97 F-350 PSD W/cruise......110k miles. Hard brake pedal feel after sitting w/ foot on brakes at traffic light , if I start to move forward I get hard pedal w/little or no braking left to stop. Brake warning light comes on, then goes off if I raise RPMs....best bet for remedy? Vacuum pump or power brake booster? Lemme know...............Thanks

 

Sounds like the brake booster but it very well could be the pump. The pump should be providing around 16 psi of vacuum, if it is really low then that would be the one to replace

 

My guess is the pump or tubing from the pump. After staying stopped at light pedal feels hard, but how did it feel when you first stopped?
Where I'm going is that your booster should have acheck valve in the big vacuum line. If the diaphragm and check valve are working, you could get a good pedal at first. However once that vacuum in the booster is lost, the pump should replenish it. If that is what's happening when yuo rev it up, your pump may very well be losing efficiency.
You can use some simple tests to help figure it out, or at least narrow it down....

Connect a vacuum gauge to your vaacuum system. Individually pinch off then release the vacuum hoses that go to the various other components. If you pinch one and you notice your gauge says more vacuum, you may have a leak or a bad component on that line.

You can cap off as many leads as possible and measure the vacuum that the pump is putting to the system.
A hand operated vacuum pump (Real good ones $50-$60) can let you put vacuum on individual components to check them. A vacuum pump in a good kit has about ten billion other good uses too.

 

Thanks for the reply guys......I'll be checking the vac pressures tomorrow.

 

most likely a failing pump... sounds exactly like the symptoms i had when my pump went out

 

Hey 96P Stroke4x4, Thanks for the reply and good words.....did you have to remove the fan to get that pump off?? Lemme' know.

 

I replaced the vacuum pump over the weekend.....it's nice to have brakes again ! A word of advice to anyone else who does this....buy a replacement pulley($18.) to install on the new pump and make sure you have the bolts in the holes BEFORE installing the pulley on the new pump or you'll never get them in. No need to remove the fan or the fan shroud either.