DOT 3 versus DOT 5
Thread Starter
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Fleet Owner
Joined: Dec 2000
Posts: 25,431
Likes: 5,439
From: Central PA
DOT 3 versus DOT 5
My Master Cylinder was leaking on my 54. I rebuilt it about 7 years ago. So I bought a new one at NAPA. I bought a big bottle of DOT 3 brake fluid.
I was expecting a new low pressure brake switch from Mid-Fifty to come in the mail this afternoon. It didn't come so I had to stop working on the truck. Don't worry I mowed the lawn, sucked up and mulched leaves and put away patio furniture instead to free up time for Saturday.
Tonight I re-read a page from the book "How to Rebuild Your Ford Truck", the page about master cylinders and bleeding. The author talks about DOT 3 and DOT 5.... He says DOT 5 is better for your system.
So, how many of you guys use DOT 5? Do you like it better than DOT 3?
And can I just put DOT 5 in my system without totally purging my brake lines? Can 3 and 5 be mixed? Or when I bleed the brakes will the DOT 3 come out?
Thanks, I want to get on this tomorrow as soon as the mail comes.
I was expecting a new low pressure brake switch from Mid-Fifty to come in the mail this afternoon. It didn't come so I had to stop working on the truck. Don't worry I mowed the lawn, sucked up and mulched leaves and put away patio furniture instead to free up time for Saturday.
Tonight I re-read a page from the book "How to Rebuild Your Ford Truck", the page about master cylinders and bleeding. The author talks about DOT 3 and DOT 5.... He says DOT 5 is better for your system.
So, how many of you guys use DOT 5? Do you like it better than DOT 3?
And can I just put DOT 5 in my system without totally purging my brake lines? Can 3 and 5 be mixed? Or when I bleed the brakes will the DOT 3 come out?
Thanks, I want to get on this tomorrow as soon as the mail comes.
Are we there yet?
Joined: Mar 2006
Posts: 1,098
Likes: 64
From: NW Valley AZ
You must completely purge the DOT 3 before you put any DOT 5 in. They do not mix.
I run DOT 5 because it doesn't absorb moisture and it won't eat the paint if there is a leak. Some people will say it's bad for many reasons but I have never had a problem with it.
I run DOT 5 because it doesn't absorb moisture and it won't eat the paint if there is a leak. Some people will say it's bad for many reasons but I have never had a problem with it.
Logistics Pro
Joined: Dec 2005
Posts: 4,644
Likes: 55
From: Moro Bay, AR
The only way I'd change from one to the other is when completely rebuilding the whole system, they will not mix. As was said, Dot 5 doesn't absorb water, but any water that enters the system will puddle in the bottom of the master and wheel cylinders. This will corrode the metal much faster than the moisture in Dot 3.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven allmost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven allmost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
Thread Starter
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Fleet Owner
Joined: Dec 2000
Posts: 25,431
Likes: 5,439
From: Central PA
The only way I'd change from one to the other is when completely rebuilding the whole system, they will not mix. As was said, Dot 5 doesn't absorb water, but any water that enters the system will puddle in the bottom of the master and wheel cylinders. This will corrode the metal much faster than the moisture in Dot 3.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven almost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven almost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
Thanks guys!
abe
Hotshot
Joined: Oct 2009
Posts: 16,205
Likes: 4,818
From: Burbank, WA
If your bottle of DOT 3 has been open and on the shelf for 7 years, you may want to consider getting a fresh can. It will have absorbed moisture just sitting around and may be a factor in a premature failure to your brake system.
Fleet Owner
Joined: Jul 2004
Posts: 27,304
Likes: 1,062
From: NM
I'd recommend using DOT-4. It is essentially higher-grade DOT-3, still glycol-based, and is totally compatible with DOT-3 (but you should flush the whole system with the new stuff to get maximum benefit). It's very little more $$ than straight DOT-3.
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Postmaster
Joined: Apr 2004
Posts: 3,005
Likes: 2
From: Santa Clarita
The difference between the DOT (Department of Transportation) ratings of brake fluid are the boiling point ranges that they achieve both dry (no water absorbed) and wet (about 3-4% water content). For DOT 3 the dry boiling point is at least 401 and the wet 284 degrees. DOT 4 raises the bar to 446 and 311. The higher boiling points are critical if you spend time on a race track. The temps on a disc rotor can easily exceed those temps and the DOT 4 and 5 would be a better option. Once you boil your brake fluid it will not recover and you MUST bleed and change out the fluid. You will know because the brake pedal will start to feel mushy as the fluid overheats and the pedal may even go to the floor. For a daily cruiser, DOT 3 has worked for several years. I would recommend changing and flushing your brake lines periodically especially if you run DOT 3. As mentioned, DOT3 and 4 will absorb moisture from the air and contaminate fluid.
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Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
I have built two trucks and used DOT 5 in both. I prefer it because my truck sits for about 5 months every winter. Sitting is the worst thing you can do for brakes. I have had other older cars with Dot 3 or 4 and had to rebuild wheel cylinders about every three years. That didn't happen with DOT 5. As said you cannot mix Dot 5 with Dot 3 or 4 and you must purge all trace of the DOT 3 from cylinders and lines if you convert to Dot 5. It is best to start with all new lines, cylinders, and/or calipers. 
Refyred
Joined: Feb 2010
Posts: 2,083
Likes: 12
From: Southern Nevada
The only way I'd change from one to the other is when completely rebuilding the whole system, they will not mix. As was said, Dot 5 doesn't absorb water, but any water that enters the system will puddle in the bottom of the master and wheel cylinders. This will corrode the metal much faster than the moisture in Dot 3.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven allmost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
I have been running Dot 5 in my 53 for 6 1/2 years and a little over 80K miles, but it is driven allmost daily. For an occasional driver, I would stay with Dot 3. Another downside to Dot 5, it does compress some, giving the pedal a softer feel.
Any way thats my experience so far. I did stay with the DOT 5 after replacing the cylinder. So I will monitor and flush the system annually, and see what happens next.
Cargo Master
Joined: Apr 2005
Posts: 3,338
Likes: 167
From: Harpswell Maine
A downside To Dot 5 is that when you bring it into your shop it is silicone based and if you prep and paint you are risking silicone contamination of to be painted surfaces. Unfortunately I have experience with this problem.
Fleet Owner
Joined: Jul 2004
Posts: 27,304
Likes: 1,062
From: NM
One thing I used to hear a lot, mostly on systems converted from -3 or -4 to -5, is DOT-5 systems that would get spongy when going from a low altitude to a high one (worst possible scenario!). DOT-5 absorbs more air than -4. Air dissolved in the fluid boils out at the lower ambient air pressure at high altitude. This usually just makes the pedal mushy, but can be extreme enough to result in a loss of brakes. Since some of my driving includes going from 5300' to 8,000' or more, just another reason to stick with -4.
Lead Driver
Joined: Aug 2006
Posts: 8,096
Likes: 656
From: Brentwood, TN
One thing I used to hear a lot, mostly on systems converted from -3 or -4 to -5, is DOT-5 systems that would get spongy when going from a low altitude to a high one (worst possible scenario!). DOT-5 absorbs more air than -4. Air dissolved in the fluid boils out at the lower ambient air pressure at high altitude. This usually just makes the pedal mushy, but can be extreme enough to result in a loss of brakes. Since some of my driving includes going from 5300' to 8,000' or more, just another reason to stick with -4.
Logistics Pro
Joined: Dec 2005
Posts: 4,644
Likes: 55
From: Moro Bay, AR
I never did notice any changes in my brakes during my trip in '12 and I had quite a few altitude changes.