Military-Vehicles: RE: [MV] Mystery Brakes

RE: [MV] Mystery Brakes

Alan Bowes (alan_bowes@phast.com)
Mon, 29 Sep 1997 12:40:06 -0600

On Monday, September 29, 1997 10:40 AM, Dan Schultz
[SMTP:schultz@inficad.com] wrote:
>
> ----------
> > From: Ben Hughes
> > My M37 and my M43B1 seem to be using a new type of evaporating brake
> fluid.
> > About every two months or so, the brake fluid disappears from the master
> > cylinders. This also happened in my '53 civilian Power Wagon.
> >
> > All three of these Dodges have NEW master cylinders, and NEW wheel
> cylinders. There
> > is no trace of brake fluid on the outside of the master cylinders, no
> puddles
> > underneath the trucks, and no leaking from the wheel cylinders!
> > Has this happened to anyone else, or am I ready for the booby hatch?
>
> Ben,
> I have the same problem with my M43! Let me know if you find what the
> problem is. I swear these things were built with gremlins in the brake
> lines!!
> Dan Schultz
>
Well, I've experienced this same symptom several times in the past. It's
always turned out to be caused by very small leak(s) that seep so slowly
that the fluid evaporates before it has a chance to build up into a drip.
This can easily happen with standard (non-silicone) brake fluids, with their
relatively high evaporation rates.

You said that the master and wheel cylinders were not leaking. Did you
happen to peel back the dust covers on the wheel cylinders and check them?
The reason I ask is that the most common place to leak without attracting
attention is around the wheel cylinder seals, where the fluid can slowly
soak into a layer of dust inside the brake drums and evaporate before it
becomes visible on the exterior of the backing plates.

I'd also suggest using a mirror and a light to check the hard-to-see places
where the lines are routed through the frame channels or other
out-of-the-way places. It's gotta be coming out somewhere.

When I tore down the brakes on my M37 a little while ago, ALL of the wheel
cylinders were leaking, though I could only see brake fluid on the outside
of one of them.

There are three problems that I've seen with new or rebuilt cylinders that
could cause a slow leak:

1) If cylinders have been sitting around on a shelf for years, sometimes the
seals lose some of their resiliency, allowing some seepage.

2) If someone rebuilds a cylinder and doesn't install a metal seal expander
behind the rubber cups, it can cause some seepage. Not all wheel cylinders
use this device, but if they do, and it is missing, it would allow a tiny
bit of leakage when there is no pressure on the brake system.

3) Some systems use a pressure retention valve in the master cylinder in
place (or in addition to) the expanders to maintain a good wheel cylinder
seal. If this valve is not working, it can result in seepage at the wheel
cylinders. This valve usually consists of a small spring-loaded ball check
valve that keeps a little bit of pressure on the brake system after you
release the brakes. This small amount of pressure is not enough to overcome
the return action of the brake shoe springs, but it is enough to keep the
rubber wheel cylinder cups pressed securely against the walls of the wheel
cylinders.

Brake lines: I had a '50 Ford pickup that inexplicably lost fluid. That
turned out to be a brake line leak in a dark, greasy area of the frame near
the transmission. It never dripped, and I had to steam clean the
undercarriage before it would show up.

Hope that gives you some ideas.

Alan
(Salt Lake City, Utah)

.

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