Mixing Brake Fluids

I changed out my front brake shoes and was considering buying some brake fluid... I knew something about my m1009 requiring Dot 5 (silicone based) brake fluid. Did a little bit of research and learned how to test the brake fluid in the master cylinder to see if it was Dot 3 or Dot 5. Turns out I have both in there!!!! the Dot 5 was floating around on top and the Dot 3 was a nasty jell-like goo on the bottom.... so now the big question- How do I go about flushing all this junk out of the system? Do I just go and buy a few quarts and bleed it all through?

I will be away from internet access for about a week, I will check back then.

use a baster or mityvac and suck as much as possible out of the mastercylinder. Then fill it back up and bleed till it all runs out clear.

In addition to the above comment I would disconnect the brake line at each wheel and let as much fluid drain out as possible into a container before refilling.

If it is already gelled figure on rebuilding the master cylinder, calipers and wheel cylinders , it is the only way to get all the residue out . There are no shortcuts when it comes to brakes . You will need to flush the lines and it may have gummed up your proportioning valve .

eric_banks32 said:

use a baster or mityvac and suck as much as possible out of the mastercylinder. Then fill it back up and bleed till it all runs out clear.

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To clarify that: Clear and free of bubbles yes, but clear in color no. The industry standard color for DOT-5 *currently* is purple/blue. This makes it easy to tell when you've bled out all the old stuff. I'd recommend you order DOT-5 from an online supplier, because it's $26/quart at my local NAPA store and they are the only place I found that carries in quantity among my local auto parts stores.

Just about every CUCV I've been under the hood of has been topped off with DOT3/4 at some point so it's a common problem.

My friend just got done dealing with a M151A1 that had DOT 3 and 5 in it for a long time. He replaced the master cylinder and cleaned tons of junk out of everything else. Now it's back to DOT 5. The above advice is good. Clean it very well to prevent problems in the future.

I think I got the same problem. When I first got my M1009 I brought it to a shop and had all fluids changed I think they put the opposite of what was in the there they were suppose to flush and refill. Looking at it a while back It looks mixed dark on the bottom and more clear fluid on the top. I will follow this thread and drain and flush ASAP

Cletus09 said:

......the Dot 5 was floating around on top and the Dot 3 was a nasty jell-like goo on the bottom...

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Hard to believe... since DOT5 has a much higher specific gravity than glycol-based DOT3/ DOT4.

Edit: .... just found out (after a little testing) that my statement was incorrect...!
Don't know where I had that misconception from... fact is that silicon brake fluid actually floats on top of water... AND DOT3, also.

Sorry for any confusion created.

G.

Heres the link to the TB on how the military converted trucks to BFS.

http://test.steelsoldiers.com/upload/M35/TB43-0002-87.pdf

You got to go with one or the other. You can run either or in the system but you can't mix them. DOT 3 absorbs water and moisture putting your brakes to possible corrosion and failure. On the other hand but a little bit more pricey. DOT 5 does not absorb water and will preserve your brake system.

Thanks everyone for your suggestions!

As for cucv1833: You can test what you got in there. Either spoon some off the top or use syringe to suck a sample of the fluid floating on top. Then put it in a clear jar with some water and shake it vigorously and let it set for 30 minutes or so.... If the brake fluid floats on top after half an hour than it is Dot 5 if it is still mixed together after that half hour, then it is still Dot 3. You can repeat the process with the stuff on the bottom if you want to.
In my case the two were very obviously different and when the fluid level was lowered the jelly stuff on the bottom even poked up through the smooth surface of the Dot 5.

I will definitely get to work on this ASAP

is that what you are talking about....?

Blew a wheel cylinder on my 1009, now I have to figure out what's in my system. Will try the water test above. Has anyone had success with the test?

If you blew a wheel cylinder. Why don't you just put in what you would like to use, since you will be bleeding the system anyway....?

gungearz said:

If you blew a wheel cylinder. Why don't you just put in what you would like to use, since you will be bleeding the system anyway....?

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To change fluids you need to throughly flush and clean the master cylinder ,all the lines and wheel cylinders not just push new fluid through it. You need to remove all residue . I would switch fluids without rebulding the master cylinder and all wheel cylinders and flushing lines with a cleaning solvent..

Correct porky, should've added that. Flushing is. Required when switching over...

While I haven't done a conversion like this, I'm really skeptical of getting all the old stuff out by flushing, even with a cleaning solvent.

I think the rebuild approach is the only way to be sure you have got it right.

We're talking about brakes here. It's no place for half stepping.

EDIT: Well, I don't know what I'm talking about. The TM linked above DOES describe blowing the system out with air, then flushing with the new brake fluid. No rebuild. I guess the military found it works well enough to commit the process to paper.


However, one thing I noticed. It specifically warns AGAINST using any solvents. From page 3:

CAUTION Do not use solvents of any type to flush brake system.

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I'd kind of agree on the not using any solvents idea. What solvent will you use that will dissolve the sludge and buildup of both the DOT 3-4 and DOT 5 type fluids? I don't think there is anything out there that will clean it out without leaving behind some sort of residue that will cause other damage. The best thing to do is remove the master to replace the seals, remove all the slaves and replace their seals and while its apart blow out the lines with air. Once you've got everything resealed and back together don't just fill the master and pump it out again. Fill it from the bottom up using a pressure system by attaching a line from you're pressure tank to the individual bleed screws. That insures that the remaining crap is pushed back up where you can clear it out and also removes most of the residual air in the system. After doing that you can do a pressure bleed like normal with a catch bottle attached to your bleed screws with the drain line under the fluid. That prevents air from leaking back up into the system. As others noted the brakes are a lifesaving or life threatening system and you get to choose which. We've used the pressure fill setup on Indy cars for many years and also on aircraft. The air is trying to go up hill so why not help it out. Filling from the top is just backwards......


Frank