MTC

All of a sudden I have had a hissing noise coming from the engine bay. I traced it down to the top black hose coming from the brake master cylinder. Its the rubber hose with the metal "weight" on it. The hissing is coming from what looks like a crimped on coupling on the hose. Is this hose an air hose or is there brake fluid in it??? It seems to me that if there was brake fluid in it, then it would leak fluid, not air.....

It only hisses when the truck has been driven for a while and is up to operating temp. The hissing goes away when the brake is depressed and comes back immediately when released. I did take it in for the issue, and of course..."can not be duplicated" was the answer I got. They didnt even drive the truck..it only had .4 miles more on it than when I brought it there.

Can anybody explain to me whats up?? I realize that replacing the hose will fix the problem, but what went wrong? Thanks

1985cucv

hoses crack and maybe its rubbing on somthing idk these things happen rubber breaks down eventually

MTC

Yeah, I hear ya...but its an '07.

I guess my main question is this: Are the two larger hoses coming from the top of the master cylinder filled with air or brake fluid? I would think brake fluid, but then why is there an air leak from the hose???

sdguy55

Well usually brake fluid is being pushed through metal lines not rubber. Did you notice any decrease in pedal feel when you noticed this hissing? If i had to guess i would guess its a vacuum line.

patdaly

Sounds more like you are confusing the Hydraboost unit with the MC.

Based upon what you say I would bet the hose goes to the PS pump?

MTC

You may be right...they are the two larger diameter rubber hoses, one of which has a horseshoe shaped "weight" on it. The hoses say "aeroquip"(or something like that) on them.

Crude dude

MTC, what did you find. I noticed that my truck is doing the same thing. Started about a week ago. Its hot in Phoenix so I rarely have the window down while the truck is running and I heard this last week. Exactly the same thing, hisses until I push the brake pedal. Im wondering what it is because lately Ive noticed that there is a slight howl from my steering and now I feel it in my brake pedal when completely stopped. What is it?

MTC

The dealer ordered the new hose for me and I m just waiting to get in there and have them switch it out. Prolly gonna get it done this week.

It is the hose coming off of the hydraboost unit like stated above. The one hose has what looks like a crimp on coupling on it and thats where mine is leaking from.

Crude dude

Was it an "air" leak? Not sure if those hoses have vacuum or not. Did you notice any difference in the way the truck ran or idle?

MTC

Im not too sure, and the dealership people seem to be dumber than I am....it has to be an air leak cause there is no fluid leaking.

The truck idles like normal and still runs as strong as usual.