I've noticed w/ my car lately that if I run it for a while (30 min at least) that the brakes will feel like they've lost power. If I go start it up right now today for the first time, it will brake perfectly. But if I run it for a while the pedal will go down almost to the floor w/ little to no effort and then I have to push kinda hard to brake. It feels exactly like the car does when I put it in neutral and roll it down the driveway... well not exactly but very close. If I pump the pedal a few times it gets full power, w/ each successive pump pedal gets higher and higher. Then the next time I brake it will go all the way to the floor. I am betting master cylinder or the vacuum servo that's connected to the master cylinder. Any help please?

Thanks in advance. Oh yeah I do have fairly new brake pads (changed about 6 months ago) in the front and was told the back pads were fine and I wouldn't need to change them until I change the front ones two times.

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1984 190D 2.2 Auto 220k
2001 Jeep Cherokee Sport 70K
2004 Lexus RX-330 ??K
2005 Chrylser Crossfire LTD 6K

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