No brake assist when it's cold out

B

BrokenAxle

New member
Hey guys!

I have a 77 Scout II here that when it's really cold out there is no brake assist. Drivetrain is all from a 2019 Chevy 1500 including the rear diff. Front diff is out of a Jeep J10. Brake master and booster are brand new. Brake calipers (f+r) and hoses are new. Brakes have been bled a lot with new fluid from a sealed container. Vacuum hose and check valve are new. The only things in the brake system that are original are some of the hard lines and the proportioning valve. When it's -20c (-4f) there is no brake assist. There is good vacuum to the booster. Had the same problem with the old booster and master cylinder. Is this something anyone has dealt with with these before? I'm at a loss on what's going on! When it's warmer out, brakes and assist work great.

Thanks!
Dave
 
I never have, primarily because I think I'd be dead at that temp. That and my brake system is stock. I doubt my Scout has seen much below 20 F. That's plenty cold enough for me, thank you very much.
 
when it was -20 i didnt have any assist either. i thought it was because the cold made everything so contracted that it just wouldnt hold vacuum. slowly it came back to working normal, as the engine generated a little heat.
 
Your brake fluid is probably full of moisture and gets congealed when it is that cold out. Brake fluid attracts moisture very easily. Hence the reasoning for it working only when it is warmer.
 
There is a tester gizmo that you stick in the fluid to measure the moisture content. I use one on my vehicles to tell when the fluid needs changing. Tester pen
 
I think Zaed might have it. Brake fluid is very hygroscopic. A non-leaking brake system should be closed off to moisture incursion, but any time the reservoir lid is removed even for just a second, that fluid is sucking humidity out of the air like a sponge.
 
The newer dot 3/4 is a lot better about moisture. The other side of no breaks when it's cold due to moisture is that there is also no brakes when they get hot due to steam. The pen tester above will let you know. Dot 5 is a silicone base fluid and will not absorb moisture or damage paint. Don't mix 3/4 and 5 without a complete flush.
 
Thanks guys. The fluid is good. Brand new and tested it anyways. It was completely flushed with the new brakes etc, then completely flushed again with the new booster/master (also the second flush was from a different container). I can't see that being the problem.

I've tested vacuum at the valve and it is good as soon as the engine starts. I drove around for a good 10 min or so and the engine was warm with no affect on the assist. I didn't want to drive much with crappy brakes. It's possible that it's a stiff diaphragm in the booster, but I'd think that everyone would have that problem? Unless I'm the only poor bastard trying to drive this thing in the cold?
 
Another theory I'll throw out there is icing up of the vacuum booster hose or check valve. If the pedal goes hard, but you still have brakes, then I would be looking at that or the brake booster as a possible suspect.
 
Another question, at -20c what is the vacuum reading on the engine? It could be that the engine is working so hard to run at those temps the vacuum is low. Lower viscosity oil might help in that circumstance.
 
ihpartsjeff said:
Another theory I'll throw out there is icing up of the vacuum booster hose or check valve. If the pedal goes hard, but you still have brakes, then I would be looking at that or the brake booster as a possible suspect.
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Yes it's a hard pedal but the brakes still work. The booster is kind of what I'm leaning towards, but it's the same with the old booster and the new booster from you guys. I did swap out the valve with a known good one from a different vehicle with the same results. The vacuum hose is brand new as well.
 
FDChappie said:
Another question, at -20c what is the vacuum reading on the engine? It could be that the engine is working so hard to run at those temps the vacuum is low. Lower viscosity oil might help in that circumstance.
Click to expand...
I can't remember the exact number, but it didn't seem low. The engine is a 2019 fuel injected engine and it runs great even in the minus jesus temps. It's warmed up a bit here so it's working right now. It's supposed to dip down again next week, I'll try to get the measument then.
 
BrokenAxle said:
Yes it's a hard pedal but the brakes still work. The booster is kind of what I'm leaning towards, but it's the same with the old booster and the new booster from you guys. I did swap out the valve with a known good one from a different vehicle with the same results. The vacuum hose is brand new as well.
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Did you use vacuum booster hose? It is different from standard vacuum or fuel hose and has a very thick wall so it doesn't collapse on itself.
 
ihpartsjeff said:
Did you use vacuum booster hose? It is different from standard vacuum or fuel hose and has a very thick wall so it doesn't collapse on itself.
Click to expand...
I didn't. I have some on there now to try it, but I had good vacuum to the booster it seemed (after the check valve). It's warmed up, so haven't been able to test anything for a while.
 
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