WARNING - DOT 5.1 Brake Fluid

I stopped by the local auto parts store today (the same one who actually has Deuce wheel bearings in stock!) to pick up a can of DOT 5 brake fluid to top off my master cylinder. No problem, he trotted out a can of brake fluid labeled DOT 5.1.

What a deal, I thought at first. They must have upgraded the DOT 5 specification. Then I read the fine print on the can. DOT 5.1 is NOT SILICONE brake fluid, and is "fully compatible with DOT 3 and DOT 4 brake fluid". Which means to me that it is hygroscopic, i.e., it absorbs water.

Sorry, but this is NOT what I wanted. I pointed this out to the clerk, and he replied G**D***, you are right! This stuff is NOT real DOT 5 fluid. It kind of caught him off guard, but he promptly placed an order for REAL DOT 5 brake fluid from his warehouse. Won't be here until tomorrow, but no big deal for me.

Just be warned though - If someone tries to sell you DOT 5.1 brake fluid, it is NOT the same as or an upgrade to DOT 5. DO NOT buy DOT 5.1 brake fluid if you already have DOT 5 in your brake system! Stick with regular DOT 5!

I was curious about the real DOT5. Since it does not absorb moisture is it ok to continue using fluid from a previously opened container? I want to buy the big jug and save some money.

a vet told me that they have used water in the brake systems before to get them back to the shop

A tightly sealed contanier of previously opened DOT5 stores just fine. Also, if you buy surplus DOT5 and the purple has turned to amber, thats no problem, it still works. I buy it in gallons surplus and as soon as I crack the last gallon, I order more!

Ethan, everything I have read on the subject says that DOT 5, being silicone based, is not hygroscopic and will not absorb water from the atmosphere. Therefor I would think that you could keep a jug of DOT 5 tightly sealed on the shelf for an indefinite period and use as needed until gone. When the silicone based fluid is in your brake system it will not disperse water like DOT 3 or 4 will so water will collect at the lowest point in the system, ie the wheel cylinders, which is another good reason to do seasonal bleeding of the sytem. The water would boil at just over 200 deg while the DOT 5 won't boil until around 500 deg. Slugs of water in your DOT 5 system could boil and cause a vapor lock at the wheel and a spongy pedal.

Worse yet, water collected at low points will freeze.

I may be alone with the opinion that a bit of DOT 3 added to DOT 5 may be beneficial to keep water from freezing. Well I said it.

Personally I like and use DOT 3, cheaper, always available and can handle water.

Good point Bjorn for those of us in the chillier parts of the world. Course being that it was a freakish 70+ here today, we might not see another freeze until next winter

Yeah I was talking about scheduling a ski trip, but what is point when I am wearing shorts in January.

So Bjorn I though that 3/4 and 5 did now mix at all. Are you mixing them in your truck? I kind of figure it would just be easier to run all 3 or 4 in your system if you are worried about moisture. These trucks used to run it in the past if I am not mistaken.

Yeah, you are not kidding Chris, I have been outside all week in short sleeved shirts. I'm not complaining.

When I was in College for advanced automotive mechanic systems, we were giving a list of household items, and was asked what would substitute brake fluid in ANY vehicle with hydraulic brakes, and not have a severe chemical breakdown or ruin valuable parts. The answer?! Coca-Cola!! ( Blew my mind also)

DOT 5 was desirable for motorcycle racers because it doesn't get water in it and doesn't have the same range for boiling when brakes are hot from use. BUT it's more expensive and harder to get. DOT5.1 was made to have the compatibility with 3 and 4 (glycol based) but have the characteristics with regards to boiling point of DOT 5.

