Hissing Sound with Unusual Timing

Hi All. I am new to the group so I hope this new post is appropriate. I have tried reading through various historical post about hissing sounds related to brake boosters and the Cadillac valve for the hmmwv but my particular issue has something that is puzzling me.

Basically, I'm getting a hissing sound that is coming from below the dash directly behind the brake pedal at the booster. When the engine is cold, the hissing will pulsate when you tap the break. Originally my thought was that it is simply the vacuum leaking in the booster, which is of course directly affected by the pedal position at the diaphragm. However, I have noticed more recently that whenever the fan kicks in, the hissing goes away.

If the brake booster was leaking vacuum, would I be correct in assuming it would be leaking all of the time unaffected by the fan being on or off? I understand that the fan is engaged through the Cadillac valve utilizing hydraulic pressure from the power steering, but is this also linked to the brake booster and some way?

Fan on - hissing stops
Fan off - hissing present from booster that will change slightly based on pedal position?

papakb said:

Since there is NO vacuum system in the HMMWV it is highly unlikely that you have a vacuum leak!

However, the Kysor Cadillac valve that controls the fan clutch is right there in the same area and hisses all over the place until the system reaches arounf 230 degrees and it shuts off to actuate the fan and cool the engine back down to ~190 degrees. Completely normal and can be alleviated by wrapping a piece of inner tube rubber around the valve under the mounting clamp.

Click to expand...

Thanks papakb. I have read up about the hissing from the Cadillac valves, but I thought it was strange that the hissing was being affected by the brake pedal.

Makes sense now as a hydro boost connected to power steering! I've owned my hmmwv for about a month and I failed to realize there was no vacuum, my bad.

I will try and wrap the cadillac valve asap and see if it quiets down.

Bone conduction/ mechanical into the foot well makes for the "snake in the cab" sound.

Lookout !!! You've been bitten by the HumV, CAMO

The brake booster is part of the same hydraulic system as the Cadillac valve, the brake booster has its own return to the pump so when you step on the brake some of the flow that would be running through the valve is diverted through the brake booster directly back to the pump

Thanks guys. I have wrapped the cadillac valve and that got rid of most of the hissing. Now that I am aware that the brake booster is connected that all makes sense!
Cheers,