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Discussion Starter #1
I have a 97 F250 diesel. My brake warning light has been coming on intermittantly and I've determined that it is because of low vacuum. I attached a vacuum guage and have determined that I can empty the vacuum reservoir at idle in three pedal pushes. My problem is that I don't know if I'm consuming too much or not producing enough.

Here are my experiments:

With no vacuum in the reservoir, I start the engine and run it at idle. The vacuum builds slowly to about 25 inches in 20 seconds and holds there. If I rev the engine, the vacuum gets to about 45 inches.

I now turn the key off. The vacuum stays constant, indicating that there are no leaks.

Starting at 45 inches, I press the brake pedal and release it once. My vacuum is now at 20 inches. I press and release a second time and the vacuum goes to 10 inches. After the third press, the vacuum is zero.

Do I need a new pump or a new booster?
 

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Discussion Starter #2
I replaced the vacuum pump and all is well. It now takes about 1/2 second to reach 15 inches of vacuum and gets to a max of about 50 inches after 10 to 15 seconds at idle.
 
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