Re: Vacuum issue for heater temp
With no check valve if you get a leak in the HVAC system it will act like a vacuum leak in the intake and the engine will run bad. Also, without a check valve the canister vacuum will rise and fall with acceleration, which is the same as just attaching the HVAC hose to the intake.
Your canister should have one built in. If not you can get one and put it on-line on the hose from the intake.
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Putts drives a 1965 Galaxie. Rebuilt 390, 4100 on a Performer, MX, AC, PS, PB w Scarebird front discs on Cragar 17" S/S.