Sprint front brake issue

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Zeets
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Location: Kensington (for now)

Sprint front brake issue

Postby Zeets » Mon May 20, 2013 8:12 am

So I recently got my 1970 Sprint back on the road and I'm having an issue with the front brake.

When I begin applying the brake, it starts to engage normally and then suddenly grabs extremely hard. It's a sharp abrupt grab and doesn't have any modulation.

To try to eliviate the problem I did a tear down and rebuild of the front brake. I pulled the drum off, took the pads off, removed the cam, used fine emory cloth to polish everything and remove all the rust, lightly greased all the moving parts and reassembled it identically. I also used emory cloth to scuff the brake shoes and inner drum. I cleaned the backing plate and all the internals before reassembly. I even went as far as to replace all the cotter pins. When I had it apart, I spun the axle and it had no disernable end play, slop or grittyness, and with the hub/wheel installed there is no noticable play. I also had previously replaced the front brake cable inner/outter as well. Both the shoes have plenty of meat on them and they have thier shims on them where the cam accuates them. They have the Piaggo stamp on them as well, so I don't believe they're a cheap import brake set.

The only thing I'm not sure of is the brake cam. when I had it out I checked it for wear and it appears to be basically brand new. (Actually everything in the frony brake and axle appears to be new, I think it is a new fork assembly, but it's all Piaggo) I removed and installed it identically, and it appears to be symmetrical, but is there a specific direction it needs to be installed? Does it have a top and bottom? It accuates the brakes without any issue it's about the only piece that I think could be causing the issue, unless the brake pads have some strange contamination on them, but they looked clean, and there was no grease from the axle seal.

Any thoughts?
Zayretow

70' Vespa Sprint (2009 LML power)
74' Yamaha RD350 (pinger)
2008 Suzuki DRZ-400SM (thumper)

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imjuanpablo
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Re: Sprint front brake issue

Postby imjuanpablo » Mon May 20, 2013 8:40 am

Do you have the proper "bend" of the front brake cable when it comes out of the bottom of the fork? There has to be enough of a 3/4 circle-shape after it leaves the fork or you will experience this instant-grab effect........very dangerous......people have gotten hurt when this little detail is overlooked.

I'll try to find a picture to show what I mean.........but for now, pull some of the brake cable out and make it curve into a 3/4 circular shape (i.e. some nice bending slack) after it leaves the bottom of the fork.


edit********************

Here's a picture showing what I mean........

Image
John F.

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Zeets
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Joined: Thu Aug 19, 2010 6:42 pm
Location: Kensington (for now)

Re: Sprint front brake issue

Postby Zeets » Mon May 20, 2013 8:55 am

Well, I used the cable from the American Scooter Center kit and didn't modify it, but that doesn't mean it is long enough. There's not a tremendous amout of extra slack, but there also does not appear to be a binding issue. The cable that came off the bike was, what I thought to be, too long. It had so much extra length at the wheel that it seemed dangerous (it had been rubbing on the tire), plus there was a big loop in the headset that was chaffing on inside of the headset. I never rode the bike with the original cable, so I don't know if there was the grabbing issue previously.
Zayretow

70' Vespa Sprint (2009 LML power)
74' Yamaha RD350 (pinger)
2008 Suzuki DRZ-400SM (thumper)

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Zeets
Posts: 905
Joined: Thu Aug 19, 2010 6:42 pm
Location: Kensington (for now)

Re: Sprint front brake issue

Postby Zeets » Mon May 20, 2013 9:00 am

The biggest difference I'm noticing is that my brake cable comes out of the speedo cable hole. That's the way the bike came and I didn't change it, since, to be honest, I didn't know. It didn't have a speedo cable installed (still doesn't) when I bought it. Maybe my issue is stemming from that. I'll try repulling the cable tonight and see if that makes a difference.

Thanks!
Zayretow

70' Vespa Sprint (2009 LML power)
74' Yamaha RD350 (pinger)
2008 Suzuki DRZ-400SM (thumper)

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imjuanpablo
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Joined: Mon Jun 19, 2006 8:27 pm

Re: Sprint front brake issue

Postby imjuanpablo » Mon May 20, 2013 9:52 am

Well, you should re-route it and not use the speedo hole in the fork..............that's why you had that rubbing issue........that doesn't happen with the speedo cable b/c it's much more rigid than an ordinary brake cable........

I'll bet money it fixes your issues............Martin, me and a couple of others have dealt with it many times before......and, at first, it's completely baffling, and doesn't seem like such a small thing would affect braking like that, but it does.
John F.

fredneckscoots
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Location: Fredneck, MD

Re: Sprint front brake issue

Postby fredneckscoots » Mon May 20, 2013 6:15 pm

[quote="imjuanpablo"]Well, you should re-route it and not use the speedo hole in the fork..............that's why you had that rubbing issue........

So it would appear Zeets puts it in the wrong hole and has a "rubbing issue". Sounds about right.
I break scooter parts

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Zeets
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Location: Kensington (for now)

Re: Sprint front brake issue

Postby Zeets » Tue May 21, 2013 7:54 am

It's only the wrong hole if you end up gettting prosecuted.

So, John, when I got home I checked the cable and sure enough, the front shock had been snagging the brake cable when the suspension cycled under braking. I rerouted the cable properly and added the speedo cable and everything is now operating as it should. Plus my speedo sort of works... it climbs, but it doesn't really comeback down. I don't really care since I didn't anticipate it being accurate, but it's fun to see how my KPH I'm traveling.

Thanks again, looks like the Sprint is pretty much ready for action.
Zayretow

70' Vespa Sprint (2009 LML power)
74' Yamaha RD350 (pinger)
2008 Suzuki DRZ-400SM (thumper)

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imjuanpablo
Posts: 1386
Joined: Mon Jun 19, 2006 8:27 pm

Re: Sprint front brake issue

Postby imjuanpablo » Tue May 21, 2013 8:11 am

Cool.........figured as much........speedos are pretty much useless on most of old scooters...........if they work, they're never really accurate.......to me, they're just decoration.

Zeets wrote:It's only the wrong hole if you end up gettting prosecuted.

So, John, when I got home I checked the cable and sure enough, the front shock had been snagging the brake cable when the suspension cycled under braking. I rerouted the cable properly and added the speedo cable and everything is now operating as it should. Plus my speedo sort of works... it climbs, but it doesn't really comeback down. I don't really care since I didn't anticipate it being accurate, but it's fun to see how my KPH I'm traveling.

Thanks again, looks like the Sprint is pretty much ready for action.
John F.


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