# 5.1 brake fluid in older systems? **URL:** \ **Category:** Factual Questions\ **Created:** [May 28, 2008, 10:40pm UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948 "2008-05-28T22:40:32Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![cedman](https://avatars.discourse-cdn.com/v4/letter/c/da6949/32.png) [@cedman](https://boards.straightdope.com/u/cedman)\ **Post date:** [May 28, 2008, 10:40pm UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948/1 "2008-05-28T22:40:32Z")
I have just put new tires, wheel bearings and brake pads on both ends of my 14 year old Honda motorcycle. The bike calls for DOT 4 brake fluid which attracts moisture. A friend got me some DOT 5.1 brake fluid that will mix or is compatable with DOT 4. I plan to flush the whole system while I’m out in the shop so, mixing is not a concern. My question is… is the DOT 5.1 brake fluid compatable with the OTHER PARTS of a 14 year old brake system? Don’t need a seal or piston swelling up and locking up on me at the wrong time or a leak starting 500 miles from help. The local techs just keep repeating that the two fliuds will mix; heck, I already know that!!! What about the REST of the braking system parts? Thanks, CedricR. ---
**Author:** ![x-ray\_vision](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/x-ray_vision/32/351_2.png) [@x-ray\_vision](https://boards.straightdope.com/u/x-ray_vision)\ **Post date:** [May 28, 2008, 11:24pm UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948/2 "2008-05-28T23:24:51Z")
[QUOTE=cedman] My question is… is the DOT 5.1 brake fluid compatable with the OTHER PARTS of a 14 year old brake system? [/QUOTE] Yes. Just make sure you do the flush you mentioned. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 29, 2008, 1:13am UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948/3 "2008-05-29T01:13:09Z")
[Here](http://www.stoptech.com/tech_info/wp_brakefluid_1a.shtml) is an excellent discussion of the different types of brake fluids. It is aimed at the race car owner/technician, but it applies. BTW Carroll Smith is the guru when it comes to race car prep and the authors of several books on the subject. He is not just some guy with a web page. I find this comment interesting > [@](#): > > I use Ford C6AZ-19542 which was developed in the early 1960’s to cure the problem caused by Lincoln Continental drivers boiling the fluid by habitually resting their left feet on the brake pedal. It is inexpensive and it works just fine. 😃 > [@](#): > > In so many words, DOT 5.1 fluids are simply DOT 4-type fluids which meet DOT 5 performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4 fluids without concern. In some circles, they are even referred to as ‘DOT 4 Plus’ or ‘Super DOT 4’ fluids because they are more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 fluid. In fact, DOT 5.1 is essentially comprised of Borate Esters. So it looks like, from here, you should be good to go if you decide to do the change. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 29, 2008, 1:17am UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948/4 "2008-05-29T01:17:58Z")
Two things I forgot. First off DOT 5.1 will also absorb moisture. Only DOT 5 (silicone) won’t absorb moisture. I would not recommend DOT 5 for a high performance application. I would only use it for antique and classic cars where parts are hard to find, and lack of corrosion is a major concern. A complete flush should be done to every car or motorcycle on a regular basis, to remove fluid that has absorbed water, and dirt. Refer to your owner’s manual / service manual for recommended intervals. ---
**Author:** ![cedman](https://avatars.discourse-cdn.com/v4/letter/c/da6949/32.png) [@cedman](https://boards.straightdope.com/u/cedman)\ **Post date:** [May 29, 2008, 10:53am UTC](https://boards.straightdope.com/t/5-1-brake-fluid-in-older-systems/450948/5 "2008-05-29T10:53:34Z")
Thanks for all the responses. I feel confident that I’ll be doing the right thing.