OP
UTC

Member
GTS250ie
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Location: Fort Lauderdale
 
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GTS250ie
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UTC quote
After noticing my rear brake was a bit soft, I decided to change the fluid.

With the bleed valve open, I was adding fluid to the master cylinder while depressing the brake lever.

The fluid was cycling through and then I felt a "pop" and then a total loss of resistance on the lever.

Since then, I have replaced the master cylinder with a brand new unit along with new crush rings. Bled the lines. But still cannot build pressure (and yes, the bleed valve is closed).

I am also not seeing any fluid seeping.

Anyone have an idea of what could have failed or be the root cause for this issue?
@adri avatar
UTC

Atypical Canadian
2009 Vespa S50(LX150 motor swap), 2006 Vespa GTS250ie
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Atypical Canadian
@adri avatar
2009 Vespa S50(LX150 motor swap), 2006 Vespa GTS250ie
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Posts: 2287
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UTC quote
Ugh, I'm gonna take a stab in the dark because no one else is...
Asperix wrote:
With the bleed valve open, I was adding fluid to the master cylinder while depressing the brake lever.
Let's start here... why were you adding fluid with the bleed valve open?

AFAIK the bleed valve should only be open WHILE the brake lever is being squeezed to bleed fluid out.

THEN you shut the valve.

THEN you release the lever.

THEN you add the fluid.
Asperix wrote:
The fluid was cycling through and then I felt a "pop" and then a total loss of resistance on the lever.
If you're adding fluid with the brake valve open, you can introduce air into the system. The pop sound could have been a bubble or something, which would indicate air in the system, which is not what you want.

When air gets in the system, it can compress, which leads to a spongy or completely non-responsive brake lever (or brake pedal whatever the case).
Asperix wrote:
Since then, I have replaced the master cylinder with a brand new unit along with new crush rings. Bled the lines. But still cannot build pressure (and yes, the bleed valve is closed).

I am also not seeing any fluid seeping.

Anyone have an idea of what could have failed or be the root cause for this issue?
I'm... not... technical, so hopefully someone can come in and educate you/me/us/either, but... I don't understand why you're replacing parts. It seems to me, if I'm understanding correctly, like you got air in your system from not bleeding things properly, and you just needed to bleed them properly, not have the valve open if you aren't squeezing the lever, and everything would have been okay.

I don't think you had a parts issue, I think you had a practice issue.

Again, I have sweet fuck all for technical training, just taking a shot here because I saw you had zero replies. Hopefully someone more knowledgeable will step up.
@adri avatar
UTC

Atypical Canadian
2009 Vespa S50(LX150 motor swap), 2006 Vespa GTS250ie
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Location: Toronto, Canada
 
Atypical Canadian
@adri avatar
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Posts: 2287
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UTC quote
Video from micbergsma who always teaches me new stuff, just hit 100,000 subscribers last month, and inspires me to do better with my own content:

I have brake bleed videos on my channel for other bikes (Honda Fury, Honda CB350, Suzuki Boulevard, etc.)... but Mitch's is both more relevant, and better made, because he is the man Clap emoticon
⚠️ Last edited by adri on UTC; edited 1 time
@petercc avatar
UTC

Molto Verboso
Piaggio Beverly 300 ie - 2012
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Molto Verboso
@petercc avatar
Piaggio Beverly 300 ie - 2012
Joined: UTC
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UTC quote
Asperix wrote:
After noticing my rear brake was a bit soft, I decided to change the fluid.
(...)
I see you have a 250ie, is it with or without ABS?

Did you check the fluid level in the first place? And was it too low or OK?

Why did you decide to change the fluid?
@dariusz avatar
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Hooked
2007 MP3 250ie / 2022 MP3 500HPE SA
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@dariusz avatar
2007 MP3 250ie / 2022 MP3 500HPE SA
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Location: CCS-Vzla
UTC quote
For me the "pop" sounds as a o-ring broken in the process. This can explian the sound and that after you have zero pressure.

In your place I open the brake pump and check all and do the bleeding process again after that.
@adri avatar
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Atypical Canadian
2009 Vespa S50(LX150 motor swap), 2006 Vespa GTS250ie
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Atypical Canadian
@adri avatar
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UTC quote
PeterCC - I think in North America we only got the GTS250s without ABS, and didn't get ABS on the Vespa in North America until the 300s arrived, whereas in Europe you guys got the GTS 250i with ABS as an option.
@safis avatar
UTC

Ossessionato
1979 P150X, 1983 P200E, 1987 PK125XL Elestart, 1988 T5, 1995 PX200E, 2011 Yamaha Fazer 600 S2
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@safis avatar
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UTC quote
The rear brake is a bitch to bleed. The "bend" created at the floor area keeps air because the line elevates going to the engine. You need to keep it as straight as possible and try again or use a bleeding system…
@petercc avatar
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Molto Verboso
Piaggio Beverly 300 ie - 2012
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@petercc avatar
Piaggio Beverly 300 ie - 2012
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UTC quote
adri wrote:
PeterCC - I think in North America we only got the GTS250s without ABS, and didn't get ABS on the Vespa in North America until the 300s arrived, whereas in Europe you guys got the GTS 250i with ABS as an option.
OK, thanks for the information. I did not know that.
So Asperix has a normal, straight forward braking system.
@petercc avatar
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Molto Verboso
Piaggio Beverly 300 ie - 2012
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Molto Verboso
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UTC quote
dariusz wrote:
For me the "pop" sounds as a o-ring broken in the process. This can explian the sound and that after you have zero pressure.
(...)
It surely can, but would you not expect then fluid should be leaking, either at the master cylinder (brake lever) or the slave cylinder (the caliper)?

