[TR] Dot 5 brake fluid

Sat Apr 29 16:56:30 MDT 2023

Well Phil, 

it is an engineering thing called “minimum standards” because when hydraulicly actuated brakes were first designed, anyone would put literally any liquid from boiled horse piss to castor oil in the hydraulic system without regard to cause and effect, and that of course caused people to get injured or die or cause major property damage when the brake line failed or the fluid boiled.  So much so that the government had to step in and regulate the automotive industry and establish minimum standards.

DOT numbers typically reflect changes in the minimum standards.

When you buy fluids for a “modern” car (mid 60’s onward), they specify the SAE standard number with its quoted release date.

It is the same for motor oil.  Most people buy their motor oil by the brand and viscosity rating like Valvoline 10W30 or Castrol 20W50, but the way people should buy their Motor oil is by the SAE specification, SAE Service range and equipment ambient temperature operating ranges as established by the vehicle manufacturer, and as stated in the owner’s manuals and TSB updates from the manufacturer. 

So when people argue about which brake fluid or motor oil is better, “better” is a relative term.  Each J170x standard represents improvements in the brake fluid formulations mostly designed for higher boiling temperatures and more consistent viscosity to reduce the potential of cavitation.

DOT 4 was created to reduce the potential of the fluid to cavitate when passing through a rapidly stroking valve in the ABS system.

For an LBC, my experience is either use Castrol LMA DOT3 Crimson, or DOT5, because you do simply do not know what the seals in the system are made of and those two fluids seem to not cause seal deterioration of seals made of a wide range of rubber materials – in my experience.  

My personal preference is DOT 5 (purple) because it does not eat my paint, I know it will not react with my seals, it has a higher boiling temperature than DOT 3, and is not hygroscopic which would cause water to pool at the system low points right at the seal to metal location and cause corrosion.

Let the “fluif” arguments continue!

Glenn aka StagByTriumph Garage

From: Triumphs <triumphs-bounces at autox.team.net> On Behalf Of Philip Ethier
Sent: Saturday, April 29, 2023 2:09 PM
To: Don Hiscock <don.hiscock at gmail.com>
Cc: triumphs at autox.team.net; Joel Justin <j_bar_j at hotmail.com>
Subject: Re: [TR] Dot 5 brake fluid

On Thu, Apr 27, 2023 at 3:32 PM Don Hiscock <don.hiscock at gmail.com <mailto:don.hiscock at gmail.com> > wrote:

The nomenclature around DOT5 (silicone) and DOT5.1 (glycol) never made any sense to me...

The stupidest decision imaginable.  Why not DOT6? Or DOT4.1? Just ridiculous. 

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