I posted on this previously - extreme cold causes brakes malfunction 200 1986 - but it was a little long-winded, so I thought I'd restate it here, a little more briefly.

My brake booster is malfunctioning in extreme cold (-20C; -4F), at least until manifold vacuum has "primed" the seal for a while. My qualifying test is to use a hand vacuum pump on the hose leading to the check valve (as per Bentley). When my unit has been under manifold vacuum for a good while, even in extreme cold, it holds 12 in. Hg fine. If I test it after the car has been sitting however, it won't build a vacuum.

I went to purchase a unit from a wrecker, but when I tried the vacuum pump test directly on the brake booster check valve seal (unit had no check valve), it wouldn't build a vacuum, even with the unit at room temperature (I've concluded that any unit would be better at this in warmer temperatures, since the seal would be more supple. My experience with my faulty unit supports this too, as I don't have problems when the weather is warmer).

I did the same test (at the brake booster seal) on my "parts" car sitting in the cold at -20C. It too failed to build a vacuum.

Can anyone tell me how I can go about qualifying a used brake booster as being better than the one I currently have? I don't want to do all this work and find I'm no further ahead; plus the weather's going to get warmer in a couple of days, so I won't be able to verify the cold performance of a newly installed unit. I could buy a new one from FCP Groton, but that'll end up costing over $250, compared to $40 for a used one, plus it'll take 2 weeks for Canadian delivery. I was hoping to avoid this, but maybe I can't (or shouldn't).

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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near