Military-Vehicles: RE: [MV] Brakes and my rookie opinion...

RE: [MV] Brakes and my rookie opinion...

Alan Bowes (
Wed, 1 Oct 1997 12:59:28 -0600

On Wednesday, October 01, 1997 7:57 AM, Scott Strance
[] wrote:
> Yesterday, I removed the heat shield backing plate just under the exhaust
> pipe
> on my M38A1. I was amazed at the heat generated at that point in the
> exhaust
> system. The metal plate was a bit warped and the asbestos was pretty well
> destroyed. I found it very interesting that just on top of that plate
> the master cylinder and input lines for front/rear brakes. (I'm down to
> drivetrain & engine only on the frame so yes, it was very easy!)
> I propose that regardless of the load of the jeep and it's carrying
> capacity,
> stopping under full force would not generate near the heat that the
> manifold/pipe generates while accelerating to and driving at speed. This
> would be true on even short trips. Therefore, I would lean more towards
> the
> evaporative aspects of the fluid over time with the heat generated as the
> primary driver to the process.
> Is this a workable theory???
> Scott
> Austin, Texas
> 53 M38A1

Hi, Scott,

The master cylinder should not heat up to any significant degree as the
result of hard braking, although the wheel cylinders can certainly heat up
(and overheat) as a result of the heat generated from braking friction.

However, if the master cylinder is heated from its proximity to the exhaust
(or any other source), it will, as you suspect, accelerate the evaporation
to SOME extent, although I doubt that it would cause a rapid loss of fluid,
even in a non-sealed system. Part of this depends, of course, on just HOW
hot it gets. If it really is getting too hot, the exhaust might need more
heat shielding, although even a single-layer heat shield can greatly reduce
radiated heat.

Fluid losses always result from some combination of:

- Evaporation
- Normal seepage around seals.
- Unwanted seepage and leaks from bad joints, worn or damaged seals,
corrosion, or other physical damage.
- Perhaps the odd molecule or two that manages to penetrate a membrane.
- Midnight brake fluid thieves.

The better the condition of the system, the less seepage there is. Unsealed
(vented) systems will have somewhat higher evaporation losses than sealed
systems. There will always be a tiny bit of seepage and evaporation at the
wheel cylinders, even with just-broken-in seals and factory-perfect cylinder

(Salt Lake City, Utah)

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