1997 F-350 POWER STROKE 8 . MY BRAKE PEDAL IS HARD AND THE AIR DOES NOT BLOW OUT THE VENTS . A MECH. TOLD ME THAT IT IS NOT THE VACCUME PUMP ON A 97 . IS THAT CORRECT?

Most likely a failed master cylinder or vaccume leak to the booster. Here is how to check for a bad brake booster first with the vehicle off depress the brake pedal usually once or twice is sufficient but however long it takes to make the pedal as firm as possible now with your foot on the brake pedal pushing down start the car if the pedal moves your boosters not at fault it's your master cylinder or another problem if it doesn't move its most likely a booster problem

There is generally a cylinoid under the dash that runs the flaps in the car's vents to change whitch way they go, check that and also check if it has a relay in the fuse panel or under the dash it may be out. To chech the relay you can look on the back for the numbers and they will match with others in the pannel on the car just swop it out with a nother and then try if it workes then you know that the relay is bad and you need a new one. If not then you know that the cylinoid is bad and needs replaced.

intake manifold gasket.

under the hood you have a bad vac. line that feeds the hvac syt.

1. Check the fluid level in the reservoir after bleeding each wheel. Add DOT3 fluid, if necessary.
2. If the master cylinder was disassembled or if the reservoir becomes empty, bleed the air from the master cylinder as follows:
   1. Disconnect the brake lines from the master cylinder.
   2. Slowly depress the brake pedal and hold it.
   3. Block off the outlet plug with your finger, and release the brake pedal.
   4. Repeat 3 or 4 times.
   
   
3. Bleed the brake starting with the one furthest from the reservoir and then the next furthest, etc.
   
4. Depress the brake pedal several times, then loosen the bleeder plug with the pedal held down.
5. At the point when fluid stops coming out, tighten the bleeder plug to 11 Nm (8 ft. lbs.), then release the brake pedal.
6. Repeat until all the air in the fluid has been bled out.
7. Repeat the procedure to bleed the air out of brake line for each wheel.
8. Check the fluid level and add DOT3 fluid if necessary.

Check the vaccume line going to the brake booster. BUT if you can hear a hissing sound from inside and the pedal is hard to press more than likely your going to need a new brake booster and a new brake master cylinder.

If it's really air in the lines it will take two people to bleed it out, and if you're not careful you can make the problem even worse. You will need a bucket, a rag, and a small wrench (usually 3/8 or 7/16, and a box wrench works best). The lucky person needs to pump the brake pedal several times until the brakes are rock hard. Then the unlucky person needs to open the small bleeder valve of the back of a wheel (near where the brake line enters the wheel) and let the combination of air and brake fluid spray out - hopefully into the bucket. Once the pedal hits the floor, tighten the valve. Repeat this process two or three times on each wheel, or until the flow is all fluid - no air, making sure that the bleeder valve is retightened each time before the brake pedal is released. If the pedal is released with the valve open, air will be sucked into brake system, making the problem even worse. Oh yeah, the rag is for the brake fluid that misses the bucket and ends up all over the pavement and the guy working the bleeder valves.

sounds like the actual booster...can you pump the pedal several times while it is off and then start it with foot on pedal and feel any difference?

switch is bad or a bad vaccume to the vent doors that control the air flow

When you press on the brake? Have your brake vacuum assist checked. It,s that bigspaceship looking thing between the brake master cylinder an firewall. good luck and be safe.