hissing air vacuum line
 

I have a 1986 F150 XLT v8 302. My truck runs great, but before dozens of expensive trips to the a master mechanic it was old and neglected with a lot of wiring problems and it appears some other things just wore out. Right now there is a air hiss where the brake pedal goes through the firewall. The brakes work fine, soft and great stopping power. I am not a mechanic and don't mind taking this to one as a last resort but would like to work on fixing this if I can as the mechanic I use is really really expensive for some things. I read a post on here about possibility of cruise control vacuum line. Mine has the cruise. How do I check this and if not that what else...step by step?

GJ

Not sure that the cruise control vacuum line goes through the firewall. Thinking maybe the vacuum supply for the HVAC controls? If this were a significant vacuum leak, would think the idle speed would be increased, so thinking it's one of the smaller lines.

If you can hear the hiss - suggest to start feeling around and wiggling hoses until you hear the noise characteristics change.

Sounds like the brake booster seal around the rod to the pedal is at fault. Unless the noise gets so loud and annoying shouldn't be a problem. Here is a simple test you can do your self. Go out to truck and start engine, shut off the engine. Now depress the brake pedal, release the pedal, repeat this and you should feel the pedal getting harder and also not going down so far. After 4 or 5 times the pedal is at the top, hold the pedal depressed and start engine. Pedal should go towards the floor and come to rest at what is your normal brake feel. This is a normal brake booster assist. Do this several times just to get the feel as to what I'm saying. Ok repeat this test tomorrow. But do not start the engine, we want to see is there is any vaccuum left in the booster. If the pedal is at the top, all the vaccuum has leaked out and is time to replace this puppy. Get back to us if I'm on tract with your concern and want instructions to replace the booster. Being younger with good eyes helps, also basic hand tools necessary. Sorry George, both WDE and I seem to distracted, it is Miller time! Welcome to the site

It could be the brake booster, they usually hiss when your foot is on the brake pedal. It could be the a/c control vacuum lines also, check those out. I'm not sure the type of brake pedal cruise control shut off switch your truck has, but some do have a vacuum line through the firewall to the switch. Check it out also, if it's the problem and you don't mind not having cruise control you can cap it off under the hood until you get the new switch or hose.

I've also seen the automatic emergency brake release switch that is mounted on the steering column under the dash make that noise when it begins to leak. It is operated by vacuum also. Sounds like a vacuum leak under the dash area. Not sure by your post if you are hearing it outside the cab or inside.

Jim:thumbsup:

It is coming straight from the brake through the firewall.. I will try the suggestions from Bill Wiltshire and post results. I don't have young eyes, 40 year old ones will have to do... I have patience and a flashlight.

Thanks for the help!

hiss
 

Looks like Bill Wiltshire may be correct. It does actually leak from there immediately after turning off the engine and the vacuum is leaking out. I still do not see a line going through there though. Just what appears to be a gasket. I also only see one air line type hose under the dash and I pinched it and did not hear any change to the hissing. Is there a definitive way to check other than depressing the brake? How expensive and difficult a fix is that? Thanks!

GJ

The line to the brake booster is under the hood, but that's not where it's leaking. You are saying you hear it through the rod, so booster replacement is required. You will be spending some time on your back. First, remove the master cylinder from the booster under the hood. Leave the lines attached so you won't have to bleed the brakes. Then pull the vacuum hose off, also under the hood. Then 4 bolts under the dash to remove the booster, but first unhook the pushrod from the pedal. Throw on the new one, reassemble and you're done.

It might help if I knew what the the master cylinder looked like, what the booster is, which lines, which vacuum hose, what 4 bolts under the dash how to remove the booster and how to first unhook the pushrod from the pedal. How to throw on the new one, and reassemble. Sounds like a job for the mechanic again... Think I am out of my element... I can build a boat but I can't fix the brakes. We all have our limits. Thanks!

Hi George, Sean wasn't being a wise guy, although I have seen him call himself the "Truck Wisperer". He has figured out some pretty tough problems and I bow down to him for that. So lets get you aquainted with what is what. Replaced my booster, NAPA about $75.00. The job is what I call nuts and bolts. As sean mentioned, and I will phrase it differently, do not disconnect the two brake lines from the master cylinder. By doing this, there is no chance of messing up the brakes hydraulic system. I would go to auto parts and buy the booster and aquaint yourself with it and what is under the hood. Follow the truck wisperers instructions and should be good to go. We are here to help anyway we can. You mentioned boats, lost my left vent window, 1981 Sea Ray 210 Cuddy Cabin, you don't by chance have one in your hip pocket do you?:)