'90 mustang/hard brake pedal, hissing sound

[fords4me]

Leaving work the other night, the first time I applied the brake, it was very hard almost like it was pushing back and I hear a fairly loud hissing. Hissing sounded like it was coming from under the dash. also for a few days before this happened my idle has been slightly surging and I would smell raw fuel and the engine would stall after a few surges. Can anyone help?

[Davescort97]

Welcome to AF!

Sounds like a vacuum leak to the brake booster or the check valve mounted inside the brake booster vacuum hose is bad. To check it, disconnect the hose from the engine and brake booster. Try to blow air through it both ways. Air should only flow towards the engine side of the hose. If it blows both ways or neither way replace the hose and check valve.

The brake booster itself could have a vacuum leak. But, before you replace the booster, check the vacuum hose for leakage. If there are no leaks in the vacuum hose, the diaphragm of the brake booster is faulty. Put on a new power brake booster.

[fords4me]

Thanks Dave. I ended up replacing the booster and the hiss under the dash when i hit the brakes is gone. Im still having a idle issue though. Any ideas?

[Davescort97]

There may be a vacuum leak somewhere else.

[fords4me]

Turns out it wasnt another vacuum leak. Just to reiterate idle was surging. It would rev then drop, rev and drop. Sometimes it would stall, other times it would even out and stay steady, depending on how long I stayed still. I noticed too while I was driving, it would feel like it was missing almost. Anyway I replaced the fuel pressure regulator today and my problem is solved.
I believe the vacuum was sucking raw fuel into the intake and flooding the engine or at least upsetting the air/fuel mixture. I got this below off a website about a test of and symptoms of a bad regulator.

Fuel Pressure Regulator Test
This test checks the operation of the fuel pressure regulator to make sure it changes line pressure in response to changes in engine vacuum. This is necessary to maintain the proper operating pressure behind the injectors and to compensate for changes in engine load.
With the engine running, disconnect the vacuum hose from the pressure regulator. As a rule, fuel system pressure should increase 8 to 12 psi with the line disconnected. No change would indicate a faulty pressure regulator, or a leaky or plugged vacuum line.
If the diaphragm inside the regulator is leaking, engine vacuum will suck raw fuel into the intake manifold through the vacuum hose (look for fuel inside the hose).