Asperix does not mention any fluid leaking.
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UTC quote
Hello everyone and thanks for the replies.

1. The brake fluid needed to be changed. Had not changed in 2 years. (adding annual changes to my vehicle maintenance schedule)

2.My front brake master cylinder had developed a problem and caused the pad to not retract so I decided to replace it after confirming it was the the source of the problem. I decided to purchase 2 new master cylinders and keep them at the same age. They were not very expensive anyway.

3. Prior to replacing the rear master cylinder, the rear brake had progressively become more "spongy", thus giving me another reason to change the fluid.

4. I admit that my bleeding technique was incorrect and have since "upped" my knowledge on the proper technique.

5. This GTS does not have ABS.

My next step is to do the hand vacuum pump bleeding technique to see if I can get any remaining air out of the rear brake system. I still am not sure what the "pop" was, but should get a better idea after I apply a vacuum to the system.

Thanks again
@wbdvt avatar
UTC

Molto Verboso
'16 Sprint S 150, 2 x '06 GTS 250, '12 GTS 300, '74 Vespa 150 Super
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@wbdvt avatar
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UTC quote
Asperix wrote:
2.My front brake master cylinder had developed a problem and caused the pad to not retract so I decided to replace it after confirming it was the the source of the problem. I decided to purchase 2 new master cylinders and keep them at the same age. They were not very expensive anyway.
I think this would be more indicative of a frozen caliper not a master cylinder. Did you remove the caliper and try to pry back the brake pads with a large screwdriver? Sometimes that will be enough to loosen up the caliper.
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UTC quote
Wbdvt... The issue with the front brake system was a clogged master cylinder return hole. This was confirmed by opening the bleed valve, which released the pressure and the pads retracted. It was further confirmed by replacing the master cylinder and returning the front braking system back to normal operation.

Btw, the first step in my process was just as you recommended.
@safis avatar
UTC

Ossessionato
1979 P150X, 1983 P200E, 1987 PK125XL Elestart, 1988 T5, 1995 PX200E, 2011 Yamaha Fazer 600 S2
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@safis avatar
1979 P150X, 1983 P200E, 1987 PK125XL Elestart, 1988 T5, 1995 PX200E, 2011 Yamaha Fazer 600 S2
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UTC quote
Asperix wrote:
The issue with the front brake system was a clogged master cylinder return hole.
You could have taken it apart and clean it. I've done it many times…
@cadbury64 avatar
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Hooked
2019 Primavera 150
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@cadbury64 avatar
2019 Primavera 150
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UTC quote
I can't say that I have ever heard a pop sound when working on brakes. A pop would be the result of something either bursting (ie a rubber brake line) or a seal being forced out of place. A burst hose should be very obvious. There are seals at the calliper but you would also definitely see fluid leaking out if that were the case.

There are also two seals on the master piston, one that keeps the fluid in the master cylinder, and the other which applies pressure to the calliper. If the inner seal has pushed out of its groove then you could have a sealed system but no means to generate hydraulic pressure.

My suggestion would be to disassemble the master and check the seals are correctly installed on the shaft. Frankly I would be very concerned if they have moved, as this should not be possible and is extremely unsafe.
Forum member supplied image with no explanatory text
@dooglas avatar
UTC

Veni, Vidi, Posti
GTS 300ABS, Buddy 125, Buddy Kick 125
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@dooglas avatar
GTS 300ABS, Buddy 125, Buddy Kick 125
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UTC quote
cadbury64 wrote:
My suggestion would be to disassemble the master and check the seals are correctly installed on the shaft. Frankly I would be very concerned if they have moved, as this should not be possible and is extremely unsafe.
Asperix description above says that he replaced the master cylinder.
@abner_bjorn avatar
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Molto Verboso
2007 GT200, 2008 Yamaha C3, 2009 BV250
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Molto Verboso
@abner_bjorn avatar
2007 GT200, 2008 Yamaha C3, 2009 BV250
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UTC quote
There's 3 parts to that brake system, You've already replaced one. You either have a compromised brake line, or compromised caliper. You are seeing no fluid, so It's probably not a split. Open up the bike inspect every inch of the line. Or buy this rear caliper and you will have eliminated 2 of 3. https://scooterpartsco.com/gts-250-brakes-c-3637_3642_3779/rear-brake-caliper-for-vespa-gt-gts-gtv-p-396.html
@cadbury64 avatar
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2019 Primavera 150
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@cadbury64 avatar
2019 Primavera 150
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Posts: 133
Location: Auckland, New Zealand
UTC quote
Dooglas wrote:
Asperix description above says that he replaced the master cylinder.
Good point, thanks for pointing that out.

Given the pop and the symptoms, my other suggestion would be that one of the rubber brake hoses has burst through the woven shell and is now ballooning when under pressure; that would explain a pop, and an inability to get any line pressure at the calliper.
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