Brake vacuum

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Mar 13, 2005 | 11:07 AM

RhinoVirus

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Brake vacuum

93 F250 7.3 IDI non turbo.
When I am backing my trailer down my driveway (160ft) just wide enough for the truck. If I'm having a bad day and I need to stop, pull forward to straighten out a few times I lose vacuum in my brakes. If give it gas the vacuum builds back up.

Idle is 750-800

Is this normal, or should I be looking at my vacuum pump. or leaks

Mar 13, 2005 | 11:15 AM

bigblu78

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Joined: Dec 2004

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From: Mackinaw, IL

I am having the same problem with my 88 F-250. Brake pedal is real hard if I have to hit the brakes twice. I changed my vaccum pump and that helped for a day or two. Back to the same problem now.

Mar 13, 2005 | 12:30 PM

Dave Sponaugle

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Joined: Nov 2003

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From: Nutter Fort, WV

You should see about 20 inches of mercury vacuum at curb idle within 30 seconds of starting the engine.

If you check with the vacuum guage tee'd into the line and you are below that you should remove the line from the pump and check it directly to eliminate the possibility of leaks in the rest of the system.
If you get the same low reading straight from the pump the pump is bad.
If you get a higher reading you have a leak in the system somewhere.

When you apply the brakes, the maximum drop in pressure is 3 inches of mercury.
More than that and the vacuum booster is probably bad.
I saved a test proceedure somewhere, but I can not find it right now. The above is the general proceedure though.
I have bad luck with the vacuum pump on my truck as well, I am getting ready to go test mine again as soon as I get all dressed up for the nice 30 degree weather we have.
Mine has always been a bad vacuum pump every time I have had to work on it.
I am getting ready to switch mine to a 96 Super Duty hydraboost system when I get a couple of hydraulic hoses made for it.

Mar 13, 2005 | 12:35 PM

bigredtruckmi

Lead Driver

Joined: Jan 2005

Posts: 7,782

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From: Huntington Indiana

rhinovirus, there is no tank to hold vacuum so what is in the booster is all you have. So if you press several times you probably have used up all the vacuum. Best way to check is to beg or borrow a vacuum gauge. Disconnect from booster and see what the reading is. Can't remember off hand what the vacuum # is suppost to be. Also you could have a small leak in the vacuum system. This also runs your heater duct door.

bigblu78 yours might be same but go to string "bad brakes" we had a terrible problem but got it resolved in a 92 at work.

Mar 13, 2005 | 11:13 PM

RhinoVirus

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Thanks guys I'll look into it the first chance I get

Mar 14, 2005 | 04:36 PM

bigblu78

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Joined: Dec 2004

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From: Mackinaw, IL

Well, pretty sure the vaccum pump went out on mine today seeings how it locked up and snapped off the block!! New pump and some EZ Outs and I should be on my way.. Thanks for the info guys

Mar 14, 2005 | 11:44 PM

PLC7.3

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Joined: Nov 2004

Posts: 5,638

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From: Manitoba

Do not expect more than 3-4 pumps on the pedal before total loss of vacuum and braking backing up. An extra reservoir may help, search the wreckers for a round vacuum reservoir about the size of a large grapefruit.