"Whoosh" sound when stepping on brakes?

[boy&hisdogs]

My truck has been like this since the day I got it. The pedal is really spongy and when I step on the pedal there's a little "whoosh" sounds that reminds me of when you're rolling up a camp mat and push the air out. I bled the brakes when i first got it, and it improved a tiny bit, but there is still a squishy dead zone I have to push through before the brakes actually start working. It's not bad enough to cause alarm, it's just annoying, particularly when driving something else and then coming back to my truck after.

Is this a common thing? Is there something actually broken or should I just do a NBS master cylinder swap? I'm planning on going to a 9.5" rear end and doing the 2500/3500 caliper swap to match. Will that take car of it or do I need a master cylinder? Or something else?

Edit: The truck is a 98 K1500, has a factory hitch (tow package?) but is not F44.

 

[woody31]

it sounds to me that it could possibly be the brake booster?????? it may be bad?????
others will chime in im sure but thats my first thought,,good luck with the repair and please keep us updated,

 

[IOWNJUNK]

Yeah, that sounds like a bad booster.

 

[boy&hisdogs]

I saw something in another thread that said if you put the truck in park and step on the brake and the pedal sinks to the floor then it's a bad master cylinder. Mine does this, could it also be indicative of a bad booster as well?

 

[IOWNJUNK]

Yes, could be, but only if you are sure everything else is good. The pedal will go to the floor for any number of reasons.

The only booster I had fail made the same sound you explained. It also would not allow the pedal to return and I would have to lift it with my foot.

 

[19trax95]

Mine made the same sound when the master cylinder went bad. It also happened to be the same time my rear line blew out.

 

[98_k1500]

Sounds like you need to change the booster and do the nbs master cylinder swap.

 

[boy&hisdogs]

I think I'll just go ahead and do the NBS master cylinder swap for now. Brake boosters are expensive. If the problem clears up then great, if not, then it's a upgrade anyway. Should I use the regular half ton MS? Or the heavy half? Bigger? I'll be swapping to a 9.5" rearend and 2500 calipers soon, should I get a bigger MS too?

Another thing it does is that if I have to panic stop then the brakes don't work so good for a few minutes after. Like there's a lot of slop in the pedal and I have to really stand on it to get what would normally take just a moderate step.

 

[19trax95]

I would get the most similar weight rating nbs master. The brake boosters are expensive. So if yours is bad (it may not be) I would go to a junk yard and get one. Much cheaper but also not new. Also,if the master cylinder did fail completely, make sure that no fluid got into the booster. That's bad and would ruin the booster.

 

[boy&hisdogs]

I would get the most similar weight rating nbs master. The brake boosters are expensive. So if yours is bad (it may not be) I would go to a junk yard and get one. Much cheaper but also not new. Also,if the master cylinder did fail completely, make sure that no fluid got into the booster. That's bad and would ruin the booster.

Should the master cylinder match the chassis weight rating (regular 1500) or to the brakes I'll be using (heavy half/LD2500)?