1982 CB750F...Better Devil

adventurco

Nick Ol' Eye
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From what I've read, 5 is a silicone based brake fluid and has a higher boiling point than 3-4. Because it is silicon and not glycol based (like 3-4) it does not absorb water and reportedly does not ruin paint. People have been running it in older systems for years without problems. The only thing you'd have to worry about is if it eats the seals, which I would think would happen pretty quickly if it were going to.

https://www.thesamba.com/vw/forum/viewtopic.php?t=409242&highlight=dot5
 

Jimbonaut

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I've done a bit of research into the make-up and advantages and disadvantages of the different fluids so I now know a bit more about their application. What I'm trying to understand is the first line of that link Mike - "Always refer to vehicle owner's manual for what the manufacturer recommends or warns against". The manufacturer (Nissin) is recommending against DOT 5 (or says that DOT 5 is not compatible) but I'm trying to figure out why.
 

adventurco

Nick Ol' Eye
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Jimbonaut said:
I've done a bit of research into the make-up and advantages and disadvantages of the different fluids so I now know a bit more about their application. What I'm trying to understand is the first line of that link Mike - "Always refer to vehicle owner's manual for what the manufacturer recommends or warns against". The manufacturer (Nissin) is recommending against DOT 5 (or says that DOT 5 is not compatible) but I'm trying to figure out why.
Jim, does it say "DOT 3 or 4 ONLY" or "DO NOT USE DOT 5"?

Like I said the only worry would be it eating seals, so maybe they know that the rubber they used isn't compatible with the silicone based fluid. But I've never heard of that happening before.
 

Jimbonaut

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advCo said:
The only thing you'd have to worry about is if it eats the seals, which I would think would happen pretty quickly if it were going to.
That's what I'm thinking - DOT 5 may bugger up the seals in the master cylinder. Of course there's one way to find out, but the cylinder I'm eyeballing is not exactly cheap :eek:

Would DOT 5 really attack the seals in a master cylinder? Seems counter-intuitive that brake fluid would attack rubber seals in a brake system, bit of a design flaw, amiright?
 

adventurco

Nick Ol' Eye
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Jimbonaut said:
That's what I'm thinking - DOT 5 may bugger up the seals in the master cylinder. Of course there's one way to find out, but the cylinder I'm eyeballing is not exactly cheap :eek:

Would DOT 5 really attack the seals in a master cylinder? Seems counter-intuitive that brake fluid would attack rubber seals in a brake system, bit of a design flaw, amiright?
I honestly doubt it would, but that's the only potential issue I could see. I've never heard of it actually happening before. I would just put it in and go with it.
 

doc_rot

Oh the usual... I bowl, I drive around...
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i think mastercylinders usually say don't use DOT 5 because is not compatible with DOT 3 & 4 and that is what they assume the system to have been bled with from the factory. if its a fresh system i think you're good to go.
 

Jimbonaut

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advCo said:
Jim, does it say "DOT 3 or 4 ONLY" or "DO NOT USE DOT 5"?

Like I said the only worry would be it eating seals, so maybe they know that the rubber they used isn't compatible with the silicone based fluid. But I've never heard of that happening before.
Good question - it depends what website I'm looking at. The lid on the cylinder itself says "Use only DOT 4 from a sealed container" rah rah rah, and the item description (on some websites) says it's not compatible with DOT 5. I'll be purchasing the cylinder from a different (cheaper!) site, but here's another link -

https://4into1.com/nissin-retro-master-cylinder-black-w-black-lever-5-8-piston/
 

Maritime

Well-Known Member
Maritime said:
Did the bike it came off of have ABS originally, that would be why it would say no DOT 5.
Just saw your link. it's new so that wouldn't be the issue. the article I linked said DOT 5 compresses different and reacts different so the piston design in this may be the issue, won't compress the same, to much i.e. instant lock, or to little, smooshy brakes or none at all.

I doubt it's an issue with eating seals. or like Doc said, they are covering their butts assuming you have DOT3-4 already and you can't mix 5 in with that. It may be just fine if you have an all dry system to start and go 5. email 4-1 and see what they say to that.
 

adventurco

Nick Ol' Eye
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I thought DOT5 was designed for use in ABS systems?

Anyways, if it says specifically on there that its "not compatible with DOT 5", then why risk it?

Product info on a master cylinder doesn't assume that you have DOT3/4 in your brake system. They don't care. If they say "not compatible with DOT 5," then theyre probably saying their internals either haven't been tested with DOT5 or have been tested and are not compatible.
 

irk miller

You've been mostly-dead all day.
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Wow, this brake fluid conversation may just trump the internet's plethora of motor oil conversations.
 

Jimbonaut

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My understanding Nick is that it's the other way round - DOT 5 is not compatible with ABS.

Why risk it? Good question. I don't want to risk anything, especially when it comes to stopping this thing when it's hurtling down the highway. But from what I now understand DOT 5 does have several advantages over DOT 3 and 4. It's better in brake systems that sit unused for long periods of time (like mine do, over winter), it does not absorb moisture unlike DOT 3 and 4, and it doesn't attack paintwork. There are other benefits too - but it cannot be mixed with DOT 3 or 4 at all and so can only be used in an absolutely clean (or new) brake system.

I guess the "non-compatible" DOT 5 warnings must have something to do with the internals of the master cylinder, but as has also been pointed out it may just be that the system hasn't been properly checked with DOT 5.
 

Jimbonaut

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irk miller said:
Wow, this brake fluid conversation may just trump the internet's plethora of motor oil conversations.
Ha! Indeed, but worthy nonetheless I hope.
 

Jimbonaut

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Ok, spent hours looking into this and have emailed Nissin to ask for their input. The only plausible reason I can find for Nissin saying not to use DOT5 in the system is because it can affect natural rubber seals in the master cylinder (if indeed that is what their seals are made of). Apparently - although this too has been disputed. If Nissin says don't use DOT5 then I'm sure they have a reason - I hope they reply so I can move on to something far more interesting
 

pidjones

Well-Known Member
Could it be that Nissin assembles their MCs with brake fluid lubricating all rubber and sliding parts (the correct way) and therefore already has DOT 3/4 on all those parts?
 

Jimbonaut

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That’s a very good question. And it would absolutely make sense too.


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Jimbonaut

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The cylinder block is at the machine shop - I wanted to make sure that the bores and piston clearances are still in spec before honing and reassembling. There's a significant possibility that the piston gap will be very close to the upper limit and so I may have to spring for the 0.50 oversize set (the cylinder has already been bored to 0.25 by a previous owner). I've had a decent look online but was wondering if anyone has bought these in the past and if so have a good supplier?

There is a cheap aftermarket brand out of Australia called Forseti but there is no info on them at all so I'll probably steer clear. Dime City, Vintagecb750, 4into1 - all the usual go-to's don't carry oversize pistons for the DOHC's. Before I hit up ebay thought it best to ask you guys.
 

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