I went to HD main site and looked up the Chrome Rear master cylinder for my wife's 2008 FLSTN Deluxe. They showed two different part numbers that would fit her bike (41775-04b and 41777-05a). I ordered the 41775-04b and when I received it I looked at the instructions and it is a Dot 5 master cylinder. The 41777-05a is the Dot 4 version. Her bike is currently Dot 4. What I am trying to find out is, will the 41775-04b actually fit and can I just use Dot 4 fluid in it? She has a Dot 4 Chrome cap I can use already on the stock master cylinder. I have done some research and I find it is about 50/50 on the subject. Some say yes no problem using Dot 4 and others say the seals are different. I am confused if this will even fit and if I can just use Dot 4. Thanks.

 

Yes you can use the DOT 4 in that master cylinder. Just don't mix the two together and you will be fine.

 

Since the master cylinder is brand new and never had DOT 5 in it there should be no problem running DOT 4. The big problems occur when DOT 5 is mixed with other brake fluids. Even a tiny amount like in a line or such. It turns to jelly and clogs and so forth.

 

No, No, No!!!! I just went threw that nightmare!!!!

Am I getting across?????????

I had apes installed by an Indy and he used a brand new DOT 5 master. The second I put DOT 4 in that sucker, the seals desintegrated. Completely lost the brakes. You could see little black flakes from what used to be the o-rings floating to the top when I squeezed the lever. I believe a DOT 4 master can do DOT 5 if its new, but DOT 4 trashed MY brand new DOT 5 master. And with a quickness!

 

What bikerlaw said!!
Dot 5 is a silicone based fluid and the others are petroleum based. They will eat Dot 5 rubber. Dot 5 can work in the other systems but, as noted, NOT mixed! You would have to completely flush and clean Dots 3 & 4 systems.
Unless you have high life insurance on your wife or want her out of your life, don't do it!

 

You'd be better off to return the MC and get the right one.
To use the Dot 5 MC you will have to take the caliper apart to make sure no traces of the D 4 remains inside. Along with the light switch (if hydraulic) and flush all lines and dry them. A lot of work.

 

flush the brake caliper , and use dot 5..
better stuff, and wont corrode the other parts it touches..
I use dot 5 in my 1978 , and it works fine
it had the old dot 3 in it before the flush