DOT 3 to DOT 5 conversion

[vernz]

I'm finally doing some work on my 1960 brakes. I'm going to install rebuild kits on all wheel cylinders and the master cylinder. I'm thinking of converting from DOT 3 to DOT 5 silicone fluid when I do the brakes. The main reason for this is to avoid water absorption by the fluid (the main reason I have to do the rebuild now). Has anyone done this? Is it recommended?

Thanks, Vern

[simplyconnected]

DOT-5 doesn't get you out of the woods. You cannot stop condensation. In DOT-3 it gets absorbed (until the fluid saturates with water). In DOT-5 water collects and sinks to the lowest areas, causing steel to rust in the bottoms of calipers, wheel cylinders, etc.

OEMs still use DOT-3 because it's simply the best and cheapest fluid to use BUT it needs to be changed every few years. Your whole system only holds about a cup of fluid so that much will not break the bank.

Never mix DOT-3 and DOT-5. Three is glycol-based and five is silicone-based. When mixed, it gels and packs the lines. You can wash your system with alcohol but no trace of three can be left behind. All your rubber parts must be made for five or they will bloat. Also, change your brake light switch because you will never get all the fluid out and it will certainly fail.

I change my DOT-3 every other Fall at oil change time. - Dave

[Yadkin]

I had DOT5 in my hydraulic system for about 25 years, after I rebuilt the brakes to get the car driveable. The car was in storage most of that time; I drove it from my rental house in Syracuse to my storage barn 20 miles away, then a decade later to my house in NC, where it sat another 18 years.

I put in a new master cylinder a few months ago, flushed the lines with brake-clean, followed by DOT3, and switched everything over to DOT3. The old DOT5 was clear; there was one little spot of rust on the inside bottom of the old master cylinder where moisture had settled. I've used the brakes hard over the past month to test the system, and have found no problems.

My wife's 2011 BMW recommends draining the master every three years, filling with new DOT 3 the bleeding the brakes to get all new fluid in the system. That's the approach that I am taking with all my cars from now on.

So would I recommend DOT5? If you are going to store the car for a long time like I did, then consider it. For a car that you plan on driving, even occasionally, and especially one that you may make modifications to (disc brakes, etc) so that you'll be going through a lot of fluid? Stick with the far less expensive DOT3.

[vernz]

Thanks for the replies. I think I will stick with DOT 3. My main interest is DOT 5 was the fact that it is NOT hygroscopic, however I did not think about condensation in the master cylinder which can take place in the air space above the fluid.

When you change out the fluid annually, does that include bleeding the brake lines/cylinders?

Vern

[simplyconnected]

Vernon, all brake fluid is water-clear when you buy it. If your reservoir is brownish-red in color that is from rust INSIDE your brake lines.

With all brake fluid, keep petroleum products (and water) far away. That includes 'brake clean'. You want to use alcohol because it breaks down water and oils, it is compatible and it won't cause your seals to swell.

Go to the dollar store and get a turkey baster. Suck out as much brake fluid as you can from the reservoir.

There is no good reason to introduce air into your brake system just because you are changing fluid.

Fill your reservoir with fresh DOT-3 and go to the longest brake line run, usually the RH rear wheel. Open the bleeder and watch what comes out. When it starts running clear, you're done with that wheel. Tighten the bleeder screw. If it won't come out easily, have someone gently press the brake pedal as you work the wrench, just like you would bleeding the brakes.

Keep the reservoir full and go to the next longest run and repeat. The lines don't hold much fluid so it should run clear quickly.

If you happen to spill any, plain water washes it away just like washing anti-freeze away because they are both glycol-based (only brake fluid is not diluted).

New DOT-3 is a rust inhibitor until it saturates with water. Saturation usually takes just over two years. Think of it... All those cups and seals going back and forth thousands of times is the only barrier between air (that is full of moisture) and brake fluid. I have friends who won't open the reservoir cap on a rainy day. - Dave