Bizarre problem. The car runs erratically, but predictable. It revs high then low high low. It's auto so when you put it in gear it stops revving but runs at a higher than normal rpm.

I mention this with a note of caution because it can make you a candidate for the Darwin Awards.

With the engine at idle, spray some either or brake parts cleaner at the hoses and connections and listen for the idle speed to go up.

The ether gets sucked into the vacuum leak and changes your fuel air mixture momentarily, and lets you know about where the leak is.

This would probably not work with a F/I engine since Lamda Sond is supposed to smooth that stuff out, and the ether might fry the O2 sensor.

I learned that trick from an old mechanic with no eyebrows.
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'70 145, Webber converson, trying to keep it stock other than that.

Its fuel injected B20E.

What an annoying problem... I had 142 that did that with about a 2 second frequency. ANd the fan belt would squeek every time it reved. At least it wasn't an automatic.

My guess is with the brakes... easy enough to test that just be squeezing the hose to the booster with a pair of vice grips and see if that makes it go away.

Could be the one way air valve where the hose meets the booster..

-Matt
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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC

Did you look it over? Carbs or FI? B20 or B30?

Could be any of several hoses, brake booster, etc or even a manifold gasket.

If FI it could be injector seals, if carbs maybe a throttle shaft leak
or not bolted up good and tight. Could even be a loose butterfly in either one.
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George Downs, Bartlesville, Oklahoma, Central US