Hey guys,
So I am used to older vehicles, but my '79 F150's big red BRAKE light came on at the same time I started getting a hissing sound as I pressed the hydraulic brakes. I thought that meant Emergency brakes were on? Anyway, there are a few strange things going on here:

1) the brake fluid from the smaller reservoir was empty when I checked, and when I added new stuff, I saw a thin top layer about 1/8" deep what looked like water droplets on oil, and two layers like it wasn't mixing.. but when I sopped up the top layer, it was oil. And what I added was (brake) oil (the same stuff I bled with a year ago), the layer below, so I'm kinda like wth?? (maybe it's very old brake fluid coming back up, but not mixing with the new?)

2) the brakes make a hissing or sighing sound when I press them, only when the motor is on, and for a short few pumps after, then it goes solid again. (sound from the pedal/booster area from inside the cab)

3) the darn Brake light is on on the dash. So what the heck is going on, any experience? Should I just replace the booster and bleed again, and is this what the brake light means, that I'm losing vaccuum or something?

Thanks,
-breton79

 

Answers:

1) If at any time the master cylinder is completely empty, the entire system must be bled because this means it took in air. You can not simply top off the system. I would actually recommend bench bleeding the master cylinder too. However, you first need to understand why the system emptied out in the first place. This HAS to happen first. Fluid separation in the reservoir usually means some sort of petroleum product has contaminated the brake fluid. This is common when someone accidentally adds power steering fluid to the master cylinder. What do you mean you added brake oil - do you mean brake fluid? If a petroleum-based fluid ever enters the system, it's recommended that all rubber components are replaced (master cylinder, hoses, calipers, wheel cylinders) - but you would first need to make sure that actually happened.

2) The booster should go solid if the pedal is pumped after shut-off because there is no more vacuum. The hissing sound sounds like a vacuum leak depending on how it actually sounds. Some sound is expected. None of it is related to the light on the dash.

3) The BRAKE light is not related to the booster or the emergency brake. It only comes on when there's a pressure difference between the front and rear braking circuits. It also comes on briefly as the key is turned to START to prove out the bulb. In your case, the light is on because of whatever caused the reservoir to empty, the fact that you topped it off after it took in air, or both.

 

I would add that brake fluid is hygroscopic, even in the brake system; it will draw moisture from the air. Technically, you arent supposed to use brake fluid that has been opened after so many mos, idk the exact shelf life. A system flush is also recommended every 2 years or so (I doubt this happens often lol), the fluid drawing moisture from the atmosphere is what causes the internal rusting of lines, wheel cylinders etc.

 

Many thanks, I was convinced it was the booster, but it looks like now that I completely severed the line and emptied the rear res fluid, it goes down like butter, so that's a relief. Thanks for the info on the light, I didn't know they were that technical in 1979. My last car was a '65, with one reservoir, and nothing fancy like that light. Turns out it was a hairline leak from a rust-split line in the rear -- very slow leak but that did it! Thanks and learned something new. I'm going to drain out all fluid and start over (after a few minutes of adding new fluid both reservoirs turned opaque dark blue, I'm not having any more of whatever's going on!!!)