How do i change a power brake booster on a 1993 ford bronco

must be air in the lines still. was the master bench bled before installing it? power bleeding at a good shop is the best bet

have you checked for vacum leak at brake booster ? if leaking vacum , when braked pressed . will make hissing sound .

You may have lost you brake assist. Look for any disconnected or broken hoses and vacuum leaks especially the one going to the brake booster. make sure that the check valve is good it should allow air through in one direction but not the other. if all checks out good apply vacuum to the power brake booster and make sure it hold if not replace the power brake booster.

Sounds like a vacuum leak coming from either the HVAC controls or the brake booster. Does the noise get louder or change pitch when you depress the brake pedal? If so then you need a new power brake booster. If not then check for cracked or broken vacuum lines under the dash.

make sure that when you replaced the power brake booster, you have the rod that runs from the booster hooked up right, and not in a bind.

Take it back to where you had brakes repaired and have them fix it correctly.

Check the vacuum brake booster and check valve maybe there is a problem there that is amplifying the vacuum inside the booster and applying the brakes. I hope this helps, let me know.

yes master Cyl

I would POWER BLEED the brake system. Hope this helps

if the booster is bad the pedal would be hard sounds like the e-brake is stuck take off the drums and make sure the arms are all the way back then did you adjust the rear brakes after changing the shoes