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Usually it is not the modulator either , although possible. Did you check the rubber double connector on the adjsutment valve on top of motor? This is often cracked. It can be replaced with two single hoses.

There is the modulator on the tranny, and vacuum adjustment valve to the left of IP on top of engine, this can be adjusted by loosening screw rotating it some. The vacuum pumps do occasionally go bad, but I'd double check everything else first, those pump heads are getting hard to get last...