Hearing a lot of hissing coming from the brake pedal, slowly fades after engine is turned off. Master cylinder seems ok, pedal doesn't compress anymore after they engage.

Is this the brake booster? The pedal doesn't seem hard to press but I have no prior to compare to.

I've included a video.

Thoughts?

 

Sounds like a leaky booster. Is there rust on the booster.

After turning the engine off, how many times can you slowly press the pedal before it becomes heavy? If you don’t touch the pedal after turning off the engine, how many soft presses do you get after letting it sit overnight.

Sixto
07 E350 5.4 183K miles

 

I can press the pedal 1 or 1.5 times before whatever air assist in there is gone, and it also makes the hissing noise stop, so yeah that's gotta be the booster.

Booster itself doesnt have much rust on it. I've included a couple pictures of the booster and cylinder. Looks new enough to maybe have been replaced.

I have records on this thing going back to the original owner though, and while the master cylinder was replaced twice, I don't see the booster anywhere, so maybe this is original and the fluid from those leaky cylinders finally wore this thing out?

 

Do you hear the hiss louder in the cabin or in the engine bay? If louder in the engine bay, it might be a tired check valve or worn gasket between the master cylinder and booster. If in the cabin, it might be a worn seal around the brake pedal rod into the booster. I’m guessing from other experience since I’ve never worked an Econoline booster.

Sixto
07 E350 5.4 183K miles

 

It's loud in the cabin, I hear almost none of it from the engine bay.

Is it worth trying to replace just that shaft seal, or better to swap entire booster? I can get a rebuilt one for under 100.

 

I don’t see parts for the booster to repair in place other than the check valve in the engine bay. If the booster has to come out, might as well replace it.

Sixto
07 E350 5.4 183K miles

 

Ok ordered, thank you!

 

You are aware that when changing the brake booster or master cylinder, that you have to check and perhaps set the brake booster push rod length?
There is a tool just for adjusting the length or you can measure the length or make your own tool. Youtube is full of videos on the specific tool and a few on how to measure the length.
I didn't see where you posted what van you have of engine size but your profile says 1991. 1988-199 use the same booster and master cylinder depending on the GVW. This diagram is from my 1988 Ford service manual showing the measurements and should be the same for your 1991 if its a E150 or E250




My brake booster has been hissing inside when you shut the engine off and press on the brake peddle for about a year now. The diaphragm probably has a very small hole in it. As it gets worse, you will eventually loose the power brake assist.. But it can also cause rough idle and maybe stalling when you come to a stop because you can loose a lot of vacuum through the booster. I bought a rebuilt one from the parts store but haven't installed it yet because its just too hot outside and mine hasn't gotten any worse. I had to buy a rebuilt one because my van was built at the end of 1987 and uses a different booster then if it had been manufactured in 88 and none were available new.
Do not throw your old one away or return for a core. There are a couple of companies that can rebuild it for you. i think all of the parts stores use rebuilt Cardone units for our older vans, at least on my 88 anyway. I am not sure about how many your booster was used.

One of those companies will actually buy some old boosters because they are no longer manufactured. Below is a good video with a lot of good info. Even though they are talking about older boosters its still good info. Also shows measuring tool for a Ford.

 

When it comes to power brake boosters
The most common complaint is a hissing noise
The Lincolns have a bunch more foam than all the rest of the Fords and Mercs
Used to double up the foam on the Fords when replacing
Replaced a whole bunch of boosters just for hissing that still hissed
If you ordered one for a Continental there was 3 inches of foam between you and the booster
Good luck

 

Yes that's right, a 91 E350. I was not aware of the need to measure or set the rod, the videos I've watched just show a simple swap. Thank you!

I've received a rebuilt unit from Cardone. I'll hold on to the old one and see if I can find a buyer.

 

Ok! Well then I won't freak out if I still hear something after swapping the booster. Thanks for saving me some potential anxiety.

 

OK, a E350! The diagram I posted above was from a section in my manual that says "Applies to E and F 150-250. So I am not sure if those measurements apply to a E350. I will have to check later this afternoon and see if there is a different section that applies to your E350.. Maybe you will have to use a different method to set the gap. Most people say 1/16"

 

I just found the same image in my manual. So it applies to E350 as well.

 

These threads always lack closure so figured I'd report in on completion.

New brake booster is in and has eliminated hissing. System holds vacuum after engine is off.

I had to remove master cylinder and a couple other things but managed to wedge it past remaining obstacles.

On to the next thing!

 

Good of you to update your own thread---very helpful to others at some point I'm sure!