Hissing Sound That Stops When Brakes Applied.....

[92Apocalypso]

gentleman,

so my 92 LX had a 5 lug disc conversion from previous owner, utilizing mostly parts from an 04 GT. I've had issues with the setup in the past, but just got some squared away when I replaced the factory prop valve with the MMS delete kit and added the ford racing adjustable prop valve. I've always heard a hissing sound coming from the dash, but I swear it sounds more prominent now. there's always a hissing noise, but once I press the brake to the floor it stops. is it the brake booster? I'll upload photos, but I think the master cylinder and the brake booster are the factory 92 LX variety. any insight on this?

 

[OneWheelPeel]

This could be completely wrong but when I think of hissing I think of a vacuum leak and I know the brakes tie into a vacuum line for the brake booster I'm pretty sure so I would look in that area.

 

[92Apocalypso]

This could be completely wrong but when I think of hissing I think of a vacuum leak and I know the brakes tie into a vacuum line for the brake booster I'm pretty sure so I would look in that area.

I agree. I originally thought vacuum, but I've been all over and haven't spotted anything. just strange that it does it until i press the pedal in

 

[Mustang5L5]

Shut engine off. Wait five mins and press the brake pedal. Do you still have vacuum?


I'm guessing it's a brake booster leak. It goes away when you press pedal because you've shifted the diaphragm slightly and it could be sealing then.

Do you have a vacuum pump to apply vac with engine off?

 

[92Apocalypso]

Shut engine off. Wait five mins and press the brake pedal. Do you still have vacuum?


I'm guessing it's a brake booster leak. It goes away when you press pedal because you've shifted the diaphragm slightly and it could be sealing then.

Do you have a vacuum pump to apply vac with engine off?

I do not have a vacuum pump. also, excuse my ignorance, how exactly will I know if I still have vacuum when pressing brake after car is off for 5 min.

if I do need new brake booster, what do I upgrade to? also, should the master cylinder be upgraded as well? thanks for your help as always..... that MMS kit worked out great

 

[Mustang5L5]

Usually after you turn the engine off, you have 2-3 presses of the brake pedal before it becomrs hard as a rock due to lack of vacuum assist. If you have a leak, the pedal might be hard as a rock immediately after turning off the engine.

As for what booster to install. Let's see what you have right now. Can you take pic?

 

[92Apocalypso]

Shut engine off. Wait five mins and press the brake pedal. Do you still have vacuum?


I'm guessing it's a brake booster leak. It goes away when you press pedal because you've shifted the diaphragm slightly and it could be sealing then.

Do you have a vacuum pump to apply vac with engine off?

I do not have a vacuum pump. also, excuse my ignorance, how exactly will I know if I still have vacuum when pressing brake after car is off for 5 min.

if I do need new brake booster, what do I upgrade to? also, should the master cylinder be upgraded as well? thanks for your help as always..... that MMS kit worked out great

Usually after you turn the engine off, you have 2-3 presses of the brake pedal before it becomrs hard as a rock due to lack of vacuum assist. If you have a leak, the pedal might be hard as a rock immediately after turning off the engine.

As for what booster to install. Let's see what you have right now. Can you take pic?

currently, I had these photos in my phone. I'm away from the car right now, but will try that out tomorrow.

 

[Mustang5L5]

I see an F4 part number. So I assume it's a 94-95 booster.

I'd replace it with the same

 

[OneWheelPeel]

I once had a brake booster fail in a jeep in traffic... That was a scary moment. I'd figure out if that was your problem or not and then replace it asap.

 

[92Apocalypso]

are there any other ways to check this out aside from that brake pump test after car is shut off? the brakes especially worry me because the E-brake is not set up.... I don't want to be without brakes

 

[Mustang5L5]

Carb cleaner? Smoke test?

 

[92Apocalypso]

I recently sprayed a bunch of cleaner all over looking for vacuum leaks when i was having issues with inspection a couple of months ago, and couldn't find anything, I'll try again though near that booster. how about the master cylinder? was it factory, and would an upgrade help with anything, not necessarily the hissing sounds but performance.

 

[92Apocalypso]

Usually after you turn the engine off, you have 2-3 presses of the brake pedal before it becomrs hard as a rock due to lack of vacuum assist. If you have a leak, the pedal might be hard as a rock immediately after turning off the engine.

As for what booster to install. Let's see what you have right now. Can you take pic?

drove the car to work this morning. waited a few minutes after turning if off and I pressed the brake pedal a couple of times. the brake was harder then usual, but def not rock hard. I can still pump it a few times and it moves 3-4 inches no problem.

 

[92Apocalypso]

Usually after you turn the engine off, you have 2-3 presses of the brake pedal before it becomrs hard as a rock due to lack of vacuum assist. If you have a leak, the pedal might be hard as a rock immediately after turning off the engine.

As for what booster to install. Let's see what you have right now. Can you take pic?

I took a video of the noise while the car was running, but says file too large to upload....

 

[Mustang5L5]

Stick it on youtube and share link?

 

[92Apocalypso]

Stick it on youtube and share link?

View: https://youtu.be/AFBPQ1QtvBo

 

[mikestang63]

my guess is either

• check valve on booster
• booster diaphragm or case has a leak

 

[Mustang5L5]

my guess is either

• check valve on booster
• booster diaphragm or case has a leak

ditto

 

[Mustang5L5]

BTW, your MC is a later model 2-port MC for rear disk brakes. Which one, I can't tell.

 

[92Apocalypso]

BTW, your MC is a later model 2-port MC for rear disk brakes. Which one, I can't tell.

good to know. so I should be ok with that MC right? it doesn't need to be replaced? should I just order a 94-95 brake booster and just replace it? also. due to not having an emergency brake, should I not drove this thing much if the booster is bad.... thanks