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Discussion Starter #1
For lack of a better place to start this thread, I'll see if this works.so we had a huge temperature change in the last few days, mid sixties to -14 in four days. Yesterday I had the D250 out and went for the brakes and the pedal headed straight to the floor with some resistance, but on a second pump it felt like normal. A quick look revealed no leaks, the master cylinder was full on fluid, and the problem repeated itself all the way home. I changed the front brake pads and turned the rotors early this summer, never messed with the rears, and so my suspicions are with the master cylinder. Open to all ideas, thanks!
 

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Are they power brakes?

Does it do it with the engine off?

Cold temperatures can reek havoc with the rubber diaphragm in the brake booster.

Check the vacuum line, and check valve.

If the booster checks out then the master may have failed prematurely. The seals may have become stuck and then frozen damaging them and causing an internal leak between chambers (Rear to front) so there's no pressure when applying the brakes.

It sounds to me like the master. If it were the booster they're usually hard as a rock but do nothing.
 

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Discussion Starter #3
Not hard as a rock, yes power brakes. As far as premature wear, the truck is 25 years old :). All the booster lines are good, just recently replaced and secured.
 

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With the truck off press the pedal once or twice. Test it for pedal resistance.

Then do it with it running. If it's the same with the truck running I'd say booster.

If it goes express to the floor with it running, I'd say master.

The only other time it should go to the floor is if there's a break in the lines, and you said there were no leaks.
 

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Discussion Starter #5
With broken lines it shouldn't be able to pump up pressure with a second pump of the pedal, it would just stay mushy while it pumps out, been there.
 

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With the truck off press the pedal once or twice. Test it for pedal resistance.

Then do it with it running. If it's the same with the truck running I'd say booster.

If it goes express to the floor with it running, I'd say master.

The only other time it should go to the floor is if there's a break in the lines, and you said there were no leaks.
I thought we had covered that it wasn't a broken line.
 
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