Provided you verify that the rubber (seals and hoses) are compatible you could
use DOT 5 - provided that you flush and clean all remnants of the DOT 3 or 4
fluid. I've seen some references to having to replace all the seals due to the
fact that they will hold small amounts of the old fluid.   Some also advocate
cleaning the brake system with alcohol to remove all the old fluid.

Most references I have read state that with DOT 5 fluid you will get a spongy
pedal feel over DOT 3 and 4 which most people do not like. In addition, DOT 5
does not mix with water, so any water in the system will migrate to the lowest
part of the brake system - potentially causing corrosion or boiling problems (if
near the caliper).   Mixing DOT 3/4 and DOT 5 will also cause a precipitate (aka
sludge) in the brake system. My impression is that with the exception of the US
military and Harley-Davidson, most everyone is moving away from the DOT 5
fluids.

Todd

Coby Smolens wrote:

> ANOTHER NOTE: Please be aware that silicone based brake fluids are NOT to be
> used (Bentley: "Do NOT add or mix DOT 5 silicone type brake fluid with brake
> fluid in car as severe component corrosion may result. Such corrosion could
> lead to brake system failure.") in normal circumstances, where existing
> brake parts are not replaced. Does anyone know if DOT 5 silicone can be used
> if you are using all new stock brake hydraulics and hoses? Or do the rubber
> components have to be a special type (silicone-based, perhaps)? This
> shouldn't be an issue, really since, as Todd brings up, there are
> glycol-based fluids available that match or out-perform DOT 5, so there
> isn't much incentive to make the switch. I think the silicone stuff is
> non-hydrophilic which means parts might last longer, but you'd want to
> change the fluid every couple years anyway, due to the inevitable
> accumulation of micro-particulate due to normal wear of internal parts.
>
> Coby Smolens
> Valley Wagonworks
> VW Bus and Vanagon Specialists
> "Intimately acquainted with VW Vans since 1959"
>
> 1535 SF Drake Blvd, San Anselmo, CA 94960
> 415-457-5628
>
> -----Original Message-----
> From: Todd Last [mailto:Rubatoguy@mindspring.com]
> Sent: Thursday, August 23, 2001 8:39 AM
> To: Coby Smolens
> Cc: vanagon@GERRY.VANAGON.COM
> Subject: Re: Reducing brake fade on '88 Westy?
>
> Great post - I'd add one thing. Which brake fluid you use *does* make a
> difference. The DOT specifications only set a MINIMUM boiling point
> standard, so
> you can avoid Colby's second situation to some degree by using a
> high-quality
> fluid.
> <<snip>>