Brake hissing sound as pedal depressed

Hi all, happy new year! First let me just say that since joining this site back in Feb. 2010, I can't tell you how much this B-body Mopar forum has been an enormous asset in helping out with my car fixing problems and motivating me in my musclecar obsession. I continue to find tons of value added info on here all of the time. Let's keep this thing going as the #1 go-to forum for sharing Mopar information with fellow enthusiasts.

So today on the first of the year we had a break in the rain here in northern california and I went out to tinker with the '68 Satellite. I haven't been driving it much as I'm trying to get all of the underside work complete and have just started to work on some steering issues. So, in mid-2010 I upgraded the front brakes to a disc system from Master Power Brakes. After having the system installed by a local no-name shop it felt like there were some issues, but they double-checked it and said it all checked out. Fast forward past a few things I did on my own, then I had the car back into another shop to do some suspension work and alignment. I asked them to check the brakes too while they were at it. They also said they couldn't find any issues.

So here's the problem - and I'm still convinced it's a problem - when I get in the car without the engine running and push the pedal, I hear a hissing whoosh sound and the pedal feels soft for the first few inches and then firms up. If I hold the pedal down and start the car, the pedal lowers slightly like it's supposed to. If I let off the pedal and press it again and again, the pedal feels soft at the upper range of travel and makes the same airy hissing whoosh sound, but then half way down the pedal feels firm and stays there.

I kept trying to convince myself that the hissing whoosh sound is normal, but I can't. In addition, when driving, the brakes feel worse in stopping than before as far as pedal feel and engagement. I used to have some spindles and front discs on there from a 73 E body (or was it A body?) and they felt great - really snappy and no whoosh or hiss. I only switched them out because I added some Hotchkis upper control arms and they told me the only spindle that wouldn't work with their stuff was the one that I had from the '73. (Figures!) So I do have a good frame of reference on how good the disc brakes could feel. Now I'm just really disappointed. By the way, the caliper bleeder screws are facing up at a 45 degree angle and I don't see any fluid leaks anywhere.

what do you guys think? Could it still be a bad booster or have a vacuum leak somewhere? There's also a proportioning valve that was installed and for the longest time I thought it could be that, but now I'm leaning towards the booster. Here are some photos of my system. If you see anything that looks really wrong, please let me know.

A little more info - the proportioning valve is mounted upwards below the booster on the frame. (see photo) The top of the valve has a bleeder nipple. The bottom is where the rear drum brake line hooks up. On the side of the valve body closest to the rear brake line connection is a connection that goes to the master cylinder front smaller reservoir. The top connection on the side of the valve body is connected to the larger rear master cylinder reservoir. ( I know the master cylinder is supposed to have a 10-lb residual valve in the connection seat and I would have thought the connections to the valve should be reversed so the front reservoir is for the front brakes and connected closest to the 2 front disc brake connections, with the other valve connection going to the larger rear brake reservoir on the MC, but mine is the opposite of this. Which is correct?)

Also, see the photo that shows the dent in the bearing cap. Don't think it affects the brakes, but I heard this is not good. Darn guys damaged this brand new part when they put it in. It's hard to get good help sometimes!

IMO it's the vacuum booster/power assist that sounds as though it's making more noise then it should and is your problem. anybody else?.

Sounds like like a vacuum booster to me too except that it should stay leaking if the diaphram's busted.I've had some that made noise and lasted a long time though. As far as the engagement, a vacuum leak will only make the pedal harder to push, just like a manual brake set up. The dent in the cap, it shouldn't affect anything unless it's rubbing the shaft. And, by the looks of it you're probably fine.Anyone else?
Don't want to get off the subject, but what proportioning valve did you use for your disc conversion?

Yup sounds like the vacuum booster... Mine did the same, hissed only when the brake was pressed and quickly went bad hissing most of the time with a very hard peddle to apply.
Replaced and problem solved.

Brake Booster.
Just replaced one on my girlfriends car. It was making gurdle farts. Progressively got worst til it even did it when your not braking.
Peddle alway felt ok, just very embarrassing when friends were in the car.

seems like the consensus is unanimous for a power booster, which is what I thought. The only thing that doesn't make sense is the pedal feels fine except for that and everything in the brake system is brand new including the booster and proportioning valve, which came with the kit from Master Power brakes. I keep trying to tell myself that instead of the parts being bad in a brand new kit, it must be installation error, but two shops have looked at it and found nothing wrong that they could identify.

Thanks for the quick replies guys. I'm going to call MP brakes tomorrow and talk to them about a booster exchange.

Talked with MP Brakes today - the tech guy was very helpful. I sent him some photos and he said the proportioning valve is fine in that orientation. With the description of what's going on with the booster, he said it's normal for it to make a little sound as the pedal is depressed and since my pedal feels normal and the car stops, he didn't think it was a booster problem, but rather could be a hyrdraulic issue. The brakes feel nice and firm though once they engage. It's only the pedal travel on the way to engagement that has that spongy feeling. Maybe nothing is wrong and I'm just comparing too much to my old disc brakes on the car. He did also mention that since my rotors are cross drilled that they have a zinc coating on there which needs to be worn off with a little longer than normal bed-in procedure. He advised 200 miles of stop and go driving. I've only got 15 miles on them so far.

Is there anything else that can go wrong at the firewall side of the booster where that push rod from the pedal linkage engages the booster?

Addressing soft pedal at first then gets harder, check your wheel bearings, if they are a little loose the rotor will wobble slightly & force the caliper pistons back slightly. (we do that on purpose when qualifying circle track cars)

phil70GTX said:

Addressing soft pedal at first then gets harder, check your wheel bearings, if they are a little loose the rotor will wobble slightly & force the caliper pistons back slightly. (we do that on purpose when qualifying circle track cars)

Click to expand...

Will check that - thanks!