Fading vacuum?

[needlenose]

My vacuum is coming and going. Wouldn't a leak be fairly consistent? The brakes and AC work fine for days, then the brakes will suddenly go rock hard and the truck won't stop. Today the blower quit blowing even though I could still hear it running. There was a little air coming out of all the outlets. I'm guessing the door wasn't being shut all the way and it wasn't being forced out a specific set of vents. When I got home I stopped and idled in the driveway and turned the AC back on, I heard the door slap shut and it started blowing hard again.

I've got the head off my Civic, so I haven't had a chance to put my manifold gauges on it yet and do a leakdown. They're the only thing I have to measure vacuum. Does this sound like a leak or a pump issue?

 

[ToughOldFord]

Sounds like a tired pump.

 

[tbiagent69]

It is a sealed system, no? Can't you apply vacuum with a pump and do a leakdown test?

 

[mu2bdriver]

My vacuum is coming and going. Wouldn't a leak be fairly consistent? The brakes and AC work fine for days, then the brakes will suddenly go rock hard and the truck won't stop.

I'm having a similar issue: About 25% of the time my brake pedal will go rock hard and the truck won't stop. With my foot on the pedal it will gradually soften up (5-7 seconds later) and I'll feel the brakes get applied. The other 75% of the time, the brake pedal and brakes grab fine. Visually, the lines appear in good shape, the master cylinder isn't leaking, the booster looks good, and I don't hear any hissing sound from either if I apply the brakes at idle. Speed drops off about 5mph during a climb on cruise control. I just borrowed my friend's vacuum gauge tonight and will be testing vacuum Thursday morning or afternoon. I know my vacuum pump is original but I'm curious to see how much vacuum is actually being produced at idle. I'm leaning towards it being a fading pump but want to make sure. Will report back...

 

[mu2bdriver]

Here are my results:
After start, I have 23" at idle which builds up to 25" at 2000RPM. Upon reducing power to idle, sometimes the vacuum will drop to about 15" before building back up to about 20", sometimes it will drop slightly, and sometimes it will reduce back to about 23". I did several idle to 2000RPM to idle sequences to see if there was any consistency in vacuum and it was only under power where it would stay in the mid 20"s.

I know the minimum vacuum at idle is 21"; is there a maximum or target suction for idle or under power?
How quickly should vacuum return to the minimum (or higher) setting after reducing power to idle?

I think I found the culprit (bad pump) but I don't understand why it would be fluctuating so much. I've never experienced a failing vacuum pump before and don't know if this is indicative of it.

I don't mean to hijack the thread, but I hope the OP finds the discussion helpful.

 

[FarmerFrank]

I agree with the pump.

Mine took a dump on me a couple weeks ago so I threw a used one on. the new was doing the same thing as mentioned by the OP. some times brakes were fine, next time both feet.

I tried another used one and so far everything is ***** dory

 

[needlenose]

Here are my results:
After start, I have 23" at idle which builds up to 25" at 2000RPM. Upon reducing power to idle, sometimes the vacuum will drop to about 15" before building back up to about 20", sometimes it will drop slightly, and sometimes it will reduce back to about 23". I did several idle to 2000RPM to idle sequences to see if there was any consistency in vacuum and it was only under power where it would stay in the mid 20"s.

I know the minimum vacuum at idle is 21"; is there a maximum or target suction for idle or under power?
How quickly should vacuum return to the minimum (or higher) setting after reducing power to idle?

I think I found the culprit (bad pump) but I don't understand why it would be fluctuating so much. I've never experienced a failing vacuum pump before and don't know if this is indicative of it.

I don't mean to hijack the thread, but I hope the OP finds the discussion helpful.

No problem at all, this is all new to me. Mine fades out over time. I just drove it for 45 minutes at a pretty constant 65mph (no traffic). My blower started out strong with frosty cold ac. After about 30 minutes the blower is still running, but there is almost no air coming from the vents. When I turn it off, wait 5 min, and back on, I can't hear the door flap shut.

My pump looks pretty old, it's just weird that it doesn't do it all the time.

 

[chris142]

can you t into the vacuum line at the pump and drive around with a vacuum guage taped to the windshield?

 

[79jasper]

Yes.

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