2001 Ford Escape hissing noise

The condition of the diaphragm inside the booster is also important. If cracked, ruptured or leaking, it won't hold vacuum and can't provide much power assist. Leaks in the master cylinder can allow brake fluid to be siphoned into the booster, accelerating the demise of the diaphragm. So if there's brake fluid inside the vacuum hose, it's a good indication the master cylinder is leaking and needs to be rebuilt or replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced

i have recentally had my master cylinder replaced for $260.00 and the noise came back after 2 weeks. should i call the shop back or do you think its something else?? Also i have a right rear brake hose cut could that have anything to do with it?? My check engine light has been on and going off after about 2 days, i did a diagnoses on it and got the code p1401...i have changed the egr pressure valve 3 times in the past 2 years also..what am i to do???? this is my first car and i just paid it off...i am at 89879 miles..

P1401 DPFE Sensor High Voltage

For the hissing sound call the shop, the cut on the brake hose has nothing to do with it-