Been asking a lot of questions about a chopped '98 Softtail I just got: here's another.

The back brake has air in the line - I need to pump it for it to work. I plan on flushing it out, bleeding and refilling the master cylinder. My question: The cylinder is an aftermarket type, the brake itself may or may not be original. I asked the local HD and they said to use dot 5 fluid as opposed to Dot 4, but without any reasoning. What is the differance and what would you think I should use? (Incidently, I have no idea what fluid is in there now, but it looks as black as old motor oil).

 

Clean it out good (master cyl plus caliper) use dot 5

 

Cool. Thanks. Out of curiousity, what's the dif?

 

DOT 5 is silicone based and doesn't absorb water. DOT 4 seems to be the new "standard" for HD. DOT 4 will actually give a better feel to the brakes, but only an expert rider would probably be able to tell the difference. DOT 4 eats paint, while DOT 5 doesn't. DO NOT MIX DOT 4 and DOT 5; bad things will happen.

 

Since you don't know what is in the system, it would be best to disassemble the master cylinder and the caliper, clean them good with brake cleaner (get two or three spray cans of the stuff) and rebuild them. Next, flush the brake line absolutely clean using brake cleaner and the spray wand that comes with the spray can of brake cleaner. Then, you can use whatever you want...but always use the same rated fluid, like pococj sez, bad things can happen otherwise. I prefer DOT 5 also.
You will probably want to do the same to the front brake system and use the same fluid in both front and rear systems so you only have to remember one type of fluid for your bike.

 

Thanks. Understanding what I "should" do, can I get away with just flushing a new can of Dot 5 through the system, refill with Dot 5, bleed, top off with Dot 5 and be okay? Do I really need to disassemble cylinder and caliper? (And if the answer is no, what are the 'bad' things that can happen).
Thanks.

 

I thought most people preferred DOT 4. You don't want to spill it on your paint, but I wouldn't want to spill DOT 5 on my paint, either. DOT 4 is less compressible than DOT 5 so it provides better and more consistent feel in your lever and pedal. It does absorb moisture, but moisture is going to get in your brake lines to some extent with 5 too. DOT 5 gets bubbles in it, feels mushier and is incompatible with anti-lock brakes for that reason; it also costs more. There's some evidence that 5 is harder on seals. I think HD went back to DOT 4 in all models.

 

Please read this stuff carefully as I jump around talking about the different fluids. I proof read it and I think I said what I wanted to say.
I read that DOT 3 or 4 mixed with DOT 5 can form a gummy residue. I have not seen it, but I have never tried mixing the 3 or 4 with 5. If it does form a residue, I don't want it in my system so I would just stick to what I know is in a system or completely rebuild the system and go from there (I would not just flush it, I would clean it and rebuild it). Like I said, I prefer DOT 5. I prefer it because it doesn't damage paint. Seriously, use any fluid you want. It'll do the job. Just don't let 'em mix. I have no idea what would happen if 3 and 4 were to be mixed together but a mechanic told me that they will mix just fine. I would not go on just his word alone unless I was in a real pinch (read: had no choice).
DOT 4 is supposed to be a better fluid than 3. I have experienced the slight mushy feeling using DOT 5 that others have claimed but I have used it for years without failure and my brakes feel plenty firm to me. I am a believer in flushing a brake system every two years with fresh fluid to get rid of the contamination that does get into a brake system.
Since you don't know what you have, perhaps a shop can identify it for you. Just proceed with caution...
I just had a thought (I should stick it in a glass case and put it on a pedestal since I don't have too many good ones), have you spoken to the previous owner about what kind of fluid is in the system?
I just went out to the garage and read the label on my genuine H-D silicone (DOT 5) brake fluid bottle. It says DO NOT mix it with other types of fluid and use it with only H-D brake parts and rubber hoses that are designed for silicone brake fluid.
I can tell you that I used DOT 5 back in the 80's in my '66 Shovel's brake system (combination of new and rebuilt parts) with not a single problem. I was using braided stainless steel lines with nylon liners (had to make them myself back then), a new Performance Machine rear brake caliper and H-D 1973 or '74 11" front disc, caliper and matching master cylinder. I completely cleaned and rebuilt the master cylinders (rear was the old 3/4" bore cast iron/steel model), and the front caliper using DOT 5 as the assembly lube but no special "DOT 5 only" rebuild kits, just what the local aftermarket H-D parts store had for 'em. They performed flawlessly for many years and I sold the bike to a buddy of mine that never had problems with those brakes.

 

Yes, DOT 5 mixed with other stuff will gum up. Personal experience speaking. And some types of fluid will eat some types of rubber, as in the seals. But have not heard of DOT 5 eating anything; can't say the same thing about DOT 4. Whatever you decide, it's the brakes. For some reason they seem to be relatively important.

 

? test for DOT 4 before disassembly and cleaning: place a few drops of the resavoir fluid on a painted surface.