DOT 5 & Seals

Hello All,

I'm new to the site, and thought I'd post my question under the mods section as it is abit out of the realm of MV stuff given my vehicle.

I have a 1970 Land Rover Series IIA, civilian spec, not military. I'm doing a frame-off and that means a brand new MC, brand new lines,new wheel cylinders, etc. - all of which are getting harder to find and more expensive. I'd like to fill the brake/clutch systems with DOT 5 and thought I'd come to the place that has a large collection of folks who have been using DOT 5 fluid for the longest.

What I am concerned about is DOT 5 attacking seals and hoses - rubber or synthetic.

I've read online that this can be a problem. Can anyone tell me if there is truth to this? With so many new parts, it's hard to know if seals will be rubber, silicon based man-made stuff.

Just so no one needs to waste alot of typing, here's what I do know :

1.) DOT 5 is hydroscopic
2.) it is GREAT for vehicles that see alot of down time - limiting rust in wheel cyl, etc.
3.) it is verboten in ABS systems due to the potential for foaming and it's inherent compressiblity.
4.) Not relevant, but mixing DOT 3/4 or 5.1 with DOT 5 will result in jelly and real problems.
5.) Bleeding is still necessary, but yearly is definitely enough.
5.) DOT 5 has a high boiling point.

So, I know abit, but not what really counts. That is, can I use it without worrying about damage to rubber or synthetic rubber.

Thanks in advance!

you can inter change dot 5 and 3 you just cant mix. Seals and lines ect will be fine.

ryan244 said:

I've read online that this can be a problem. Can anyone tell me if there is truth to this? With so many new parts, it's hard to know if seals will be rubber, silicon based man-made stuff.

Just so no one needs to waste alot of typing, here's what I do know :

1.) DOT 5 is hydroscopic
2.) it is GREAT for vehicles that see alot of down time - limiting rust in wheel cyl, etc.

Click to expand...

I'm in the same boat. I'm trying to decide if I should use DOT 5 in my M-37 but I have no information about what the seals are made of so I don't know if DOT 5 is safe or not and I haven't found anyone else that's using it. I may be wrong but I thought DOT 5 was NOT hydroscopic and that's what made it desirable for collector type vehicles that didn't get driven frequently since it doesn't absorb moisture that could cause rusting of the brake system components.

Couple of cons. Dot 5 doesn't absorb water. Any mositure that gets in will settle at the lowest point, or wheel clyinders in a deuce, and cause rust. Rubber seals swell a little with dot 3, and less prone to leak. Dot 5 can be cheap, but only by buying 1 gallon surplus cans. More expensive than dot 3.

I switched both of my trucks to 3 and am happy with them.

I too use dot3.

I always replace DOT 3 with DOT 5 because there aren't enough days on the calendar to bleed 20 vehicles every two years. Try to run an old lattice boom crane that uses brake hydraulics to control all of the clutches and brake bands, after the government didn't bleed the system for many years! DOT 5 is formulated with 1% tricresylphosphate to give proper compatibility with regular rubber seals. Without the TCP, seals shrink slightly. When DOT 3 and 5 are in contact, the 3 steals the TCP from the 5, and then seals shrink. DOT 3 is hydroscopic, (not DOT 5), and absorbs water right through the rubber lines, but DOT 5 is a great water repellent. A thin film of DOT 5 will prevent rust better than petroleum oil, because it does not evaporate as easily. I love DOT 5.

hygroscopic - Means to take up and retain moisture.
http://www.merriam-webster.com/dictionary/hygroscopic

I installed Dot 5 silicone in my M135 GMC during a brake overhaul back in 1983 and to this day all 12 wheel cylinders, the master, and the air-pack work great, do not leak and are not corroded. This truck gets driven very rarely.

Thanks NDT and Trogg. That's what I thought.

That's difference between brake fluid that is hydroscopic (DOT 3, etc) and the fluid used in a DOT 5 system. Prior to Dot 5, the fluids were hydroscoptic and would absorb moisture right out of the air and eventually rust ALL the steel components in the brake system. That's why you were supposed to periodicly flush the system, to eliminate any accumulated water. And you also had to periodically replace all the steel components, even the brake lines! With DOT 5, you don't have nearly as many rust problems because it doesn't absorb moisture. However if water is added to the system (highly unlikely IMO) then it will settle to the lowest point and will rust any steel found there. That's still a lot better than rusting everything in the system IMO! Since DOT 5 doesn't absorb moisture, periodic flushing should be completely eliminated unless the system is open and exposed to liquid water. If you have a habit of driving your vehicle where the brake system might be submerged it might still be a good idea to flush the system just in case any water managed to get in but other than that I think you could go many years with no problems.

