/pivots?

BB7s FTW 😉

Replace the caliper bolts first?

Good thread, I may have to do this to mine when I get it back from Italy that is..

is the lever an assembly with the master cylinder or separate? Could it be flex in the lever/pivots?

Nope. New Master cylinder is the complete unit including lever.

Replace the caliper bolts first?

That’s occoured to me too, but it’ll be easier to pay £35 for a used caliper and bolt it on IMO, although I’ll check them for tightness before I commit

This is really bugging me now. It’s not a matter of money, becasue I can replace the entire brake (M/cly, hose, calliper, disc) for just over £200…. It’s just that I NEED to know what the problem is!!!!

I completely understand PP.

I had a similar problem with a bike disc brake. Replaced the hose, hope, replaced the lever, nope. Turned out it was the caliper and one of the seals was letting fluid out / air in, but only slightly. Hopefully swapping the caliper out will fix it. Failing that I’d look again at the master cylinder.

Failing that I’d look again at the master cylinder

As above (somewhere) the M/cyl is brand spanking new…..

🙂

Replace the hose and calper.

Triggers new brake innit. 😆

Flexi Hose would be my last resort if it has one that is.

I had it on my Honda years ago, was de-laminating inside and causing the hose to expand. Didnt notice because it was hidden inside the stainless braid.
Once I replaced it and cut off the covering it was obvious.

Worth a try.