My dad's '08 has a brake pedal that hisses from under the dash whenever it's released; when the brake is applied at all the hiss goes away. This hiss is constant when it happens, but it's came and gone a couple times now; we've had a couple weeks of constant hiss, a week of no hiss, and are now on another couple weeks of hiss.

Anybody else have this problem? TSB?

What would it be?

Begle1's Sig:

A custom-made electro-digital-hydraulic water injection system with a 1990 D-250 wrapped around it that on a good day might break 15 seconds in the quarter.

man i dont know. mine dont do that

diesel pap's Sig:

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i had an f150 that hissed til u stepped on the brake, there were some bad seals in the booster.

cummins95's Sig:

95 dodge ram 2500 rc lb 4x4, nv4500 swap, southbend 3250 dual disc, rough country 2.5" leveling kit. no plate, starwheel cranked, 4k GSK, afc forward, wastegate semi restricted, bhaf, primeloc remote fuel filter, 4in dp to 5in straight pipe with 5 inch black stack, 2095 mack rack plug.
2001 sterling triaxel dump truck. 600 horse N14 Cummins, 8LL transmission

There was a gentleman that had a similar issue with his 07.5 in another forum that I remember. This turn out to be a failed brake hose, the liner which is Teflon had failed and was extruding in to the booster. He finally replace this hose and the hissing went a way. I believe that this was the return hose that failed. He posted pictures of the failed hose on two different forums, check them out.
This may or may not be your problem but it would be a worth while for you to check.
PM me and I will look for the thread if you can not find it.

jim w's Sig:

2500HD Mega Cab SLT, 6.7L CTD 68RFE,Optix Gauges Pyro, Boost, Trans Temp A Piller mount, Silver, Westin Side Steps, Line-X Spray in Bed Liner over the rails, Bed Cover, Prodigy BC, 15K Reese hitch and pulling a 11K 5er

I had the failed liner in the power steering hose to the booster. Basically it was as described with the liner out of the hose coming loose and causing a blockage to the booster. I was concerned it was the booster but relieved it was only a hose. 2 Dodge techs on the TDR diagnosed it over the web in minutes... Seems to be a common failure..

If you don't mind heading to another thread/site I have photos etc.

http://www.dieseltruckresource.com/d...s-t255765.html

or

Power Steering/Hydroboost hiss - TDR Roundtable

Lil Dog's Sig:

97 Dodge CTD 2500 4X4 LWB CC SLT Laramie 5 spd LSD 3.55. Goodies from: South Bend Clutch, PEAK, PDR, Scotty, DieselPowerProducts, Solid Steel, Roda Deaco Positive Air Shutoff. Espar D5WSC heater. B-Quiet sound deadening inside. Dyno 08-2009/ 405 RWHP/1100 Ft-lbs

08 Dodge 2500 4X4 Mega Laramie - 6.7 CTD, 68RFE, 3.73 LSD, MADS S-67ME POD.

I have the same hissing noise comming from under the dash. I hope this is the only problem. I have lost my job now for a few months and the dealer is saying my warrinty don't cover this issue. I hope this won't lead to a bigger problem. I have also had the turbo issue but after the fixno more problems. Send pic's of what line is the problem maybe I can fix it myself.

IT'S FIXED!!! It was the brake booster or better known as the hydrolic seperator well this is what the dealer told me anyway they replaced the booster and the booster line and there is no more noises ah quite again.