It's hard to making a sensor that is both easy to make, calibrate, and not subject to being damaged by large pressure swings.
I would recommend trying a small garbage bag.
1. take a small garbage bag. (bathroom type)
2. Tape the end over a piece of cardboard so that the channels in the cardboard can be used as the input pipe.
3. tape hose, ect to fill the bag.
4. Lay sheets of paper on the bag until it's roughly flat and barely puffs up.
5. Weigh the paper sheets and divide by the square inches.
The real question is why you need to measure the pressure?
If all you are doing is finding leaks, just having pressure is enough.