DOT 5 Brake Fluid

[oldroadiedog]

Dan wrote...

On a semi-related topic, to replace all the rubber in the system you need to get the fluid res-master cyl; master cyl seals; front piston seals; front piston dust covers; rear wheel cylanders; and 3 rubber hoses. Does this sound right? I'm doing all my brake work and switching to Dot5 so I want to make sure I get all the rubber out of there.

This came from the Ramps or Jacks topic. I'm about to rebuild my master cylinder and had planed on using DOT3 fluid. Is this the correct fluid? What's the difference in DOT3 and DOT5? The Opel service manual calls for Delco Supreme No. 11 or equivalent.

[kwilford]

DOT 3 and 4 are glycol-based fluids, which absorb water ("hygroscopic"). DOT 5 fluids are silicon based, and while they don't absorb water, they are incompatible with many older brake component rubbers, and also with glycol based fluids (they can't be mixed).

The merits of converting to DOT 5 fluid was discussed at length on the earlier version of this site, and the consensus (if there was one) was that DOT 5 fluid should ONLY be used if ALL the brake system rubbers (including wheel, disc and master cylinder seals, & all flex hoses) had been replaced with new (not NOS) components. It was also imperitive that the old fluid was thoroughly flushed out by using butyl alcohol in the lines.

The problem with water in brake fluid is two-fold. First, it can cause premature corrosion of the components, especially caliper and wheel cylinder bores. Secondly, when glycol based fluids gets saturated with water, the boiling point of the fluid gets reduced dramatically. Heavy braking boils the fluid and creates steam, which is "compressible" (versus the fluid itself which is incompressible) and the brake pedal gets soft, or even ineffective. This is really only a concern among the racing crowd, since it is VERY rare (unless you do a lot of mountain driving) to get on the brakes this much.

Glycol based fluids, if changed every two years, will perform very satisfactorily for everyday use.

An earlier post gave this address:

http://www.shotimes.com/SHO3brakefluid.html

In it, one of the writers suggest that while silicon fluids don't directly absorb water, they have the disadvantage that if water DOES get into the system, it will "pool" and cause excessive corrosion. It also lists the "dry" and "wet" boiling temperatures of various fluids.

HTH

[oldroadiedog]

Excellent, Kwilford. Thanks for the low down. I'll stick with DOT3.

[kwilford]

I forgot to mention that the site above describes a "LMA" (Low Moisture Absorbency) fluid. Since most of our cars are low daily mileage cars, or even parked for the winter months, that would seem to be a good choice to minimize brake system corrosion. And even at that, the system shouild still be drained, flushed and re-filled with new fluid at least every two years. And not even with old bottles of fluid, since humidity in the air will penetrate the plastic bottles and absorb into the fluid.

JM2CW

[TheCollector]

An additional merit of DOT5 fluid is that it does not damage paint. This means that you can spill it all over your engine compartment and suspension components without it stripping the paint.

This was my primary reason for using DOT5 in my Austin Healey after rebuilding the brake system. However, one disadvantage to DOT5 is that it tends to reduce pedal feedback and make the brakes feel spongy. At first I thought I just needed to continue bleeding the lines, but discussions with others suggested that it was a byproduct of using DOT5.

-Bob