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Author Topic: '76RD 400 brake drag  (Read 509 times)

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Offline jamz50

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'76RD 400 brake drag
« on: December 04, 2009, 07:03:13 PM »
I went for a ride today and noticed the rear brake is dragging, it is noticeable when coming to a stop as the bike rapidly decelerates; when I place the bike on the center stand and try to spin the wheel it takes a great amount of effort to do so. Disc is too hot to touch after riding. Caliper rebuild?

Thanks

Offline HeartlessMachine

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Re: '76RD 400 brake drag
« Reply #1 on: December 04, 2009, 08:00:12 PM »
My understanding is that there's a teeny tiny hole that allows fluid to return where it belongs when you let go of the brakes.  If that hole gets clogged (and it happens a lot on these old bikes) the brake will stick.  After you ride around for a while, the brake will stop opening up when you let off of them, and it will act as if you have the brakes half-applied all the time.  (this happened to the PO of my bike, who just removed the rear brake altogether, rather than try to fix it.)

When I bought the bike (a 76 RD400), I started with a really clean-looking rear brake assembly, and it worked great for about a year, but it's getting to the point where the rear brake is hot as hell at the end of a leisurely ride, and I think it might be time to clear out that little hole again.

So, make sure (once you get everything apart) you clear that hole before you put things back together, and you should be fine.
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Offline Surewin

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Re: '76RD 400 brake drag
« Reply #2 on: December 04, 2009, 08:11:00 PM »
Might want to double check that the brake linkage is fully returning first, if you have rearsets.  However the same thing happened to my RD400, rear caliper locked up while riding.  All of a sudden I noticed the engine power going down, about a min later I could smell the rear brake.  I rebuilt both the caliper and rear master.  It worked fine afterwords and was shocked how rusted the rear caliper pistons were.

HM, are you referring to the hole in the master cylinder that allows the fluid to return to the reservoir?

Offline HeartlessMachine

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Re: '76RD 400 brake drag
« Reply #3 on: December 04, 2009, 08:16:38 PM »
HM, are you referring to the hole in the master cylinder that allows the fluid to return to the reservoir?

That's the one!  DINGDINGDINGDING!   :thumbsup:

(to be honest, I don't know where the hole is, or where the fluid goes when it goes through the hole.  I've never rebuilt the brakes on this unit.  I'm only parroting what others have told me when I described the same symptoms.  Is that the only tiny hole in the brake assembly?  Would that hole being plugged cause the symptoms described?  If so, then yes.  I'm sure there are people who know much more about this that can help further.)
« Last Edit: December 04, 2009, 08:20:40 PM by HeartlessMachine »
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Offline jamz50

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Re: '76RD 400 brake drag
« Reply #4 on: December 04, 2009, 08:21:25 PM »
Would a rebuild of the master cylinder be the first priority? The situation that both of you describe are exactly what I am experiencing. I'll get out the shop manual and try that first. After a leisurely ride of about 30 minutes today, the rear disc was so hot it would blister your finger, like touching a red hot stove burner. This would also affect handling, correct?

Offline Surewin

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Re: '76RD 400 brake drag
« Reply #5 on: December 04, 2009, 10:11:18 PM »
If the above theory about the brake pressure remaining is correct you could disconnect the brake line and see if the caliper releases. If it does rebuild the master, if it doesn't rebuild the caliper.  I tend to think that if the seals in one component went bad and/or corrosion is present, the other component is similar.  Just rebuild them both IMHO.

Offline poppaweely

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Re: '76RD 400 brake drag
« Reply #6 on: December 04, 2009, 10:58:28 PM »
if you search "brake pump" you'll see several relevant posts
2- RD400D
GT380K
B105