Honda brake fluid required?

[NH123]

When I bought my NSX about 7 years ago, the previous owner had been using Motul RBF 600 Dot 4 fluid, which I have continued to use (from a new sealed container) when I replace the fluid every other year.

Several weeks back my 2011 TL with 80k developed a seized caliper. I had previously replaced the brake fluid with a Castrol Dot 3 about 30k before this happened. After replacing both front calipers as well as the rotors and pads, I found in the TL owners manual that only Honda Dot 3 fluid should be used. It went on to state that an aftermarket Dot 3 or 4 was acceptable only as a temporary replacement (sealed container) and should be replaced as soon as possible with Honda fluid to avoid corrosion. So I installed Honda fluid.

Now it is time again to replace the brake fluid in the NSX. The NSX owners manual makes the same statement about only Honda fluid to avoid corrosion. Do I need to only use Honda fluid?

 

[jwmelvin]

Poppycock

 

[Old Guy]

All brake fluids save DOT 5 (which you can't use) are hygroscopic (absorb moisture out of the air). That is why regular replacement is a good idea. Honda brake fluids are not immune to the problem. Maybe Honda DOT 3 has better, or they think they have better corrosion inhibitors. However, Honda still recommends replacement every 2 years and if you respect that recommendation, you are probably OK (reliability wise) with any respectable brand brake fluid.

Was it confirmed that corrosion was the cause of the seized caliper? The only cases of calipers seizing up due to corrosion that I am familiar with are on cars that have been parked for a significant period of time. If the car was being used regularly, I struggle to envision a scenario where the caliper could be working and then seize up due to corrosion within the caliper bore.

 

[Vanishing Point]

Use Motul 5.1 full synthetic fluid, higher temp range and no water absorption worries.

 

[Old Guy]

Use Motul 5.1 full synthetic fluid, higher temp range and no water absorption worries.

Be careful with the terms. To quote from Motul's product description:

Long life fluid:
The high wet boiling point (185°C / 365°F), superior to DOT 4 (155°C / 311°F mini) and DOT 3 (140°C /
284°F mini) conventional fluids enables to use this product longer. Indeed, DOT 3 / DOT 4 and DOT 5.1
brake fluids have the property to absorb humidity contained in the air, which reduces their boiling points
and consequently security.

DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol based fluid. Polyethylene glycol is hygroscopic and will absorb water from the atmosphere. I don't know what full synthetic means (since polyethylene glycol is not a naturally occurring chemical, perhaps all brake fluids are synthetic?); but, DOT5.1 is not a silicone based brake fluid and most definitely suffers from moisture absorption. It is compatible with DOT 3 and DOT 4 and definitely not compatible with DOT 5. The DOT 5.1 specification, as noted by MOTUL has a higher minimum boiling point than DOT 3 or DOT4.

DOT 5 does not suffer from the moisture absorption problem; but generally, cannot be used in cars with ABS braking systems because of its viscosity. It also has some other characteristics which makes it undesirable for racing / high performance applications.

Sometimes I think the DOT would have done everybody a favour if they had called DOT 5.1 something like DOT 6 to eliminate confusion with the DOT 5 standard. It is definitely not an overlapping product.

 

[NH123]

The TL is driven daily, although it sat for about a month at Acura, due to the air bag recall. The brake problem was noticed on the way home from picking the car up.
I replaced the calipers myself and found with the drivers side caliper that both pistons were seized. The rubber boots looked fine (not cracked or damaged).

All brake fluids save DOT 5 (which you can't use) are hygroscopic (absorb moisture out of the air). That is why regular replacement is a good idea. Honda brake fluids are not immune to the problem. Maybe Honda DOT 3 has better, or they think they have better corrosion inhibitors. However, Honda still recommends replacement every 2 years and if you respect that recommendation, you are probably OK (reliability wise) with any respectable brand brake fluid.

Was it confirmed that corrosion was the cause of the seized caliper? The only cases of calipers seizing up due to corrosion that I am familiar with are on cars that have been parked for a significant period of time. If the car was being used regularly, I struggle to envision a scenario where the caliper could be working and then seize up due to corrosion within the caliper bore.