I should probably know this but as I've moved on to newer cars I've forgotten the old ways. My old bucket 2004 Ford Focus Wagon (231K miles) has a on again off again rough idle. I've replaced the fuel pressure thingamabob but it didn't really help. I noticed while holding my foot on the brake that when I turn the car off the air pressure immediately goes out and the pedal turns hard. Does this mean there's a leak?
Does it come up? If you turn your car off with the brake on, it should stay at the same height, as long as you maintain pressure on the brake. If you pump it with the engine off, it should get progressively harder to press, until you really can't press it anymore.
Yes! I tried holding steady pressure on the brake pedal and then turned off the car. You could hear the air leaving and the pedal came up and became firm.
It's definitely not supposed to do that. Your brake booster is supposed to use engine vacuum to boost your brakes, when there's a leak in the booster, it can cause a rough idle, because there's a lot of unmetered air being let in, causing your engine to run lean.