Military-Vehicles: RE: [MV] scary brakes - fluid compatibility issues

RE: [MV] scary brakes - fluid compatibility issues

Alan Bowes (
Wed, 1 Oct 1997 12:14:39 -0600

On Tuesday, September 30, 1997 7:30 PM, Tony Standefer
[] wrote:
> I have read the recent brake thread with interest. However being a
> layman when it comes to brake fluids and their relative properties. I
> need someone to bottom line all of this mumbo jumbo for me. All of the
> literature and articles i have read point to silicone brake fluid as
> being the way to go for restored vehicles that are not driven a great
> deal. Now we have some postings about silica abbrasion and flash
> steaming! how often should silicone fluid be changed? Now that i have
> my bantam trailer finished i will have that much more weight to stop
> with my brakes.
> Tony Standefer
> 1944 Willys MB
> 1943 Bantam T-3
> Seattle, Wa USA

I know I'll probably be accused of being overly cautious...again...but I
think I'll remain on the cautious side and not make any "official"

Each type of brake fluid has its own unique maintenance requirements and may
also raise compatibility issues. You have to "analyze" your own brake system
and decide which fluid type would be better or safer for that system. What
works great in one system may destroy another. I've seen rubber brake parts
turn to mush from using the wrong fluid. Military vehicles can have a very
checkered history, with parts added from a variety of sources and locations.
So, how does one decide what is compatible?

Well, first of all, you really have to know who made the brake parts in your
system (seals, dust boots, hoses, etc.). Unfortunately, this is not always
easy, unless you are rebuilding the system.

Then, you should ALWAYS CHECK WITH THE MANUFACTURERS of the brake parts AND
the brake fluid to make sure that there is no compatibility problem. Using
an incompatible fluid can have disastrous results. For example, there are
some systems that cannot use DOT 3, 4, or 5.1 fluids due to the composition
of the soft parts in the system. These systems may require a special fluid.
Perhaps they could also use silicone fluid. Again, check to be sure.

That said, here is a crude summary of some basic characteristics that I have
gleaned from several sources (I've probably missed a few, which someone else
is welcome to fill in):

Silicone Fluid Characteristics:

1) It doesn't absorb water, so the fluid doesn't have to be changed on a
regular basis (unless you manage to get some water in it from fording
streams or washing the vehicle or ?). If you have a master cylinder which is
not completely sealed from the outside air, then you have to be VERY careful
when washing the vehicle to avoid getting any water in the master cylinder
through the vent opening. (Naturally, you don't want water in a glycol-fluid
system either.) Some non-sealed systems have a master cylinder with a long,
raised vent line, which helps.
2) If you do manage to get water in a silicone fluid system, it's a royal
pain in the posterior, because you can't just bleed it all have to
disassemble the wheel cylinders because the water remains in blobs that can
collect in low spots, such as the bottom of the wheel cylinders, and the
bleeder screws are at the top of the wheel cylinders.
3) It doesn't dissolve paint.
4) It has a higher boiling point than glycol fluids, but not too much
different than DOT 5.1 glycol fluid, as I understand it.
5) It should be compatible with most systems. Check to be sure!
6) The system must be thoroughly drained of the old glycol fluid and clean.
Perhaps some silicone fluid could be sacrificed to flush out the remnants of
the old glycol fluid. To be sure, check with the fluid manufacturer to see
what their recommended flushing methods are.
6) It costs a few coppers more, but no big deal in the overall scheme of
things when restoring a vehicle.
7) No matter what fluid type you use, you should label the fluid type in the
system (especially with silicone fluid) to avoid having some "mechanic" dump
in the wrong type of fluid.
8) It can produce a spongy feel when the fluid is hot, but below boiling
point. Silicone fluid is slightly compressible, especially when hot.
9) It can trap air bubbles, requiring slow pouring and bleeding when filling
the system.

Glycol Characteristics

1) It absorbs water...even from the air. It is recommended that you change
the fluid every two or three years to keep the water percentage low and to
maintain the effectiveness of the built-in corrosion inhibitors.
2) Since water disperses in the fluid, bleeding the old fluid out should get
rid of the absorbed water along with it, so you shouldn't have to
disassemble the system.
3) It dissolves some kinds of paint. Solution: Don't spill it on your paint.
If you do, wash it off immediately with water (don't get any in the system)
and gently wipe it dry.
4) It may be incompatible with some brake systems because of its softening
action on certain types of synthetic rubber. Always check!
5) It's cheaper.
6) It doesn't store well in open or partially full cans (absorbs moisture).
Buy small cans and leave them sealed until you're ready to use them.

Some other factors to consider:

If your master cylinder is sealed (or set up so that it can't get any water
in it under normal driving conditions), and you're careful when using water
around the master cylinder (especially with unsealed systems), and you're
starting with a completely clean system, and there are no compatibility
problems, then perhaps silicone would be the way to go. Some vehicles have
raised master cylinder vent tubes that do a good job in keeping out liquid
water. If the vent tube on your vehicle has been removed, you should replace
it, no matter what kind of fluid you're using.

For vehicles that don't have sealed systems or raised vent tubes, and will
be driven in normal conditions, and there are no compatibility problems,
then perhaps DOT 3, 4, or 5.1 glycol fluid would be good. Just change it
every two or three years. Naturally, you still want to avoid getting water
in the system.

I know that I keep saying "check with the manufacturers." This is really
pretty easy to do, yet very few people ever seem to do it. Labels,
instruction sheets, warranty forms, etc. usually have a telephone number.
I've been pleased to find that quite often I've been able to speak with an
engineer about technical issues, and they're usually happy to find that
someone actually wants their opinion.


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