Determinining which is in system Dot 3 or Dot5 empty master cylinder

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EO2NMCB

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M813 that came from GSA had working brakes, 6 mo later I move the truck and pedal goes to the floor. With the master cylinder now being empty how does one determine if it has DOT 3 or Dot 5 in the system? It had been a VFD truck, is it possible they received the truck before the Military switched over? Has NO stencil on the dash pertaining to Dot 5.

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doghead

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Use a vacuum pump and suck some out of a wheel cylinder bleeder.

Anything is possible...
 

Tow4

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if you are going to work on the brakes later just put Dot3 in as it's easier to find. Having mixed Dot3 and Dot5 I found they don't mix. They stay separated but don't interact with each other. I have converted all my Dot5 trucks that needed brake work to Dot3.
 

Ajax MD

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I dunno if I agree with this sentiment.

Yes, you can pull into any parts house or even grocery store to purchase DOT 3 but DOT 5 isn't difficult to find. It's available from all the common vendors on this site, and eBay. Just be proactive and purchase a gallon to keep in your spares locker.

DOT 3 absorbs moisture, DOT 5 doesn't. DOT 5 offers a higher boiling point which reduces the risk of "fade out" during prolonged, heavy braking incidents. I don't necessarily do things "because the Army did it" but this is one instance where I think the military was correct. Just my opinion...which is worth exactly what you paid for it.

As far as how to tell what kind of fluid is extracted from your brake system, I don't have a clear answer. The DOT 5 from our vendors usually has a purple color, but this may be eliminated after sitting in the brake lines for years.

You could try mixing a sample with some new DOT 3 in a glass jar. If they mix, it's all DOT 3. If they do NOT mix, then you have DOT 5 in your system.
 

Tow4

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I dunno if I agree with this sentiment.

DOT 3 absorbs moisture, DOT 5 doesn't.
This is the problem. Water sits in bottom of the wheel cylinders and no amount of bleeding will remove it because the bleeder is at the top of the cylinder. My M35A2 required the replacement of all 6 wheel cylinders that were not rebuildable because they were severely corroded on the bottom from water sitting in them. Dot3 suspends the water and it goes out when you flush/service the brake system as you should.

I stand by my statement.
 

Ajax MD

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This is the problem. Water sits in bottom of the wheel cylinders and no amount of bleeding will remove it because the bleeder is at the top of the cylinder. My M35A2 required the replacement of all 6 wheel cylinders that were not rebuildable because they were severely corroded on the bottom from water sitting in them. Dot3 suspends the water and it goes out when you flush/service the brake system as you should.

I stand by my statement.
The part about not purging the water by the bleed screw is worth noting and I will take that into consideration. I may end up changing my position, in time.
 

Elijah95

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This is the problem. Water sits in bottom of the wheel cylinders and no amount of bleeding will remove it because the bleeder is at the top of the cylinder. My M35A2 required the replacement of all 6 wheel cylinders that were not rebuildable because they were severely corroded on the bottom from water sitting in them. Dot3 suspends the water and it goes out when you flush/service the brake system as you should.

I stand by my statement.
This right here, and furthermore for the folks that claim anything that dot 5 isn’t up to the task for a heavy truck and boiling point of the fluid being too low, they now make a DOT 5.1 fluid that is compatible with dot3/4 but NOT DOT 5. So you can obtain proper boiling point and freeze protection without silicon fluid.

Simply put, I don’t run DOT 5 in anything I own, in the past as soon as I got something home I flushed the system with fresh DOT 4 and would run it for 20 miles in stop/go then come home and flush it through again.

Annually I like to flush a few OZ through every wheel cylinder. IMHO if you keep things clean, exercised, and don’t have a brake stick that overheats the seals, these things should last a LONGGGG time.


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Elijah95

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@Elijah95 Based on what you do with your trucks, are you saying that DOT4 and 5 are compatible? Is that how you're able to flush DOT5 out of the system?
Dot 4 and 5 do not mix at all, if you put the two in a cup you’ll see a clear separation and when agitated under a vacuum I found it will begin to create heat.

On an m35 for example, I take a shop vac and suck out the master cylinder then fill with DOT 4, suck it out again, repeat 2x. Then fill it up, and bleed all 6 wheels continuously topping off as needed until every cylinder comes out clear with new fluid. There will be some water left in the cylinders from the old DOT 5 but that’s what the 2nd flush gets rid of after the drive and absorption into the DOT 4.

Some folks say “you’re going to ruin your truck!” Or “it’s going to cost you to replace everything!” But over several trucks and 15k+ miles I’ve yet to experience a single failure yet along with quite a few other members here that are in the same boat


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Elijah95

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Very interesting, thanks.
You’re welcome, with that said, I personally wouldn’t recommend dot 3 even in an unloaded truck because what might be acceptable here in my state might be different in another when it comes to long downhill braking.

If you have the coin and want to upgrade over dot 5, I’d swap to dot 5.1 and if you’re ever on the road any parts house or Walmart will stock dot 4 which is compatible and can bail you out. If you go backwards mixing the two just assume DOT 4 protection specs.

Unless you’re working one of these things like a dog in the mountains, or very heavily loaded with constant braking going on, DOT 4 will fit 98% of the applications for these trucks. Most 5 ton guys could probably use it as well, but if there was the slightest doubt, I’d step up to the 5.1 which trumps 5 in every aspect including price. A gallon can be had currently <$40 while it seems Silicone keeps going up every year
 

royalflush55

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This is the problem. Water sits in bottom of the wheel cylinders and no amount of bleeding will remove it because the bleeder is at the top of the cylinder. My M35A2 required the replacement of all 6 wheel cylinders that were not rebuildable because they were severely corroded on the bottom from water sitting in them. Dot3 suspends the water and it goes out when you flush/service the brake system as you should.

I stand by my statement.
Where does the water come from in the bottom of the wheel cylinders below the bleeder screw if Dot 5 does not attract moisture?
 

Elijah95

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Where does the water come from in the bottom of the wheel cylinders below the bleeder screw if Dot 5 does not attract moisture?
No brake system is water tight, water can condensate into the master cylinder just like a fuel tank if low, and with DOT 5 it will settle at the lowest points


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Larry Weibert

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When I picked up one of my trucks years ago I had to use some dot 3 to get it home. Later on I blow air through the lines with the bleeders open. It seemed to get all the liquid out of the cylinders. I then power bleed it with fresh dot 5. I agree that the power bleeder wont move enough fluid quick enough to get it out of the cyl that's why I used air. I like dot 5 I think you can get it for 28 a gal somewhere on this site so its not that bad.
I like the power bleeder it takes 15 mins and the whole truck is fresh again with purple.

I found the place Iris Industries out of NJ $27 a gal 135 for 4 gals shipped i just ordered 4 gals.
 
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