DOT3 vs DOT4 Brake fluid?

[GreenPat]

I've got a 26 year old Toyota for when there's too much luggage for my bike. It's a beauty, I've had it for 8 years myself, driven it across Australia twice, and up and down the East coast several times.



Rather than sign up to a cager forum, I'm sure the collective experience here could tell me if it's ok to put DOT4 fluid in the brakes? The manual says DOT3, but I have some DOT4 in the garage I think I used to top up the clutch a year or so ago, and I'm wondering if it would be a drama to top up the brakes with the same? For all I know there is already DOT4 in the brakes, but I can't be certain. So if it was, on the off chance, DOT3 in there now, and I mixed a couple of drops of DOT4 in with it, would it grow legs and kill my brake system?

[tbirdsp]

DOT 3 and 4 are compatible, no worries mixing them.

[GreenPat]

Thanks mate.

[JAB]

Nice truck. Keep your containers sealed tight, so no moisture gets in. NEVER mix DOT 5 with anything or vis-versa. 3-4 is OK.

[JimVonBaden]

Originally Posted by tbirdsp

DOT 3 and 4 are compatible, no worries mixing them.

But if your fluid you have to put in is a year old open container, dump it and get new! Brake fluid is hydroscopic and will retain water even though you sealed the container.

Jim

[jeffk14]

Originally Posted by JimVonBaden

But if your fluid you have to put in is a year old open container, dump it and get new! Brake fluid is hydroscopic and will retain water even though you sealed the container.

Jim

+1. As cheap as I am, I don't keep old brake fluid around.

It has always been my understanding that when a new DOT or SAE designation comes out, it's always backwards-compatible with the previous designation. EX: GL5 vs GL4 grease, SH vs SG oil, etc.

[Yossarian™]

Originally Posted by jeffk14

It has always been my understanding that when a new DOT or SAE designation comes out, it's always backwards-compatible with the previous designation. EX: GL5 vs GL4 grease, SH vs SG oil, etc.

Most of the time, yes.

However, you do NOT want to use DOT5 brake fluid in a system along with DOT3 or DOT4 (or DOT5.1)

[P B G]

Originally Posted by jeffk14

It has always been my understanding that when a new DOT or SAE designation comes out, it's always backwards-compatible with the previous designation. EX: GL5 vs GL4 grease, SH vs SG oil, etc.

With some exceptions, DOT 5 is actually not a new thing, not really atleast, it is a completely different beast that actually does not suspend water (for better or worse) and should just be completely avoided. 5.1 i'm not really familiar with.

As far as SH vs SG oil being back compatible, not truely. Just because something new comes out does not make it preferable, for instance, the new Diesel oils now come with a lower level of ZDDP, a metal additive that has long been in oils to prevent metal on metal wear of parts like cams. Unfortunately this additive tends to ruin catalytic converters at the same time. So while the new oils might meet certain specifications they aren't the same/better than the old ones in some instances. The choice to eliminate them is often driven by other concerns, in this case lowering the environmental impact of diesel trucks, which is where this low sulphur fuel issue comes from as well.

Others will argue about things like gear oil, where the old formulations were perfect for the old designs, going newer could possibly not work so well in the long run.

If you are getting to the point of topping off the brake fluid in your truck it is possibly the perfect time to completely drain the system. Just like your old bottle of DOT 4 shouldn't be used because it probably has picked up moisture your entire brake system in your truck probably has water in it. If you purge it fully you'll prolong the life of your brake lines/master cylinder.

[jeffk14]

Originally Posted by jeffk14

It has always been my understanding that when a new DOT or SAE designation comes out, it's always backwards-compatible with the previous designation. EX: GL5 vs GL4 grease, SH vs SG oil, etc.

Found this. Seems to partially support my assumptions. Not entirely though. Now my head hurts.

[Inane Cathode]

As i understand it, dot 3-4-5.1 are all glycol based, and dot 5 is silicone based. Glyco based fluids dont compress as much as silicone, but thats hardly even felt.

The most important part is how they both interact with water. Glycol will pull moisture out of the air and absorb it homogeneously, so the boiling temperature will go down over time. Silicone fluid isnt hygroscopic so it wont pull moisture out of the air, but (huge but) if water does enter the system it wont be distributed evenly and will remain a droplet. This droplet can stay in one place and corrode components, or make its way to the caliper and boil the second the fluid around it gets to the boiling point of water (or more really, if its under pressure)

The key difference between 3, 4, and 5.1 is the boiling point and to a lesser degree the hygroscopic nature of the fluid. I wish i could remember exactly what the difference is, but it has to do with the glycol base, i almost want to say one is ester based. Dot 5.1 boiling point is something like 100-150 degrees hotter than dot 3.

[markjenn]

Originally Posted by jeffk14

+1. As cheap as I am, I don't keep old brake fluid around.

If you do the calcs, the amount of moisture you introduce into a brake fluid container via a single opening and then resealing is minuscule. Personally, I think the key is to make sure the container stays sealed after use so the air can't circulate and continually introduce new moisture to the fluid. And to avoid too many openings/closings which would keep replenishing the moisture supply. To always require brake fluid be from a brand new sealed container seems over the top to me.

- Mark

[GreenPat]

Jeebus, I haven't been back here since my last and was about to go and do the top-up. Thing is, I'm not all that cheap, but I like to spend all my money on my bike and only the bare minimum on the ute. Water in the brake fluid from a topup of maybe 5% of the total fluid in there out of a bottle that may or may not be a year old and was only opened once doesn't concern me as much as say; the air leak in my fuel line, or the amount of rust in the vehicle...

Thanks for all the advice though.