Brake pedal rock hard

It started a couple of days ago but now it's frequent. Every now and again my brake pedal goes rock hard right from the top. After a minute it goes back to normal and then after 10 more minutes it goes rock hard. It doesn't affect the braking power, only the feel of the pedal. Any ideas???

Master cylinder valve fooked? Maybe?

steve-o said:

Master cylinder valve fooked? Maybe?

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Could be... maybe?

More likely to be vacuum pump / servo unit.

instantsquid said:

More likely to be vacuum pump / servo unit.

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Right that makes 2 things to check.