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).

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

From my cold morning tests, where I put a vacuum gauge between the check valve and the booster, the differences between my known bad and 2 hopefully better units was subtle. After the engine was shut off, one however did seem to allow more "pedal pushes" before its vacuum was depleted, so I installed this unit.

It indeed seemed to work fine in the -20C temperature the next morning. I also noticed that it had held vacuum since the previous night when I parked it. If I ever have to test another one, I think I'll forget the vacuum gauge, and simply hook it up in place of the car's existing unit, to see if the seal holds overnight on a cold morning. This is probably the definitive yet simple test.

Then again, had I depleted the vacuum after parking it, who's to say the seal would pull back in on the cold morning! The ability to pass this test - with the seal sitting for hours relaxed in the extreme cold - would give me even more confidence in the unit.
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near

OK, so I'm talking to myself.

I called Bendix today, who FCP Groton attributes as the supplier of their brake boosters. They've sold the business (apparently to Bosch), but Jay was nice enough to field my questions. I had posed the following in my original thread:

"I've concluded from all this that the only way to get the unit to seal when it is cold, is to apply vacuum to the unit using a fairly significant force for a period of time - ie. manifold vacuum - more initial force than the vacuum pump is capable of delivering. Presumably, once this significant vacuum "primes" the seal by pulling it onto the shaft and holding it there for a period of time, it re-forms to the shaft, such that if the vacuum pump is tried shortly afterwards is able to hold vacuum."

Jay confirmed that this is indeed the case; the meagre flow offered my a hand pump is not enough to pull in the seal for a vacuum test. So I guess Bentley is misleading.

I suppose my only recourse is to either hope for the best with a used unit, or hook up a rig in the vacuum line so I can compare how quickly various units build vacuum after startup in extreme cold conditions.
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near

I
ve been listening too. Does it get so bad you must stand on the pedal braced against the chair back to lock up the wheels? Does it float down an inch when starting warm but not cold?

-jerry

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

"OK, so I'm talking to myself."
Yep, but I was listening. (What would YOU say to someone who confesses they suck on brake boosters?) :)
--
1980 245 Canadian B21A with SU carb and M46 trans

Very funny.

It's nice to know someone is getting benefit from my musings, although I suppose that regardless, it's helpful even for my own understanding to write it down; tends to make you think it through more thoroughly.

I don't suppose that you have a booster and/or check valve for sale, do you? My current plan is to pull one off my parts car tonight, but that's not a lot of fun in a field at -10 to -15C! Number9 wreckers want $50 for the booster/master cylinder combination; maybe I will see if they'll separate the two, but it has no check valve. The dealer wants $55 for just the check valve! (and I can't wait to get a $10 one delivered from FCP Groton). Just for laughs, the dealer wants $900 for the complete booster! (FCP Groton comes in at about $200(US), and UAP/NAPA is competitive at $250 Cdn).

My test now will be to connect a vacuum gauge in between the check valve and the booster when I first start the car in the cold. I'll see how fast vacuum comes up, and then pull the line off the manifold every 30 seconds or so, to see how soon it holds. Then I can repeat by connecting to the other booster while it's sitting on the strut tower, and compare.

I guess it's starting to look like I'd be better off to just install another booster and hope for the best. Unfortunately, I need to get all this done by Wednesday night, since Thursday morning appears to be the last time for a while that the temps are going to -20C.


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

I have a spare booster but it's for "just in case".

$50 sounds okay for an MC and booster. Don't know where Number9 wreckers is.

Standardautowreckers.com lists a power booster for $25 in its UPIC list ($15 for the MC). But that means visiting the yard in Markham and finding a 240 (or 740?) and freezing.

"Just for laughs, the dealer wants $900 for the complete booster!"
That's why I don't bother with them and will drive miles to a wreckers or order from abroad.
--
1980 245 Canadian B21A with SU carb and M46 trans

Number 9 is on Highway 9, just west of the 400 (it runs west into Orangeville).

$50 including the MC isn't bad if you want the MC, but I wonder how good an MC exposed to air for an unknown period is anyway. I got him down to $35 because it didn't have the check valve, and I didn't want the MC, but I got both anyway.

I tested vacuum by pulling the check valve, and tee-ing a connection between it and the booster off to my vacuum gauge. The results in the cold this morning were not obvious. All 3 units (the current one, the one I pulled off my "parts" car, and the one from the wrecker) seemed to build vacuum about the same. The one from the wreckers however didn't seem to lose it as fas when the plunger was depressed, so I installed that one tonight. I'm hoping tomorrow morning is cold, so I can get a meaningful test of if its better.

For all the hassle, in hindsight, I should have just installed the replacement without testing and hoped for the best. Being methodical is sometimes counter-productive. If the new one's fine though, I'll probably justify all my efforts by convincing myself that my tests pointed me to the right unit.
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near

Being methodical is sometimes counter-productive.
LOL - story of my life.

I was going to offer a dumb idea: An ordinary household vacuum cleaner pulls an excellent vacuum with speed (volume). Maybe it would seat the seal where the hand pump won't. Just sounds like something I would try, but haven't yet.
--
Art Benstein near Baltimore

A vacuum cleaner is a good idea (a little obvious; no wonder I never thought of it)!

I on the other hand, used the car's intake vacuum this morning, at -25C, because the problem is obvious in extreme cold. Due to the cold temperature, the supple vacuum fittings become rigid and hard to control and probably introduce some leaks that didn't show in my indoor testing. Suffice to say that in the end, the differences in the results were subtle, not obvious like I had hoped. A vacuum cleaner driven test would have been easier to setup and control, but I'm not convinced the results would have be clearer.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near