M151 brake question

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Sundawg3
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M151 brake question

Unread post by Sundawg3 » November 20th, 2023, 8:30 pm

Gentlemen,

I found silicone brake fluid in Dot 5, purchased an 11 oz can this afternoon. Should have checked before buying but... that would have been the smart thing to do. Decided to run a long screwdriver in the hole and dripped a little fluid on a paper towel. The fluid looks clear or much like Dot 3, 4 or something NOT Dot 5 which I'm told is purplish or blueish. The previous owner passed so I can't ask him what he used. Everything I've read says NOT to mix. Should I leave it alone and add a little Dot 3 or flush, bleed and fill with Dot 5?

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rickf
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Re: M151 brake question

Unread post by rickf » November 20th, 2023, 11:26 pm

You need to find out what is in there to begin with. get a small clear jar and put some water in it and then drop some of the fluid from the master cylinder into the water and watch what happens. If the drops stay whole and float it is silicone DOT5 and if the drops dissipate and possibly turn milky as they do then it is DOT3. If you have 5 and you want to change to 3 you need to completely drain the system which means pulling the wheels and popping open the wheel cylinders to empty them out. Put the wheel cylinders back together but do not hook up the lines yet. Gat a turkey baster and suck all the fluid out of the master cylinder unless it gravity drained out on its own. If it gravity drained out then you are ready for the next step. Add the DOT3 and pump the brake pedal but do not push it all the way to the floor, this can damage the seal on the piston by pushing it into gunk that has accumulated in the area where the piston normally does not go. Once you start getting good clean fluid out of the lines you can hook them back up and bleed as usual. Again, try to avoid pushing all the way to the floor.

Now, Here is another thing I tell people that get a new to them vehicle that they know no history on, if it is rubber then replace it! You do not know how old it is and rubber rots. This means all hoses and belts. Brake hoses are rubber so now would be a good time for replacement. This is a single stage brake system so if anything fails and you lose pressure you lose ALL of your brakes!!!!! Also, M-151's that have been neglected and sitting seem to have a very high failure rate of wheel bearings since they are such a pain to access. I know you do not want to hear all of this, you just want to drive it. But if you just drive it and the brakes fail................ well, that is self explanatory. If the bearings fail it will be very expensive. And, you will not be driving it for a while. U-joints, check them and grease them. You have 12 of them!!!
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

steveo
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Re: M151 brake question

Unread post by steveo » December 6th, 2023, 11:56 am

I know this is an older post but I have seen
Prestone brake fluid for sale at the local auto parts store and was labeled 5.1 it’s synthetic but not silicone.
I almost grabbed it off the shelf seein that 5 and not the .1 at first till I got it in my hand.
it was the only bottle on the shelf. No straight dot5.
Not sure if that’s a clear fluid like dot3 or purple like silicone we use for the mutt.

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rickf
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Re: M151 brake question

Unread post by rickf » December 6th, 2023, 3:06 pm

5.1 is compatible with DOT 4 and 3 fluids but it would not be worth it due to it's rather high cost. DOT 5.1 is mainly used in high performance applications where very high heat will be encountered. It has a very high boiling point. It is NOT silicone fluid and is not compatible with the DOT 5. It is very misleading I agree. I am sure there have been more than one accident caused by mixing them and destroying the system.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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glcaines
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Re: M151 brake question

Unread post by glcaines » December 8th, 2023, 7:33 pm

One side note is that although DOT 5 is purple when new, it usually turns an off-yellow and sometimes brown with age. As Rick mentioned, always test the fluid prior to adding anything.
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m3a1
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Re: M151 brake question

Unread post by m3a1 » December 9th, 2023, 11:49 am

A lot of us yellow with age.

