2000 Maxima GXE, 75K mi: On extremely cold mornings (10 deg f) there is no vacuum assist on brakes for first minute. I start vehicle, I get 1-2 pedal actuations w/ assist, then no assist for approx. 30 sec, then assist returns and functions normally for duration of day. Seems like there is residual vacuum followed by a delay of establishing vacuum. Assumption is that booster is okay and there is loss of vac. ... where do I start? Has anybody seen this?