Brake pedal goes to floor when engine is on and don't when car is turned off. When off pedal is nice and stiff,, but when engine is turned on, NO pedal. It goes to the floor. Father in law thinks it maybe booster. Has anyone else had this problem.

Your father-in-law is smart--check the brake booster.

Functional Test:

1. With engine off, depress brake pedal several times; then, depress the pedal hard and hold that pressure for 15 seconds. If the pedal sinks, the master cylinder, brake line, or brake caliper is bad.

2. Start engine with pedal depressed. If the pedal sinks slightly, the vacuum booster is working. If the pedal height does not vary, the booster or check valve is faulty.

Leak Test:

1. Depress the brake pedal with the engine running, then stop the engine. If the pedal height does not vary, while depressed for 30 seconds, the vacuum booster is fine. If the pedal rises, the booster is faulty.

2. With the engine off, depress the brake pedal several times using normal pressure. When the pedal is first depressed, it should be low. On consecutive applications, pedal height should gradually rise. If the pedal position does not vary, check the booster check valve.

Check Valve Test:

1. Disconnect brake booster vacuum hose at booster.

2. Start engine and let it idle. Vacuum should be available--if not, the check valve is not working correctly. Replace the check valve and retest.

Your father-in-law is smart--check the brake booster.

Functional Test:

1. With engine off, depress brake pedal several times; then, depress the pedal hard and hold that pressure for 15 seconds. If the pedal sinks, the master cylinder, brake line, or brake caliper is bad.

2. Start engine with pedal depressed. If the pedal sinks slightly, the vacuum booster is working. If the pedal height does not vary, the booster or check valve is faulty.

Leak Test:

1. Depress the brake pedal with the engine running, then stop the engine. If the pedal height does not vary, while depressed for 30 seconds, the vacuum booster is fine. If the pedal rises, the booster is faulty.

2. With the engine off, depress the brake pedal several times using normal pressure. When the pedal is first depressed, it should be low. On consecutive applications, pedal height should gradually rise. If the pedal position does not vary, check the booster check valve.

Check Valve Test:

1. Disconnect brake booster vacuum hose at booster.

2. Start engine and let it idle. Vacuum should be available--if not, the check valve is not working correctly. Replace the check valve and retest.

All right so I know this is a tad old forum/question. However I am having the same prolem on my 2001 Jimmy. Did all the tests that you suggested and the booster is fine, any other ideas...
Thanks in advance

Oops, replaced the brakes (all 4) as well as the master cylinder... all with no prevail

If you didn't bench bleed the master cylinder first, there may be a lot of air in the system. You may also want to perform the brake booster tests below:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-12.html#post2321080

I have the sameproblem, but one up. The engine dies when I press the peddal. Just replaced the master and booster, 97 sentra. Bled the heck out of it, bech bled the master, booster tests out fine, so does the check valve. I'm serching for a vacuum leak next.