ok, i pulled off the master cylinder, empty. always goes empty. but havent found the leak. going tomorrow to get brake fluid. whats going on. i dont want my girlfriend driving this truck if i cant trust "her", {olive}, thats her name, any help would be great. thanks guys. ripman.

 

Wow! I have the same problem. I cant see any sign of a leak. All four wheels are dry, hoses are good, connections are tight, no fluid on the inside of the firewall. I figure maybe I have a wheel cylinder that leaks very slowly, so slowly the leak never becomes visible.

 

There must be something in the air. This is the second time I have typed this it two days.
If you have a vacuum powered brake booster it can fail in such a way that the brake fluid get sucked into the engine and burned.
If it is slow enough, you will not notice it at the tail pipe. But, sometimes you can smell it. I once walked past a stranger's car idling in a parking lot and smelled it. As I went by his window, I mentioned that he better check his brake fluid as it was getting low again. He thanked me, and I got almost 10 feet away before the penny dropped. He jumped out of his car so fast to ask me how I knew that, that he forgot to put it in park, and had to jump back in and stop it<G>.
He was very grateful when I explained where his fluid had been going. He had been looking for the leak for two weeks.
If you are losing the fluid fairly fast, just pull the vacuum line off the booster and check to see if is damp with brake fluid inside. A really slow leak might be hard to spot that way I suppose, but I almost always see some.
As to how the fluid gets out, I am embaressed to say I'm not sure. I just always note that it is and replace the booster. At a guess, I suppose that the failed booster lets vacuum get to the plunger and the seal is not designed to seal a pressure differentual from that directon.
BTW, the same thing can happen on an automatic transmission that has a vacuum modulator. The modulator can get a pinhole in the diaphram and let the engine suck transmisson fluid out. In that case, almost always the rubber hose that hooks onto the modulator swells from the fluid and falls off. I had a friend complain that he had put three hoses on in a month and still they fell off. We replaced the modulator and all was well.
Hope this helps.

J.

 

As soon as the weather warms up a bit, I have the same issue to diagnose myself. Would have never thought of that possibility. I am also planning to replace the front rubber hoses, as even if they do not have a slow leak, at almost 40 years old, should not hurt to replace them.

 

Leaky diaphram in the power brake booster.
Mine was sucking the brake fluid through the vacuum line into the engine.

 

I don't know about SnowHiker, but I don't have power brakes. Neither of us gave any info on what we are driving which I personally embarrased over since that is one of my biggest posting pet peeves. Sorry!

Anyhow like I said earlier I have vanishing fluid with no evidence of a leak. 1967 F100 Bone stock with no "power" anything. The way I like it.

Any ideas on non booster trucks would be great. The power brake info was great stuff. I never would have thought of that.....

 

Greg,
Make sure the firewall insulation isn't soaked from the master cylinder leaking.
All wheel cylinders look OK?
Backing plates clean?
Make sure the original brake lines aren't rusting through?
Cracked flex hoses?
Your fluid will drop over time(1 or 2 years) from the pads, shoes and drums, rotors wearing but you shouldn't notice much from this.
Good luck.
Ken

 

Sometimes the cap gasket will leak, ditto woods info.