So I just got a new chrome master cylinder that said it was a direct fit for my bike but when I got it it said only use dot 5 fluid. My stock system was dot 4. Since the master cylinder is new and has never been used with dot 5 fluid can I just run it with dot 4 fluid like the rest of my brake system has been using or is there a difference in the material of the seals and gaskets?

 

Use the DOT4.

 

So there is no concern with the seals and comparability with the dot 4 fluid? Just want to make sure and when I search the subject online all I find is info on the difference in the two fluids or info on converting from dot 4 to dot 5 when you already have dot 4 fluid in all the parts.

 

I agree with Madduc, use the DOT 4.

 

HD has flopped back and forth on brake fluid usage. My '05 Springer was marked to 'use DOT 5 only'. When I went to replace the fluid it was clear that someone had moved over to DOT 4. The brakes were working fine but I opted to go ahead and flush it thoroughly and move back to DOT 5. Again, the brakes worked fine. I am very confident you will not have compatibility problems. I am assuming your new part was never used with DOT 5. If it is a used part that has seen DOT 5 it wouldn't hurt to flush it with alcohol just like what you've read on the web about converting.
Good luck with getting everything up and working.

 

I did the same to my bike. Previous owner ran DOT4, master cyclinder said DOT5. I flushed the system and am running DOT5. It's been 5 months and so far so good. I like the DOT5, because it doesn't attract water like the others. And you don't have to flush it every 2 years.

 

Don't use DOT 5 in ABS brake systems.

 

Yeah the master cylinder is brand new and has never been used with any brake fluid. Honestly since I know dot4 has corrosive properties I just wanted to make sure there wasn't any concern with the soft parts, gaskets and seals, in the new master cylinder if I did put dot4 in it.

I also don't think it actually says on the master cylinder use only dot5, I think that was just on the packaging but it had big warning labels which is what spurred the question. If it didn't have that then I wouldn't have thought twice about filling it with dot4, which is already in the other parts and stock for the bike. The master cylinder, in case that matters is a Bikers Choice chopped master cylinder with the same as stock on the bike, 9/16 bore.

It also doesn't have ABS so no worries there.

 

There are no requirements for different seals between the use of DOT 4 or DOT5... DOT 4 is glycol based and DOT 5 is silicone based. If you mix them, you can create sludge in your brake system.. I suspect your master cylinder says "DOT 5" because it was made for a bike model during the DOT 5 years...

DOT 5 was considered a progression from DOT 4. It's a better brake fluid in terms of higher wet/dry boiling points, it won't absorb moisture readily, and won't harm painted surfaces if spilled on them..... Many early HDs (I forget when they started using DOT 5), use DOT 5... my '01 & '03 Both use it.. my anti-lock brake '16 bagger uses DOT 4.

It was discovered that DOT 5 didn't play well with anti-lock brake systems... so around 2006ish, when Harley started using anti-lock brakes in a couple models, they switched all models back to DOT 4....

Don't use DOT 5 with anti-lock brakes, or mix the glycol based with silicone based, and you will have no problems. Some people switch from one to the other, and it's OK, as long as they properly and completely flush their brake systems of the "other" fluid...

If you want high performance brake fluid with anti-lock brakes, there is a high performance glycol based brake fluid called DOT 5.1. It rivals, or beats, the silicone based DOT 5 in wet/dry boiling points, but still absorbs moisture and damages painted surfaces....

 

Thanks, this is very helpful. I will be running dot4 in it since that's what's already in the other brake parts and it will match what's in my rear cylinder. I just wanted to make sure there was no difference in the seals.