Hello everyone I've been having a problem since I bought my truck about a year ago it's a 94 3.0 but Everytime I cold start it to go to work the brake pedal is rock solid along with a hissing coming from the dash but as it warms up it goes away I have a hunch it's my brake booster but I'm not to Shure I've been told these trucks have slot of vacuum leaks the older they get any suggestions where I should start???

 

Its for sure the brake booster


The way a power brake booster works is to use outside air pressure to assist the driver in pushing down on the brake pedal
The booster is on the engine side of firewall but it's center is on the inside of the cab, the brake pedals push rod is in the center part
The engine provides vacuum to the larger part of booster which is Negative pressure, i.e. 20" vacuum = about -9psi
When you press down on the brake pedal that opens the Atmosphere Valve, the center part in the cab, this lets in the +15psi outside air pressure, and since there is Negative pressure in the booster it adds to the power of the driver's leg force
What you hear in the cab is the leaking atmosphere valve, so vacuum takes awhile to build up in the booster, so you need a new booster

FYI, for safety a power brake booster should hold vacuum for 3 to 4 pedal presses after the engine is shut off, this is in case of a stalled engine while driving, the driver can safely pull over and stop while still having power assist
So simple test for a leaking booster is to shut off the engine and press the brake pedal 3 or 4 times until it gets hard to press, if its not at least 3 then booster is leaking
OR
If you get into a vehicle that has sat for over an hour and brake pedal is hard then booster is leaking

Booster's do have a Check Valve, usually its the 90deg fitting on the booster that the hose connects to, but some have it on the hose to booster, not expensive to replace
But in your case that won't be the issue since you hear vacuum being released in the cab