Now if I could just find out if the seals, etc in my M-37 are compatible with DOT 5!

Trog & NDT,

When you converted to DOT 5, how thorough did you have to be about getting out the older brake fluid? Will draining the old fluid and a pressurized flush with DOT 5 do it or does the system need to taken completely apart and all the seals replaced?

I flushed the steel lines with isopropyl alcohol. Then blew them out with compressed air. The hoses were new. All the rubber parts were off the shelf standard parts intended for Dot 3.

Taking it apart, cleaning it and then refilling with DOT 5 is the best way. Because the DOT 3 is heavier, it would be very hard to convince all of the DOT 3t to leave the system through the air bleeder screws, which are mounted at the top of the wheel cylinders.

-Chuck

You want brake fluid to absorb any water that gets into the system. Hygroscopic is good!

trog said:

....DOT 5 is formulated with 1% tricresylphosphate to give proper compatibility with regular rubber seals. Without the TCP, seals shrink slightly. When DOT 3 and 5 are in contact, the 3 steals the TCP from the 5, and then seals shrink....

Click to expand...

OK, great. This sounds like there is ZERO impact on seals, synthetic or otherwise when you use DOT 5.

Yes, my bad on hydroscopic - what I meant and what I said were two different things.

I agree with the excellent point on it being a good thing that DOT 3/4 and 5.1 will absorb water, so you don't end up with a water bubble, but for infrequently driven vehicles, I don't think you can beat DOT 5 for protection of vital and hard to replace components. Unlikely that my truck will be a daily driver, and I keep hearing that folks have broken down their braking systems years down the line and found NO rust.

There is a military bulletin on the proper switchover from DOT 3 or 4 to 5, while I don't think it calls for replacing the 'soft' parts in the form of hoses, seals, etc., I definitely would. I'd think years of use would mean years of absorption of brake fluid into the rubber. Not worth the risk to me.

Thanks All. Appreciate the input.

As others have pointed out, DOT 5 fluid is hydrophobic, not hydrophylic.

DOT 3 is a glycol fluid. DOT 5 is a silicone fluid.

The types of rubber NOT recommended for use with silicone fluids are silicone rubber and fluorosilicone rubber. Like you were taught in high school chemistry regarding solubility, "like dissolves like".

I do not know what types of rubber are in the Rover brake system, but I doubt it has silicone or fluorosilicone.

EPDM rubber is the best with DOT 3 glycol and would also work fine with DOT 5...assuming EPDM is what they used.

Rubber fluid compatability reference - Parker O-ring Handbook, pages 7-9 & 7-10:

http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf

Regards

Jim

I thougth hydrophylic meant water-loving, or would absorb water. So I think DOT-5 is Hydrophobic, or water-hating, like wax. In other words, water is non-soluble in DOT-5, and that is why it will form a water bubble in the brake lines if it gets in there somehow. The like-dissolves-like is true in most instances. One exception might be with certain alcohols which may be polar, but still have a big enough chain of carbon atoms on them that they would repel water rather than dissolve in it. I also admit I could be wrong here since my last chemistry class/lab was at UF in 1993? Man, my brain hurts now...

akonitony said:

I thougth hydrophylic meant water-loving, or would absorb water. So I think DOT-5 is Hydrophobic, or water-hating, like wax. In other words, water is non-soluble in DOT-5, and that is why it will form a water bubble in the brake lines if it gets in there somehow. The like-dissolves-like is true in most instances. One exception might be with certain alcohols which may be polar, but still have a big enough chain of carbon atoms on them that they would repel water rather than dissolve in it. I also admit I could be wrong here since my last chemistry class/lab was at UF in 1993? Man, my brain hurts now...

Click to expand...

I stand corrected - hydrophylic means "water loving" - thanks.

Hydrophobic means incapable of absorbing water.

Hygroscopic means capable of absorbing water from the air.

My excuses are: (1) Last chemistry class ~1976, and (2) boozeheimers.

When it comes to the fact that silicone rubber is not compatable with silicone fluid the "like dissolves like" analogy is a stretch. Silicone rubber is a strange animal.

Regards

Jim

Notice the relationship between Si and C in the periodic table? It would have been interesting if God had decided to use Si for the basis of life on this planet, but then, I suppose, He would have had to do away with oxygen, or we'd all turn into piles of sand pretty quick.
I was hoping you would define "Hydroscopic" for me... I'm lost with that one
Here is an interesting website relating to that word: Hydroscopic and Hygroscopic

I was hoping you would define "Hydroscopic" for me... I'm lost with that one

From Merriam Webster Online:

Appears "hydroscopic" is not a valid word. An inconsistency in the naming convention.

Regards

Jim

Maybe 'cause they knew it would just be like what happened with the word "Inflammable"?