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svramselaar
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Re: M151 brake question

Unread post by svramselaar » December 10th, 2023, 6:32 am

hi

it is not the answer for the qwestion bud mabey usefull

look at the TB 43-0002-87

http://www.jatonkam35s.com/DeuceTechnic ... 0fluid.pdf

george

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rickf
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Re: M151 brake question

Unread post by rickf » December 10th, 2023, 11:09 am

This brake fluid thing has kind of gone off the rails. Lets start from the beginning. Sundawg, What is it you are looking to achieve? Do you want to convert over to DOT 3 fluid? If so and there is any question at all about what you have I would strongly suggest you pull the drums and pop the caps off of the wheel cylinders and drain the entire system. Be sure to remove the brake shoes first so you do not contaminate them with fluid. Suck out all of the fluid from the master cylinder with a turkey baster and pump the pedal a few times but no more than halfway to avoid damaging the piston. This will empty out the entire system of whatever fluid was in there. Now, put in whatever fluid you are going to use, if it is DOT 3 then that is fine. Add it to the master cylinder before you close up the wheel cylinders and pump about a half master cylinder full through the system. Make sure the new fluid is coming out of all the wheel cylinders. Now you can put it all back together, fill it up bleed it and done.

This procedure will eliminate any chance of cross contamination of fluids no matter which one you decide to use. You do not know what was done before you got it. I have seen more than my fair share of brake systems that have had mixed fluids and it is not pretty.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Sundawg3
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Re: M151 brake question

Unread post by Sundawg3 » December 10th, 2023, 6:23 pm

I had no clue what was in it when I purchased. This morning I did your test and believe it's probably Dot 3. I put water in a glass and dripped some fluid from the master cylinder in. The crappy dark fluid slowly fell to the bottom and dissolved turning the water milky white as you said it would. For the hell of it I poured a couple drops of the Dot 5 to see what you were talking about. Sure enough it floated, even with a little shake it still floated and somehow the little drops found themselves into one. Rickf, your words sunk in and I've ordered all new rubber, already replaced a lot. I have new rubber brakes lines ready to install. Now that the fluid in the master cylinder has been determined, which really doesn't matter since I'll be draining soon, I guess I need to ask what to fill it with after installing new lines and cleaning the brake cylinders? Dot 5 as the military says? Or is Dot 3 what most of you run?

Thanks

Brad

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rickf
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Re: M151 brake question

Unread post by rickf » December 11th, 2023, 10:56 am

I use DOT3, It has worked on antique cars since they went to hydraulic brakes. As long as you flush it out every five plus years depending on where you live it is fine. Contrary to what people will tell you military systems with silicone fluid in them DO get moisture in them. As brakes wear there needs to be air introduced in some way when you add fluid and if you are in a high humidity area you will get moisture. The difference being that the moisture does not mix with the fluid. It just finds the lowest spots and sits there. That would be the bottom of the wheel cylinders. There is no way to remove this moisture other than opening the wheel cylinders. Flushing does not get stuff off the bottom. With DOT3 the moisture stays in the fluid so in a few years you suck the fluid out of the master and then add new fluid, then bleed the brakes until you get clean fluid at the wheels and you are done. Moisture is gone with the old fluid.
Where do you live?
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Sundawg3
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Re: M151 brake question

Unread post by Sundawg3 » December 11th, 2023, 3:01 pm

Cincinnati Ohio, high humidity.

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rickf
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Re: M151 brake question

Unread post by rickf » December 11th, 2023, 7:04 pm

And high rust!!! I will get some pics of the wheel cylinders I just took off of my M37. They were the perfect storm. They were replaced while the system had DOT5 in it and then instead of opening the wheels back up and taking the cylinders apart the guys "mechanic" just flushed the system with DOT3 and called it good to go. Well, In addition to having the wheel cylinders on backwards the flush just mixed the two fluids together and left the water in the bottom. I have since rinsed out the goo that was in there, I though about getting pics but I was not touching my phone covered in brake fluid. But you can see clear as day where the water rusted the bottoms of the cylinders. I will try to get pics tomorrow.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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