72 shovelhead brakes - Harley Davidson Forums: Harley Davidson Motorcycle Forum
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Old 08-19-2012, 10:04 PM   #1 (permalink)
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72 shovelhead brakes

OK so I introduced myself and told my story in the new people section. I'm in the process of getting the braking back in order. And I replaced the rear master cylinder with one from SamsPowersports on Ebay. When I got it the instructions say that I must run dot5. But back in 72 they only ran dot 3. So the new lines and new drum cylinder I have are dot 3 compatible not dot 5. The guy from Sam's says just run dot5 but I did and the brand new (not rebuilt) drum cylinder is leaking out the boot. So I have to assume its because I'm trying to run dot5 through a cylinder that is made for dot3. So I have new springs, cylinder bolt, shoes, the drum is in spec and a new master cylinder. But that master cylinder is calling for dot5 and everything else is calling for dot3. What are your thoughts?
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Old 08-20-2012, 05:32 AM   #2 (permalink)
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DOT-5 can be run if there's NO natural rubber seals. I believe the seals in the slave in the rear drum are natural rubber.

I'd personally stick to DOT-3 or 4. You can get new cups for the rear brake slave at NAPA. Take the old ones out and the bore size should be printed on the cup, or take it with you and they'll match it up.

Make sure the bore isn't pitted too. If it can't be honed, then it's shot and needs repalcing.
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Old 08-20-2012, 08:54 AM   #3 (permalink)
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Just did a 70 flh with a new master cyl it called for 5 but 3 causes no problems just rebuild your wheel cyl an use 3.
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Old 08-20-2012, 10:26 AM   #4 (permalink)
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I just installed new:
Springs
Shoes
Master cylinder
Drum cylinder (brand new not rebuilt)

Ok I'm going to flush everything and clean it up and start over with Dot 3 then. Thanks everyone!
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Old 08-20-2012, 12:46 PM   #5 (permalink)
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My scooter all have rear shoes. I use DOT3. My understanding when you mix DOT5 in a cylinder that originally had DOT3 and wasn't theroughly cleaned, the mix of the two different brake fluids will deteriorate the seals.
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Old 08-21-2012, 06:09 PM   #6 (permalink)
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I rebuilt my '68s brakes with a new master cylinder, Russel braided rear line and DOT 5 about 10yrs ago and it hasn't leaked a drop. Drag Specialities has a DOT 5 rated master cylinders , and maybe even a rebuild kit for each. I'm thinking I only rebuild the wheel cylinder.

But IIRC , it only has to be new rubber pistons. It can't be ones that had DOT 3 and were cleaned. And flush the hard line on the frame with plenty of brake clean.
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Old 12-07-2012, 07:52 PM   #7 (permalink)
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72 shovelhead brakes

i used the 5.1 fluid that is compatible with 3 or 4 but takes more heat because that master cylinder is right next to the front pipe ,try it.
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Old 12-08-2012, 05:08 AM   #8 (permalink)
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check out ATE super blue raceing fluids. syn. they may come in DOT 5 on-line.
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Old 12-08-2012, 05:17 AM   #9 (permalink)
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Quote:
Originally Posted by hobo1948 View Post
i used the 5.1 fluid that is compatible with 3 or 4 but takes more heat because that master cylinder is right next to the front pipe ,try it.
Agreed. 5.1 is compatible witn 3-4, and performs better as well.
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Old 12-08-2012, 06:41 AM   #10 (permalink)
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Is 5.1 compatible with 5 ???
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