Modern Vespa

A couple hundred miles into an extended trip I discovered that I had no rear brake when I repeatedly pulled the lever. I pulled off to the side of the road and expected to see a severed line but instead found the system to be intact. Pulled on the brake again and this time it operated normally. I started down the road again but the scoot kept losing the rear brake. Limped into the closest small town with a mc shop that could fit me in. They bled the system, test rode it and found that it worked for the first couple hundred feet before it went soft. They bled the system again and had the same result. So I turned around and came home and ended up taking my van for the trip.

Now, six weeks later, I took the scoot out for an hour ride and found the brake to be working as it should. Then, 12 hours later, I took the scoot to the nearby grocery store and once again found the brake lever soft with no stoppage of the rear tire unless I repeatedly pumped the lever.

I've searched the forum but I haven't found a case in which the problem only exists sometimes. Seems like it could be air in the line but I would think a shop, with their fancy pneumatic bleeder, would get any air out of the system on their first, if not second try.

Has anyone else had this happen?

I've had that happen with old fluid when it got too hot, but it has to get really hot. Once it cools, it's fine again. But if they changed the fluid and bled it, that shouldn't be what's happening.

I've had a master cylinder failure before which had similar symptoms. Rather expensive to replace if that's not the culprit though.

I'll try a reverse bleed first to see if that sorts it out.
Does replacing the cylinder require that you bench bleed the new one?

Not with the one I replaced - I let it 'gravity bleed' first and then a few lever presses got it good and firm.

I ended up trying something I came across in the mp3 forum and tied off the brake lever and let it sit overnight. Why this would work, I have no idea. But so far so good. We'll see how the next few days go, though.

Well, balls. The zip tied brake lever as not the magic cure-all. I'll try the reverse bleed.

And reverse pumping failed as well. Before I pony up the dough for the master cylinder, is there a way to rule out the brake caliper?

Also, does anyone have experience with overhaul vs replace the master cylinder and/or calipers?