The Stovebolt Site Forums The Shop Area Driveline '51 3100 brakes: is it a leak, or does brake fluid evaporate?

The guys in the Electrical Bay helped me get her started, but, wouldn't you know, the brakes don't work. Push the pedal, no resistance. Check the fluid in the master cylinder, dry as a popcorn fart.

About 10-12 years ago, I rebuilt the brakes at each wheel (honed the cylinder or bought new, bought new drums, etc., but left original lines), filled the master cylinder with fluid and bled the system, and they worked fine for many years of VERY intermittent use. The entire time she has been stored in a weather-tight barn, though mice roam freely (how does a mouse climb on top of a fender?). The last time I drove her was maybe 2 years ago, and the brakes worked. Today, the MC is bone dry.

So, the fluid persisted for a decade, but in the last two years has leaked or evaporated or been drunk by mice. I have not done a proper under-truck inspection yet, but after moving the truck I don't observe any residue of anything on the floor near the wheels, and I couldn't tell if the MC leaked because of all the oil pan leakage (onto cardboard) that would obscure that.

After a quick but unproductive search of the forums, I have two questions:
1) If it were you, would you just fill the MC with fluid and bleed the system and hope the brakes work for many months, or would you overhaul the master cylinder (as I am inclined to do regardless of the cause of this problem)? Of course, it could be that neither of those things is the right first thing to do. I will inspect everything when I have a chance. The lines are metal; maybe they're rusted through.
2) Given these symptoms, could anyone hazard a guess as to likely cause?

Thanks for any help.

My first guess would be line rusted through...then a wheel cylinder leaking.
If no signs along the brake lines then I'd pull the wheels/drum off and inspect. If its been very long the rust, old grease or brake dust, mud, and crud may have soaked up the fluid so you may have to pour some in the M/C and pump them, maybe even bleed them to find the source.

....and yes, definitely get it to Whoa before you Go again. Lets us know what you find.

Thanks A. I like the idea of adding fluid, pressuring the system, and looking for a leak.

If it were mine, that's what I'd do.

I would start by replacing all of the "original brake lines" of which you speak, then bleed the brake system. Doing that will make trouble shooting much easier, and you need to do it anyway if you value your truck.

Carl, would you replace with original style metal lines terminating with rubber, or would you go rubber all the way? I'm trying to stay as original as possible, but I think metal lines would be a pain to install and maybe require more maintenance (since they can rust through).

I don't play around with brake lines. I order the pre-bent stainless steel lines and all new soft flex lines for the wheel cylinders. The SS lines are a little pricey, but you will never have to replace them again.

Fill the master cylinder with fresh fluid, open all the bleeder valves a little, and let the system "gravity bleed". Once most of the air in the lines and wheel cylinders is displaced that way, top the MC off and try bleeding the brakes. You'll either end up with good brakes, or you'll discover the source of the fluid loss. At least try to do a little diagnosis before you start throwing a truckload of parts at the job.
Jerry

Pull back the rubber boot on the master cyl. If it is leaking there will be fluid behind it. Fill the MC & pump it a few times. It probably will pump up. Then you can find the leak. That model will not gravity bleed as the master cyl. is lower then the wheel cyls.

[quote=ol_red]Carl, would you replace with original style metal lines terminating with rubber, or would you go rubber all the way? You can't use rubber lines all the way on brakes.
The new alloy metal tubes are easy to work with & don't corrode or rust. All the parts stores carry them now. Stainless is hard to work with but does look good.

Hello. Sorry for the delay in proceeding with troubleshooting.

To recap:
Ten years ago I "overhauled" the rear brakes. They worked fine for the few times I drove it.
I haven't started or driven this truck in 2-3 years, but I recently fired her up (thanks to the guys in the Electrical Bay).
My current symptom is my brake pedal goes to the floor with no resistance, and the master cylinder is bone dry. So I started this thread and received a few suggestions.
Yesterday I filled the MC with brake fluid. I did not bleed anything, out of laziness. The brake pedal again just goes to the floor with no resistance.
I let it sit overnight to see if I had a leak, which I do not appear to; the reservoir is still full of fluid, and there are no signs of leak on the floor.
I did remove the rubber boot and there was no fluid inside it (good thing).

I would've thought that I would get SOME resistance from the pedal, even though all I did was add fluid without opening any bleeders.

You will see from the attached picture that the bottom of the MC is very rusty, almost as if the fluid leaked out slowly over time and caused rust, without creating a puddle on my floor (although I did have cardboard down and couldn't tell a new leak from an old one).

Please advise on next step in troubleshooting. Is it invalid for me to just add fluid and pump the pedal and expect some pressure? Maybe some part of the MC is not doing its job in pressuring the system. But where did my original brake fluid go over the last 2-3 years?

Pump them up and try bleeding them. What could it hurt? Might just get lucky or at least find the leak.🛠

Last night I added fluid and bled the brakes. Now I do have pressure on the pedal; still a little spongy, but at least not straight to the floor. I need to buy more fluid and re-bleed them, as I did it very quickly and the fluid that bled out was very murky. Thanks for the encouragement.

I haven't driven it yet, as I still have the right front wheel off. Feel free to read my new thread about front wheel bearings and respond.

I'm still curious where the brake fluid that was in the system went to. No apparent leaks, and I don't think it's prone to evaporation in 2-3 years, because there's still fluid in the empty Scotch bottle I used to bleed the rears years ago.

Follow the bleeding sequence in the Shop Manual (for small trucks) - furthest wheel cylinder to closest wheel cylinder.

Use/waste a lot of brake fluid cleaning/clearing/purging the system. Presumably you bleed out of a hose that has its open end in/under the waste brake fluid in a bottle?

Do you have a helper push the brake pedal during bleeding?

Brake fluid does not disappear. You either have a lot of air in the lines, or a leak somewhere?

Good luck - keep at it.

Thanks everyone. I'm all fixed up. Tim, I bled the brakes again and did follow the proper sequence and did have a helper this time, and did have the hose end submerged in waste fluid. It seems to me that if one pump of the pedal doesn't clear all the bubbles, I should close the bleeder valve before releasing the pedal to keep the air from getting sucked back into the line. Then open the valve again before the next pedal push. Seems logical. Hope I didn't waste my time. In any case, I have brakes!

I guess I do have a leak, and it seems that brake fluid doesn't leave an oily stain, because it's glycol based. So not seeing a stain on the floor or cardboard doesn't mean I don't have a leak, I assume.