so i will say all i know about this weird brake problem, i got in my truck this morning after warming it up for about 5 min.. throw her in gear and start driving, all is well till about the 3rd stop... all the sudden real hard brake pedal, then a clunk.. best way to describe it... sorry.. i limp her home, luckily a manual so no real danger, brakes basically gone...it felt.. still a hard pedal with hardly any reaction with me standing on the pedal.. all i could hear when i got it home was an air sound coming out by the pedal, i park it and grab my other truck cause i had to run to do something... 20 min. later i come back.. no fluid leaking out of anywhere, not as much "air"coming out from the pedal area.. i jump in and take off.. i have brakes, but still seem funny, can't quit stop on a dime and still a bit weak, so i'm wondering if there is something that popped off or got caught on something then found it's place again.. it seems that i just have some air in the lines that need to be bled out now??? but i don't really know, thus my post here.. sorry for the long thread, but that is all the info i have thus far on this problem.. any ideas would be greatly appreciated as i want to get this fixed asap..

 

Sounds like a problem with the vacuum pump..

 

The sound of air from around the pedal area would usually indicate a bad booster.

 

Maybe the brake booster is intermittent...

 

How would I check to see if it is vaccum or boost? Thanks

 

The really hard pedal indicates the hydraulic section of the brake system(fluid, lines, master & wheel cylinders) is OK, but you've lost vacuum assist.

Your brake booster (don't confuse with turbo boost pressure) is vacuum-assisted... the vacuum from the vacuum pump provides the power to move the master cylinder, based on input from the brake pedal. Kind of like how the power steering box and pump take input from steering wheel to turn the wheels.

The vacuum pump is on top driver's side front of the engine; it only has one line coming off it. You could disconnect the line and see if the pump outlet is pulling vacuum.

But the air hissing sound suggests that something in the brake booster or a vacuum line is leaking vacuum... the pump is sucking well enough, but it's sucking outside air rather than pulling vacuum inside the booster assembly. Maybe a line came off or cracked, or something in the brake booster assembly went south. Since the same vacuum system also operates your heater & AC flapper door, the problem might be in that line. Capping that side of the "T" in the main brake line and seeing if you have brakes will tell you if that side is leaking.

While you're at it, take a good look at the "neck" in the vacuum pump casing, between the pulley bearing/shaft housing, and the saucer-shaped housing, for cracks. Mine broke there. Mine was a Zamack or similar casting, so any cracks are likely to fail quickly.

 

What year is your truck? It sounds like the 94 and 95 air sucker booster. It's a common problem on them. Especially when it's cold out. The seal gets hard and air gets sucked through. Look up the part number on this site for the f350 zero loss booster and get that one to put in it. It's double sealed in the back and won't ever do that again. These vacuum pumps pull more vacuum than the gas engines the original booster was designed for so they are just one big problem for a diesel. It's easiest to replace.

 

I wasn't aware the "woosh"noise is a problem . Mine makes that noise every time i press the brakes ...

 

That's the sound of air sucking out of the brake booster cylinder to generate assist when you apply brakes. It should occur only when you are moving the brake pedal down, then stop when the pedal is stationary, even when brakes are still applied. If you back off the brakes and reapply, you'll hear a light "whoosh" each time you drepress the pedal.

If you have a hissing or whooshing sound when you aren't depressing the brake pedal- either brakes not applied or applied & steady- then you have a problem. I read Bryan's problem as constant hiss... which indicates a vacuum leak rather than normal function.

 

thanks for all the responses. my first post was a bit unclear. when the problem manifested i had a hard brake pedal.. could hardly even push on it.. no brakes. got it home and parked it, looked at it briefly and all sudden the brake pedal was acting very soft an extra amount of "whoosh" sound to me..came back in about 30 and all the sudden brakes appear to be back. take it for a test drive and they are not as strong as they were.. meaning i can't lock them up, to me they are just acting like they need to be bled. which is very weird to me. now they are acting somewhat normal without the hard pedal symptom and i do have some brakes how am i suppose to go about finding the problem.. i am going to start by bleeding them out.. like i say seems like a very weird thing to have happen but the best i can think if right now is trying that

 

The front calipers could be sticking also. That was the case with my hard pedal issue. The brakes still functioned though.

 

That is definetly a possibilty... certainly with the symptoms I'm having. But when you had the issue did you get the feeling the brakes weren't 100%? Like I say it feels like I have air in there now, they won't really lock up and get the truck to stop good and hard.

 

Yep. That was the case. I ended up putting all new rotors, calipers, pads, rear cylinders and shoes, and hoses. I don't go half way with brakes.

 

ok new info on my problem.. so i just bled out the system.. the extra amount of whooshing seems to be gone...still not 100% sometimes doesn't want to stop at all sometimes almost normal.. but this is the weird part a couple times when i pressed real hard at about 20mph.. it was soft then bottomed out and actually pushed back on me?? everything in the brake area up front is already new minus calipers, i'm about to the point to take to a local shop that i know the guy is honest and fair, but i would much rather not... but it is brakes.??

 

At this point I suspect the calipers are sticking. Of course thats just speculation on my part.