Boiling points

Minimal boiling points for these specifications are as follows:
Boiling Point Ranges
Dry Boiling Point Wet Boiling Point
DOT 3 205°C (401°F) 140°C (284°F)
DOT 4 230°C (446°F) 155°C (311°F)
DOT 5 260°C (500°F) 180°C (356°F)
DOT 5.1 270°C (518°F) 191°C (375°F)

Ryan - I don't have a problem with the boiling points, but I do have a problem with DOT 5.1 apparently being hygroscopic. Considering the M35 is a 57 year old design, and has been out of production for 20+ years, I don't see any advantage to introducing brake fluid which will absorb water into a brake system which in my case is 30 years old. Especially since the DOT 5.1 fluids cost as much as the regular DOT 5 fluid.

Oh, Granted, the 5.1 is made to be compatible with 3 and 4 because as I understood it a conversion from 3 or 4 to 5 required a complete system flush because the glycol based fluids aren't so compatible with the silicone based DOT 5.

For our MV's the best thing to go with is Silicone based DOT5.

Re: RE: WARNING - DOT 5.1 Brake Fluid

cranetruck said:

Worse yet, water collected at low points will freeze.

I may be alone with the opinion that a bit of DOT 3 added to DOT 5 may be beneficial to keep water from freezing. Well I said it.

Personally I like and use DOT 3, cheaper, always available and can handle water.

Click to expand...

Don't know about the antifreeze effect, but it will create one gelled mess when mixed.

Smitty

The studies done at length by the U.S. Army prior to the adoption of BFS (DOT 5) showed no adverse affects of mixing DOT 3 and DOT 5, other than negating the benefit of DOT5

I had tests done at a reknown lab on the same subject - which interestingly showed that when DOT 3 & 5 were mixed, the colors exchanged - that is, the DOT3 became purple, and the DOT5 clear (or nearly so) - but no other problems with the combination.

The Army studies (both of them) specifically say that there is no problem mixing them. Does anyone here have specific, first hand knowledge (not "my neighbor", "my cousin", "the guy at the parts store said") counter to this?

This is a sincere question, though I'm really tired and it may sound otherwise.

Regards,
David Doyle

Re: RE: WARNING - DOT 5.1 Brake Fluid

DDoyle said:

The studies done at length by the U.S. Army prior to the adoption of BFS (DOT 5) showed no adverse affects of mixing DOT 3 and DOT 5, other than negating the benefit of DOT5

I had tests done at a reknown lab on the same subject - which interestingly showed that when DOT 3 & 5 were mixed, the colors exchanged - that is, the DOT3 became purple, and the DOT5 clear (or nearly so) - but no other problems with the combination.

The Army studies (both of them) specifically say that there is no problem mixing them. Does anyone here have specific, first hand knowledge (not "my neighbor", "my cousin", "the guy at the parts store said") counter to this?

This is a sincere question, though I'm really tired and it may sound otherwise.

Regards,
David Doyle

Click to expand...

I guess I might have a little experience with this. I spent 26 years in the vehicle maintenance side of the house in the Air Force. We found that it would take a while for the fluid to start to gel when mixed, but it would gel. If you look at the label on a commercial bottle of Dot 5, it will have a warning stating that it should not be mixed with Dot 3 or 4.

Smitty

RE: Re: RE: WARNING - DOT 5.1 Brake Fluid

I think I'll stick with DOT 5. I really don't Need DOT 3 or 4 or 5.1 introducing water into my brake system and further corroding my brake cylinders and master cylinder cylinder any more than they are already corrupted!

RE: Re: RE: WARNING - DOT 5.1 Brake Fluid

I am going to use the coke idea and run the vent line into the cab like a straw.

RE: Re: RE: WARNING - DOT 5.1 Brake Fluid

I am going to use the coke idea and run the vent line into the cab like a straw.

Click to expand...

We need to figure out a way to cool it. Warm coke is not pleasant.

Re: RE: WARNING - DOT 5.1 Brake Fluid

1956_4x4 said:

We found that it would take a while for the fluid to start to gel when mixed, but it would gel.

Click to expand...

Any idea how long this would take? I wanna investigate this closely, and in a controlled enviroment.

Thanks for the input.

David