Hey everyone,

I'm in the process of changing out my bars and controls, and I just realized that the new chrome master cylinder says "use DOT 5 only". I've been looking around the web for the past 30 minutes, including the search function on here, and all I seem to find is an argument. Some say that using 4 in the new MC will ruin the seals, while others say there is no difference between the seals in a 4 or 5 system. Then there's people arguing over whether or not 4 will eat the chrome finish.

Everyone seems to have their opinion, but I haven't seen any posts yet that are backed up with a "yes I have done this and it works/doesn't work)."

I know you should never mix 4 and 5, but does anyone have experience with putting DOT 4 fluid in one of these aftermarket chrome master cylinders that states 5 only? I don't want to flush the whole system of the DOT 4 fluid and switch to 5, as I'd rather just use 4 since harley recommends that for my bike anyway (07 train). On the flip side I don't want my brakes to fail while I'm out for a ride either.

Any help would be much appreciated. Thanks

 

Just use what is called for and you will be golden. My 03' SE-Deuce calls for DOT-5.....and I am not sure what my 08' calls for but if its DOT-5 thats what it will get. How hard could it be to bleed the old fluid through and add the DOT-5.....what 1/2 hour of your time????

I dont understand.....if the MC your installing calls for DOT-5 only.....why would you question it????

 

His bike had DOT 4. His new mastercylinder says DOT 5 only on it. He wants to know whether he can put DOT 4 in the MC without harming it.

Personally, I'd flush the whole system and put DOT 5 in.

 

Dot number will state how the liquid will resist fading, that is: the mnore you break, the hotter the whole system becomes, up to the point when the liquid boils. Now, boiling is just a common name for a liquid transforming into a gas (or vapor), and a gas is what you DON'T want in a braking system, as while liquids are incompressible, gases are famous for the ease with which they are compressed (i.e: bye bye brakes).
Water is what boils first in the liquid. So, you must keep water OUT of the brakes liquid.
The higher the DOT, the more resistant to heat the liquid is, the more it will attract water, the more corrosive it is.
Moreover, there are actually TWO dot 5 liquids: one is Dot 5.0. This is a silicon based oil which avoids the water problem by actually trapping water into its very complex molecules. This means there's almost no need to change it, and for this reason it's very popular in military applications.
BUT, it huge molecules are actually compressible, to a point. This makes yeldy, spnge-like brakes feedback. BAD.
5.1 is the actual brake fluid you get when asking for "dot 5" liquid. It has to be changed OFTEN, say once every 6 months, a year top, as it is extremely hygroscopic (that is: attracts water A LOT).
Dot 4 brake fluid is less resistant to heat, but it's not nearly so hygroscopic, moreover is easier on the seals and o-rings.
So, if a brake system is rated DOT 5, you can also put DOT 4 fluid in it. You'll be sacrificing some heat resistance to more duration of the fluid itself. It may be worth it, it may not, depending how frequently you service your brakes, and if you do it yourself or have it done by somebody (sbdy you pay).
"Dot 4 improved" is almost as heat resistant as Dot 5, but also almos as hygroscopic as dot 4, so it can be the best of both worlds.
Usually, even in sports cars you put DOT 4 Improved at best, as it not worth using DOT 5 unless it's a track day car.
On a bike, I really don't know.
But you must firs make sure that they REALLY mean DOT 5.1 and not, for some reasons, DOT 5.0, the silicone based oil, or you could seriously risk malfunctions in that case. Read carefully the specifications of your bike, and in doubt, ask a dealer.
So, bottom line is: DOT 5 liquid is

 

Putting DOT 5 in a system rated for DOT4 could seriously jeopardize safety.
DOT 5 rated parts can work just as well with DOT 4, but DOT 4 rated parts will be corroded by DOT 5 fluid.
So, if only the master cylinder is rated DOT5, and all the other parts (hoses, calipers) are rated DOT 4, putting DOT 5 liquid in them is not good.

 

Because the rest of my brake system is stock, and HD says to use DOT 4. Does it hurt to ask questions? No. It's not a time concern, it's the fact that most people seem to be saying to use DOT 4 fluid for my bike, and now the new part says to use 5.

99octane, thanks for all the info, much appreciated. I'm trying to contact the manufacturer and see what they have to say. I also looked through a few more posts, and saw that a couple people have just gone ahead and put the DOT 4 in the new MC that said 5 only, and they haven't had any problems.

 

I think the only thing HD changed was the fluid itself when they made the models with DOT 4. If you go to the MoCo's website, I believe that part numbers are the same for both models that came with DOT 4 and DOT 5.

 

My bike is supposed to take DOT4. My polished M/C had the DOT5 sticker on it. Flushed it out, wiped it clean, put in DOT4 fluid, rebled from M/C to calipers. Works like a champ. Coming from an automotive background, I have to agree with 99. The M/C will work fine with the 4. You place a greater risk on the other components switching to 5 on your OE lines and calipers. Now, if you changed out lines and calipers (braided and chromed, of course) that are rated at 5, then you could use 5, but unless you're blasting around a track or heavy twisties all the time, the improvement would be nominal. Actually, most people find 5 to have a bit more of a mushy feel.

 

Thanks cwickgo9,

Like I said with the exception of the new MC, the rest of my brake system is all stock and DOT 4 rated. I read about 50 different articles (arguments) on brake fluids, and i can't seem to find any proof that DOT 4 fluid deteriorates rubber seals, like some people seem to be saying.

I'm gonna try the 4 in it and just pay close attention to the front brakes when I ride.

we'll seeeeeeeeeeeeeeee

 

If you use the dot 4 fluid you will be fine.That is ifyour master cylinder isbrand new and has never had fluid in it before. I went through the same thing with my 07 fat boy. The internal parts on the master cylinder are the same. You want to make sure you never mix dot 4 with dot 5 because it will turn to a gel. I recomend you buy a new cap that says dot 4 ortake the sticker off your old cap and put it on your new cap. Therefor you want forget what fluid it has in it or if it goes in the shop for service the tech won't put dot 5 fluid in it and ruin your bike. Also make sure you never let dot 4 fluid get on any painted surfaces. If that happens you can kiss your paint goodbye. Harley tried using dot4 a while back but because of the paint problem they wen't back to dot5. In the last few years they just wen't back to dot 4 again because of the stopping demands and more performance of the bikes. Actually sport bikes have been using dot 4 for a long time because it gives better stopping power and holds up longer and like wick said dot 5 will sometimes get a spongy feel to it.