I am getting gas in the brake booster on my 1988 Bronco II. It has ruined two so far. The vehicle is getting about 12-14 MPG. There is vacuum in the booster because the last time I drove I got out and pulled the hose off and the was vauum there. How could fluid be traveling backwards in a vacuum line
There should be a check valve in the line where it attaches to the booster, if the check valve was stuck open then when you shut down the engine that vacuum tank could draw residual gas fron the intake manifold, a way to check the valve is after you run the vehicle shut it off, wait a few minutes and see if you still have vacuum boost at the brakes, there should be, if not then the check valve is probably bad and should be replaced.
If it’s fuel injected, the problem might be with a leaking fuel pressure regulator. Start the engine and run it for a minute. Remove the vacuum hose from the fuel pressure regulator, and if gas leaks from this connection the diaphram in the regulator has failed.
Here’s an image of the regulator.