I don't think there is a leak or seal failure, because the pedal does not go down when it happens. It feels as though the brakes release very briefly, then re-engage - i.e. as you might expect in a panic stop with ABS engaged.

I also don't think it is vacuum-related, because I would expect vacuum to be fairly high at closed throttle. Also, if it were vacuum related, I can't see how it would pulse in and out as it does.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7