Mysterious Disappearing Brake Fluid

[JBF43]

Hi,
New member here with a question about brake fluid on a 1976 Chevy C60. As mentioned, the truck is a 76 Chevy C60 with a 350 engine, a 4-speed manual tranny, and a 2-speed rear end. Brakes-wise, it has the single well master cylinder under the hood with the brake booster mounted under the floor of the cab. The truck is rarely used to haul a tractor on either a 16-foot car hauler or sometimes a 24-foot deck-over goose-neck trailer. Other than that, it just sits in the yard, and I run it a couple of times a week to keep the battery up and keep gas in the carb. It's not your everyday grocery getter. I bought it back in 2017 after seeing it for sale online. When I looked at it, the brakes weren't working. It started right up and purred like a kitten. No smoke, no strange noises or anything like that. Just no brakes. I did talk the owner into installing a new master cylinder, and he did that. So then it had pedal pressure, but you needed 3 full-grown men and a small boy to stomp the pedal to make the brakes work. At least I was able to get it home. I found out that the brake booster under the floor needed replacing and was able to get one through my local NAPA store for about $300 or so. Installed it, bled it, and had great working brakes. HOWEVER, after not all that long doing regular fluid checks before driving, etc, I noticed that the master cylinder was empty or almost bone dry. Refilled it and checked the entire brake system for leaks but didn't find any wet spots anywhere. Since then, I check the brake fluid every other time I start the truck, even with it just sitting here in the yard and not being driven. Over the course of between 2 to 4 weeks, it will need fluid again. Still no visible leaks. This has gone on for as long as I have had the truck. I have also had to replace the booster 3 times since owning the truck from 2017. The first and second boosters I replaced were the single-cylinder type, and the last one was the double-cylinder type. Pictures attached. The last one went out while driving. The brake pedal got very hard again. I had the tractor on the trailer when it happened and was able to creep home in low gear using the trailer brakes to stop the truck. Luckily, on all level ground all the way home. This time, only the double-cylinder type was available. Interestingly, I thought maybe a double-cylinder vacuum booster would increase the braking performance, but it didn't make any noticeable difference at all.
ANYWAY, now that I have bored you to death with this story, I am wondering where this disappearing brake fluid is going since it is not coming out on the ground anywhere. My best guess is that it is going into the brake booster and somehow possibly collecting in the vacuum chamber somewhere until it can't hold any more, and then the bad booster problem recurs. Any thoughts or ideas on this problem would be greatly appreciated.

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[Smtty58]

When you have replaced the boosters in the past, was there any evidence of fluid inside them, or the master cylinder wet at the seal? How is the hose from the reservoir to the cylinder? If it's the wrong type it will leak through the layers.

 

[JBF43]

Luckily and strangely, I happen to have the last booster still here, as I ordered the present one online and they did not ask for a core. Since you've asked and got me to thinking, I got the old one out and shook it. I can hear fluid sloshing around, i think inside the vacuum chamber, as there isn't enough room to make that kind of a noise in the slave cylinder. Not enough room for that kind of fluid movement. When I shook it, fluid came out of the hole, (see picture) where the main vacuum line connects. Note this is wet from today's shaking only. Was never wet before. I am wondering if I should open up the clamp on this one and see if that is brake fluid in there. Don't really see how it could be anything else.
Checking from the master cylinder on down, the firewall is pretty much a mess from spilled brake fluid the last couple of times I filled it, but I believe there is no leak there as there are no signs of a fresh leak. I will clean that back up and monitor it closely. There is a rubber line just above the frame that I have replaced, and it looks good. No cracks or signs of leaking. I have also replaced the brake lines from end to end, and everything is dry end to end. Had drum brakes all the way around. All wheel cylinders are dry, with no signs of leaks. Never any fluid dripping from the brake drums anywhere either. Something that you now have me wondering is if there are seal failures happening within the slave cylinder and letting fluid leak into the vacuum chamber until it destroys the diaphragm in there, and that's when I lose the vacuum assist on the brakes. However, it does not take very long at all after replacing a booster before more brake fluid is needed.
Hose-wise for brake fluid and vacuum lines, all hoses are the ones that came with the truck except for the one(s) I mentioned replacing, and those are the type that is/ was called for for the truck. All are dry, no leaks.
As seen in one of these pictures, the fluid that splashed out of this booster came out through the main vacuum line connection; I'm guessing running back from the chamber through the vacuum (fitting) connection where I assume no fluid should ever be, just vacuum. If I have this figured right, brake fluid should come in that back hole, (fluid in) and go out through the fluid-out hole to the truck only. And one more question. There is a nipple on the bottom of this vacuum port(?) where a vacuum hose goes off to what looks like either a breather or some type of filter. It just goes up under the floor on this truck and is attached there. On a side note, I have a '70 C50 with the exact same system on it. On that truck, that breather filter, whatever it is, comes up through the floorboard behind the driver's seat. When you step on the brake, you can hear a noise like an air movement or something, almost like some check valve or something allows air to move somehow to somewhere. You can't hear this on the C60 because the breather is under the floor of the cab. The C50 has never had any brake issues other than the master cylinder leaking at the firewall, and a simple cylinder rebuild and bleed job got it back in shape.
Can you clear up a long-time question on whether or not oil is supposed to be added to the vacuum chamber of the replacement boosters? I have heard and been told both ways, yes and no. The first 2 I replaced, I never added anything to, as I was told not to. This last one called for, I forget just how much, mineral oil added to it. I checked with the manufacturer of the booster as well as the seller and all said yes to adding food-grade mineral oil. So I followed the instructions and did that. Sorry to be so long-winded; I'm just trying to make the whole situation understood.

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[Smtty58]

Yes, there should be a check valve inline with the source from the engine. In the car/ light truck world, Ive never heard of adding oil to the chamber. But you followed the recommendations, so you should be good there.

 

[JBF43]

Somewhere along the way in researching these things, I learned that what makes the difference in using or not using added oil is what the diaphragm is actually made of. On this last one, food-grade mineral oil is required. I don't remember the amount for sure, as it has been some time now, but I am thinking it was in the neighborhood of 2 ounces.