DOT 4

[Johnie]

Has anyone switched from DOT 3 to DOT 4? Just not crazy with how the 3 eats paint and plastic. Thx...

[flyin900]

DOT 4 is the same as DOT3 the difference is in the temperature rating. The synthetic DOT5 is the only one that won’t absorb moisture or damage paint. To change though requires a replacement and cleaning of the entire system internally. Well worth it when your doing a full rebuild of the system and have everything apart.
You can mix DOT 3 and DOT4 with no issues, yet DOT 5 cannot be mixed with either hence the need to fully clean the system if your switching over.

[Shadowjack]

I seem to recall that DOT5 makes the brakes feel the slightest bit spongy, although I have not tried it and can't remember where I read it.

[Whiskerfish]

5's different chemical composition make it more compressible than the others so yes some report it as spongy.

[redglbx]

I did a lot of testing on all this in my job prior to retirement.

There is some DOT 3 fluid (Dow 50-4 & no longer made) that is superior to pretty much anything else in every category. DOT 4 is supposed to have a higher Boiling point but that’s not necessarily true.

Brake fluid in addition to providing hydraulic energy between the lever (master cylinder) to the pistons/pads to supply force for stopping. It provides lubrication to the parts and absorbs and suspends any moisture that may be in the system and act as a rust inhibitor.. it is also supposed to absorb heat out of the system w/o boiling.

Most good name brand DOT 3 & 4 fluids will do all of that while some cheaper nonmainstream fluids are terrible at doing any of that, Pentosin comes to mind. The bad thing about DOT 3 & 4 fluids is that both will eat paint if it comes in contact with it. Both will also become quite caustic once it has reached its maximum water absorbency and will boil at a really low point as well causing very weak braking.

Now DOT5 is a totally different animal as it’s a silicone based fluid that will not absorb water which in theory is good but the problem is that it just sits on top of that moisture so that the water just sits there and corrodes what it comes in contact with. Any contact between DOT5 & any DOT 3 or 4 will cause it to turn into a jello like substance that will not work.

I will say that I have used DOT5 fluid in some of my vehicles which has made me not a fan, because of the DOT5 sitting on top of the water which then corrodes/rusts the bottom of the m-cylinder, the bottoms of the calipers & pistons, etc , not good !! So yes it will not eat paint although it will stain the heck out of it and cause blisters. Add-in that it is very hard to find and very expensive to buy, no OE currently uses it and in the past 30yrs the only manufacturer that used it was the government in only the H1 Hummers, must be a reason, your tax dollars at work ! But it’s not used anymore.

Now there is a DOT5.1 but don’t be confused as it is the same type of fluid as DOT3 & 4 (vegetable based ) which are all hygroscopic (absorbs water) and cannot be mixed with DOT5 which is silicone based.

So with all that said I recommend staying with a good quality, name brand DOT3 or 4 which due to the water absorption thing should be regularly changed every 2-3yrs (most don’t). Anyway that’s my .02 , hope it helps.

[robin1731]

DOT5 is easy to find. Most auto parts stores carry it. I can go to my local Oreilly's, Advance Auto, and NAPA and pick it up today. I used DOT5 in all the race bikes. Never any issues. You would have be pretty observant to tell much difference in lever/pedal feel compared to 3 or 4. After several years no issues with rust or anything else either.

I've also put it in several street/show bikes to help protect the paint. I've never seen it bubble paint. Maybe if you spill it and don't clean it off right away. Seen plenty of 3 and 4 get wiped up immediately and still leave stains on the paint.