Is DOT 3 Fluid OK?

I've decided that the brake master cylinder on my m1008 needs to be replaced. However, the cover on the master cylinder states to use DOT 5 only. I didn't find any DOT 5 at the local parts store; is it acceptable to use DOT 3 since the CUCV is basically a Chevrolet truck?

NO NO NO NO NO NO

NOT UNLESS YOU COMPLETELY BLEED THE OLD FLUID OUT!!!!!!!!

So you're telling me that the CUCV's used the silicone brake fluid and if I bleed out all of that then the hoses, seals, etc. should be able to work with DOT 3?

yes, as long as you bleed out all the silicone fluid, it will be fine. But be SURE all the old fluid is out.

otherwise you will be cleaning GOO out of the master cylinder.

Thanks. What does silicone fluid look like so that I will know? I have partially bled the system with DOT 3 (rain interrupted me) and the fluid that came from the bleed screws looked/smelled like standard brake fluid.

its either yellow or purple, older fluid will be yellow.

The fluid that was already in the brake master cylinder and what came from the bleeder screws was neither of those colors; it was rusty-in-color like old brake fluid. This truck supposedly came from Eglin AFB just before I got it. Is it possible that the Air Force didn't use DOT 5?

its possible that its been replaced with dot3 sometime ago. just be sure to get all the old fluid out, unhook the lines at the wheels and let it gravity bleed. then fill up with dot3 and bleed as normal.

Would it damage the system if I opened all of the bleeder screws and pumped the pedal, (without going to the floor), until DOT 3 came from all screws? I'm in a hurry and can't wait for a gravity bleed.

you dont want to suck air into the master, thats hell to bleed out. just keep bleeding till you see only the new colored fluid coming out of the bleeder. and starte at the furthest from the MC, just like always.

A bona fide chemical lab that charges large dollars did some work for me on the subject of DOT 3 and DOT 5 a few years ago. I incorporated some of this into an article for MVM a few years ago.

I'll hit some high points for you. DOT5 is purple, DOT3 is nearly clear. If you mix them the purple dye in the DOT5 ABSOLUTELY WILL EVERY TIME leave the DOT5 and move to the DOT3. The DOT3 becomes purple, and the DOT5 clear. If you look at a sample of the two in container it will look from the color to be evenly divided - but the colors WILL be backwards. So, do NOT rely on color if you think both fluids are in your system.

The army conducted two very major, extensive (couple of years) and expensive tests on silicone brake fluid. Results of both tests were that there was no degradation of the fluid or brake system components caused by mixing the fluid. The sole problem with this is that mixing the two negates the antihygroscopic (non-water absorbing) nature of the DOT5. Hence the army issued emphatic orders that when converting the system should be completely purged.

Lab tests I had done a couple years ago resulted in the same conclusion. HOWEVER, many folks on this board like CCATLETT state that there are problems with this - therefore this winter I'll begin a controlled test over some time and report the findings I get.

You be the judge on the "goo" issue - but I can tell you with 100% certainity if you even think the two fluids have been mixed, don't trust the color. Take a small sample of the brake fluid and add a few drops of water - if they mix, its DOT3, if it doesn't, its DOT5.

Hope this helps,
David Doyle

Thanks for the info it helps a lot. At any rate I've completely flushed the system and it is now DOT 3. Thanks to both of you for your posts.

This helps me too. I have been FREEKING OUT! for the last 2 days since I read about DOT 5 , drained my MC reervior, have been researching all the posts.....
I just realized something.
I have had a mixture of d3 and d5 in my MC for 3 years and have had no problems with the braking system.

I very much appreciete your scientific method, and your taking the time to do this. At one point when I was down on fluid, I added some dot 5, all I had at time to some 3. mixing the two had worked in some old all drum brake vehicles I,d had in past.

This was a 82 chev k-10 with 6.2. The front calipers siezed up and overheated roters. These calipers had been replaced a few years before, and I replaced the flex lines at that time, so the swollen inside and looking ok outside on them was unlikely. I add this for your ancedotal data base, and will like to hear your results.

Pictures of my dot 5 dot 3 mix

The clearer jar is from the smaller reservoir /the dark is from the front res... it looks like an emulsion, and the 3 and 5 are separated. Which one is the heavier liquid?

clearer stuff

spruce, it should be a yellow color if its just older stuff in the can. the purple dye breaks down with age.

I've had zero problems with the two trucks that are running DOT 5, and recurring problems with those running DOT 3.

I'm thinking about changing all my MV's to DOT 5. I think it's possible that DOT 5 is more tolerant of long periods of inactivity than is DOT 3. Just a thought.

Does anyone else have thoughts along this line?