Modern Vespa

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?

Ugh, I'm gonna take a stab in the dark because no one else is... 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. 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). 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.

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

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?

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.

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.

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…

OK, thanks for the information. I did not know that.
So Asperix has a normal, straight forward braking system.

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.

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

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.

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.

You could have taken it apart and clean it. I've done it many times…

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.

Asperix description above says that he replaced the master cylinder.

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

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.