No power brakes on cold start

[93IDIT]

This morning I had no power assist on the brakes. I had to stand on the brake pedal to stop. The truck did this a few weeks ago and was fine later that day when I got off work. I only drive three miles to work and have two traffic lights so it wasn't too bad but I like power brakes. Could it be that ice is building up somewhere? How do I clear the moisture out?

 

['94IDITurbo7.3]

would be worth it to install a vacuum gauge to see what is going on. you can "tee" it into the little vacuum manifold thingy under the break master cylinder.

you might have a bad brake booster check valve or you could have a bad booster. if the booster is bad then upgrade to hydroboost since you need to R&R the booster and spend the money on one anyways.

 

[93IDIT]

would be worth it to install a vacuum gauge to see what is going on. you can "tee" it into the little vacuum manifold thingy under the break master cylinder.

you might have a bad brake booster check valve or you could have a bad booster. if the booster is bad then upgrade to hydroboost since you need to R&R the booster and spend the money on one anyways.

The brakes worked fine coming home. Something must be icing up If the issue repeats tomorrow morning, I'll try pulling the hose from the manifold and booster and see if I can narrow it to a pump issue, a stuck check valve, or the booster. I would love to put hydroboost on, but don't have money for that right now.

 

[Knuckledragger]

When was the last time you changed the brake fluid? It will trap water in the system and can freeze. That would explain that the brakes worked fine coming home when it is warmer.

 

[93IDIT]

I flushed the fluid until it ran clear about a year ago but I won't rule that out. I do recall that something did make a cracking sound the first time this happened. Maybe bleeding the lines loose will show if the master cylinder itself is icing? The low tonight is only 32F and it's warming a bit so I may not see the issue until Thursday or Friday.

I'll start looking at the vacuum stuff and move to the hydraulic. Thanks for the suggestion.

 

[Knuckledragger]

Start with whatever costs least first. First law of auto repair is that the cheapest parts fail first. Check the fluid, vacuum lines then the Master cylinder (all cheap) before messing with the booster or vacuum pump.

 

[79jasper]

Those are all great places to start. Could even be moisture in the vacuum pump itself. I mean everything does get pulled towards the pump. Maybe dump a little alcohol or HEAT in the line. I would try alcohol since it shouldn't mess with the diaphragm. Water can get in it from the top center piece.

 

[TWeatherford]

I have had the same problem for too long, but haven't done anything as it isn't a daily driver and there always seem to be more pressing issues. The brakes always are good after 20 minutes, and are never bad in warmer weather. I have done a complete brake fluid flush when I replaced the rear wheel cylinders, and it didn't change anything.

 

[RLDSL]

REmember rubber lines need to remain FLEXABLE to hold a seal, if they are old and dried out which happens when exposed to heat, they will not contract along with the plastic fitting when it gets real cold and they will not contain vacuum, it will take until the under hood temp rises enough to expand the hose for them to contain the vacuum

This is the same thing that effects old fuel return lines and causes air leaks overnight

 

[93IDIT]

So we finally got another cold spell. It was around 20F this morning. I had no power assist again. I pulled the check valve off the booster and checked for vacuum. It pulled a vacuum the same as it did earlier in warmer temps. I really think this is a booster issue. I did have almost as much ice on the inside of the windshield as I did on the outside. I'm going to get some desiccant for RVs and try to dry things out inside. Maybe this issue will go away.

 

[jonathan]

Does your break light come come on

 

[93IDIT]

Does your break light come come on

The warning lights do not come on, just the rear brake lights when I push the pedal. The braking system seems to be working ok other than not having power assist when cold.

 

[jonathan]

My light comes on when it's in the negative degrees

 

[madpogue]

Sounds like the booster. Booster issue like this won't set the brAKE failure warning light in this case; only an upstream vacuum issue would.

Confirmation - if you can get your hands on a hand vacuum test pump, hook it up to the booster directly and pump it up (down?) on the next cold night. Keep the engine off to keep things cold. Hop in and see if it provides any boost or not. For super-confirmation, you could set the tester to hold the vacuum, and then apply some heat, maybe with a heat gun or hair dryer, to the booster and see if it eventually starts boosting.

 

[79jasper]

Is it possible for the vacuum pump to test good with a gauge but not actually be able to suck enough volume?
If so, how would we test that?