At 04:21 PM 8/19/2010, mcneely4@cox.net wrote:
>David, I may be wrong (never happened before ;-) ), but I think
>there are some incompatibilities in materials, such that chemical
>interaction, or lube problems may occur whenever 4 is substituted
>where 3 is specified or vice versa.  There are some differences in
>chemistry, hence the different names.  If DOT 4 fluid could always
>replace DOT 3, why wouldn't the manufacturers just drop the DOT 3?

No doubt because it's less expensive.

But -- as of 2003 ChevronTexaco thinks resellers ought to only carry
DOT 4: "Widespread suitability for different motor vehicles provides
inventory benefits since all conventional systems can accommodate DOT
4 fluids, and vehicle manufacturers are increasingly specifying this
level of performance."

They cite these applications:

All hydraulically operated motor vehicle braking
systems (drum and disc types) for which a DOT 4 or
SAE J1704 fluid is specified

Make-up *or service fill* [my emphasis] of braking systems requiring
DOT 3 or SAE J1703 fluids

Vehicles with anti-lock (ABS) braking systems

Hydraulic clutch systems requiring conventional fluids

Passenger cars, commercial road transport, offhighway
vehicles, agricultural tractors and
motorcycles

For vehicles operating in extreme service conditions, or
where a DOT 5.1 fluid is specified, Caltex Brake and
Clutch Fluid DOT 5.1 is recommended.

Not to be used in systems designed for mineral oil based
fluids (LHM), e.g., certain Citroen models, or where
Silicone DOT 5 fluids are recommended.

http://japcokwt.com/pdf/Brake%20Fluid%20Dot%204.pdf

>but I'll check later to see what if anything the book states, beyond
>specifying a fluid.

I'd be extraordinarily surprised if it didn't say use DOT 3 or DOT 4
but never never never use DOT 5.  There is now a DOT 5.1 (terrible
nomenclature) that is once again backwards-compatible with other
glycol-based fluids such as 3 and 4.  Look on page 140 or in the near vicinity.

Yours,
David