Ghost Brake Fluid!?

Schurkey said:

Not my first choice. Some guys have multiple vehicles, so I don't know which of the vehicles in their signature they're talking about.

Others put a vehicle in their signature, then start a thread about their buddy's car, or their co-worker's truck...and again everything gets confused.

Putting the vehicle info in the title works...unless you're me and can't remember the title by the time the thread loads.

Put the vehicle info in the text of the first post, first thing.

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Yea, if I had multiple GMT 400's, they would be in a bigger signature. If have other GM's, like my 2005 Z71 Tahoe, that can go there too or like you said, put the info in the thread, SOMEHOW.

It is an ongoing battle at TYF as well.

It's leaking somewhere. Brakes are a closed system, if you are losing fluid it's leaking. You just gotta find out where.

Road Trip said:

Greetings Docs, and Welcome to the GMT400 forum!

Instead of a 'one size fits all' brake system across all GMT400 models, there are
different brake options driven by how much weight they are going to have to deal
with. I've attached a chart I extracted from the '97 Factory Service Manual to
show you the differences. (See attached.)

If you could go to the inside of your glove box door and tell us which JBx or JDx
code is listed there and post that, then the brake gurus in here will *know* what
you are working with, can share their experience and tell you what to look for.

Cheers --

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JB5 BRAKES!

Docs said:

You must be registered for see images attach

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Linky no worky

There’s only so many places pints of brake fluid can go.

Docs said:

JB5 BRAKES!

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Very good!

Here's a quick list of where brake fluid can disappear to:

1) Rear drums: Leaking wheel cylinder seals. Some can be absorbed by brake dust. Larger leaks
will leak down the inside of the backing plate & drip onto inner wheel/tire sidewall/ground. Older trucks
usually have neglected rear brakes -- so majority of the time if you open up the rear brakes you will find evidence
of wheel cylinder weepage/seepage/leakage.

2) Front discs: Caliper puck seals. Brake fluid leaks end up on inside of wheels. Can be really hard
on paint, bad juju. Kinda rare to find in the wild because front brakes get serviced more regularly/swapped
out for sticky slides, frozen pucks, or other issues before the seals give out?

3) Rusty/weepy steel brake lines. Also sometimes the flexible rubber lines where the rubber is crimped to
the connections. Replace.

4) Master cylinder leaks into the cab, and goes *behind* the carpeting/rubber flooring. First place to
look IF there's no signs of external leaks in the 4 corners? Messy.

5) And if you've been adding a fair amount of brake fluid, and it's been disappearing with no trace?
Because you have a JB5 setup, your power brake assist is developed by engine vacuum. If everything
fails just right, the brake fluid either get trapped inside the vacuum booster assy, or it actually gets sucked
into the intake manifold, gets burned, and disappears via the exhaust.

That pretty much covers the 5 areas you need to look for your disappearing brake fluid. Q: How often
are you having to add brake fluid, and what quantity?

Let us know what you find. Pictures when you catch it red-handed a plus!

And as always, brake issues should be addressed ASAP.

Happy Hunting --

Road Trip said:

If everything fails just right, the brake fluid either get trapped inside the vacuum booster assy

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Picture evidence of everything failing just right

This is failure of the master cylinder on a GMT800 where it leaked the fluid into the booster. The booster diaphragm didn't seem to completely fail as it filled the booster up but not to the point it began leaking externally - zero signs of that - BUT it might have been slowly leaking into the intake via vacuum and burning off; let me tell you the booster was FULL of fluid.

Popped the m/c loose from the booster and stuck my finger in there; it's wet:


Inspection mirror shows clear evidence of leak behind the m/c rear seals


Pulled everything apart and had to be extra careful to not spill the fluid out of the booster while removing from the truck, as mentioned, brake fluid will ruin your paint in a hurry!


Not saying this is what's going on with yours, but it's a possibility in the realm of "disappearing brake fluid"

Richard

Docs said:

1997 ext cab silverado 2wd 5.7

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Docs said:

Front half of master cylinder keeps going low..damn near dry!?

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It's not going into the cab, or into the booster. That would drain the REAR half of the master cylinder reservoir, connected to the rear front brakes.

Docs said:

Funny thing is, there are no leaks!?

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Of course there's at least one leak. You just haven't found it yet.

Most likely by far is rear wheel cylinders. Pull the wheels and drums, look for wetness around the wheel cylinder. Ideally, you'd pull the shoes and check the pistons in the cylinders. If the shoes are really-worn, the reservoir could be low with no leakage because the brake adjustment on the crappy 254mm Leading/Trailing shoe design is at the top, near the wheel cylinder. With the adjuster at the top, the pistons in the wheel cylinder travel farther and farther apart as the shoes wear and the adjuster takes up the clearance. But that's not going to drain the reservoir empty, let alone draining the reservoir multiple times. You have a leak in that system.

Next-most-likely is a pinhole in the metal brake tubing.

After that, it's anyone's guess. Leakage in the ABS unit, or in the combination valve, or something else similarly unusual.

Schurkey said:

That would drain the REAR half of the master cylinder reservoir, connected to the rear brakes.

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No sir, the smaller front part of the reservoir is for the rear brakes.