Confusing brake issue - need help

[doodah man]

Since it got cold out (< 50 degrees) I've noticed a strange thing with my brakes. Sometimes, after letting the truck warm up for a few minutes, I have to press much harder on the brakes to get them to stop the truck. This typically only happens the first time I need to stop. And it doesn't happen every time I get in the truck. Maybe one in ten times.

The only thing I've done on the brakes recently is change the front pads while I was doing the wheel bearings. The stopping problem didn't turn up until a month or two later.

I don't know what's going on. I tried pressing the brake pedal as far as possible before starting the engine and didn't notice any change once the motor was running. Does this indicate a bad vacuum booster or low vacuum?

Any advice is greatly appreciated.

Craig.

[Daddy Brim]

I'm having similar symptoms in mine. I think mine is the booster because it and the master cylinder are the only parts left to replace. I will be replacing my booster this afternoon.

[Stocker]

Could be a combination of moisture and cooler temps. Your new pads may be more sensitive to that than your old pads were.

[Longhorn 70]

Start the truck without pressing the brakes. Let it run a couple of minutes and press the brake a couple times. It should be easy first press or 2 and then progressively more difficut. If not, check vaccuum hose and valve to the booster.

I have also seen rotor run out or loose wheel bearings do the same thing. The rotor "wobbles" and slightly spreads the pads apart. Then you have to press the pads back with the first pump. After that they are close until they wobble out again.

Do you notice the same thing after a long highway cruise with no brakes applied?

[doodah man]

Vince, I tried pressing the brakes several times after the truck was started/running 2 mins. The pressure didn't change at all that I could tell. What's more, once I finally got going down the road, the brakes were poor again. I think this rules out the wobbling rotor/bearings.

I'll check the vac hose and valve this weekend. Is there some particular test I can do? I assume I will just listen/look to the vacuum hose for leaks, but how can I check the valve?

Daddy Brim, how did your's turn out?

Thanks guys.

[CCCC]

To see if your problem is a vac/booster issue:

First see that the line from manifold to booster is pulling good vac, then see if the check valve at booster entry is working and sealing in the gasket. If all is well so far, do the following.

From a previous poster: "With the engine off, pump the brake pedal to remove all vacuum, the pedal should be hard. Then push the pedal down and start the engine, if the pedal goes down, it's working, if it doesn't, it's (the booster) bad. "

[TBONE1964]

Another thing to consider is the engine RPM when you are pushing the brake. You stated that it does it when it is cold? Is the choke set? This will rob the booster of the vacuum it needs.

[Longhorn 70]

Another thing to consider is the camshaft. More radical camshafts have low idle vaccuum. Even some mild cams can have low idle vac.

Disconnect the vac hose to the booster and make sure you have vac that almost sucks a blister on your thumb. IF good, go to next step.

Pull one way valve out of the booster and blow through the end that goes into the booster. Then try the other direction. It should only work one way. If that works....

Mr Booster may not be happy.

[doodah man]

Thanks guys. I'll be trying these tests tomorrow AM.

CCCC, I tried this too. The pedal doesn't move at all when I start the engine. Seems to me that this could be booster, vac or valve still.

Tbone, It does seem to happen mostly when the truck is cold. It could be that the choke isn't open all the way when this happens (usually drive once the temp is near the first line). I will have a look at the choke too. It's been stalling every first crank since it got cold also so I think the choke needs adjusting anyway.

-Craig.

[doodah man]

Hey guys, sorry this is dragging out so long. I haven't had much time to look into this and it's been a little cold for my taste lately.

So, the choke seems to be working as intended.

The check valve is working and I'm getting 21-22 inches hg vacuum to the booster at curb idle. The other thing is that this happened on the highway the other day when the truck was plenty warm. In fact, I hit the brakes hard because some a$$hole was pulling into my lane. The brakes were hardly there. I tried them again immediately afterward and they were fine.

Not sure where to go from here. I may just replace the booster to eliminate that from the equation.
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[Longhorn 70]

doodah, did you start the truck, let it idle, turn it off, wait a minute and pull the check valve from the booster?

Also where do you have the booster vacuum line hooked to? It should go to the back of the carb or a BIG port on the manifold.

[doodah man]

Vince, I pulled the check valve before starting the truck and blew through it both ways (one way was open and the other closed). Then I plugged my vac gauge in to the line where the check valve was, cranked the truck and let it warm up and measured a steady 22 in Hg at normal idle. I'm not sure whether the vac line goes to the carb or the manifold, but it is a larger, heavier duty hose than the other vac lines and it goes somewhere behind the carb.

What am I looking for when I follow your procedure? Just the booster losing vacuum?

I've been concentrating on the booster because it seems to be a temporary loss of power brakes, but could this be something further down the line?

Thanks again.

[Longhorn 70]

YES. When you pull the check valve out of the booster, you'll hear/feel the suction happen. if you don't, the booster is leaking. Also when the truck is running, if you pull the check valve out, you'll hear the RPM change. When you put it back in, it runs fast for a few then settles down.

[doodah man]

My wife had the truck out so I got her to test it. She pulled the check valve from the booster and didn't hear anything. I guess that means the booster isn't holding vacuum and at least needs to be replaced.

I noticed that lots of parts houses sell the booster and master cylinder together. Is there a reason to change the MC now? I'm thinking that will require bleeding the brakes and since I have to do all my maintenance under an overpass in the city, i try to keep the jobs as simple as possible. But, if the MC should go with the booster I can do that too.