Vacuum / brake system problem
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Senior User
Joined: Sep 2003
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From: Cleveland TN
Vacuum / brake system problem
I am having a problem with the brakes on a 1997 F250 with the diesel engine? The brakes will build and hold pressure with the motor not running. But with the motor running they will build pressure for a second or two and then will fall slowly to the floor. I have no leaks and have bleed the lines very well have bleed over a gallon through the lines. I have put a new master cylinder on it and I bench bleed it also. I can hook my vacuum pump to the booster and it holds vacuum also the vaccum pump is making 25 inches of vaccum. If I run a vaccum line straight from the pump to the booster it works fine. My real question is what is the device that one of the vaccum lines off of the tee runs to it has a three prong eletrical connection it uses the same bolt to hold it to the inner fender as the vaccum tee. Any suggestions would be greatly appreciated.
If this would be better listed in the diesel section than go ahead and move it or I will list there.
If this would be better listed in the diesel section than go ahead and move it or I will list there.
New User
Joined: Mar 2010
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I have the same truck but can't get any pedal. I was told that the master was bad, so I replaced it. Bench bled it for 30 minutes! Installed it and while bleeding found the rear shoes getting low so I replaced them. Bled all four corners and still the pedal doesn't hold. I was using the old pump-and-bleed method. Used about a quart of fluid and nothing is holding. Can't find any leaks and not getting any air bubbles. Open to suggestions!
Posting Guru
Joined: Aug 2007
Posts: 1,714
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From: Canton, IL
If running a line directly to the booster solves your problem, then you have a major vacuum leak somewhere. Jumper your Vacuum pump and listen for sucking noises under the hood with the engine off. There are also steam machines designed to find these leaks. Then it becomes visual instead of auditory. May check the vacuum reservoir for holes, if you have the old coffee tin reservoir, that is..
New User
Joined: Mar 2010
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I think the vaccume leak would be an issue if the engine were running but I can't get the pedal to hold with the engine off. I keep going back t air in the system but I bled the corners out a dozen times now...help-I'm lost!
Elder User
Joined: Dec 2008
Posts: 745
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From: North Carolina
Also, i have heard that you can get air in the ABS system. The only way to get it out is to have the ABS valves cycled while bleeding. This can only be done by someone with a fancy scan tool, probably the dealer is the best bet.
New User
Joined: Apr 2010
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I have a 97 f250 diesel and the brake light goes on and off and it is not the vacuum pump but the brake vacuum switch that is shorting out. The part number is F-6TZ-9C490-A but Ford is telling me that part is discontinued? Anyone aware of a after market place that carries this?
Posting Guru
Joined: Aug 2007
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From: Canton, IL
Yep, Your local junk yard. Very rare to see one fail, so just go get a cheap one from the junk yard.....
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I had a very similar problem in mine... and this will be written up about shortly.
It boiled down to bad vacuum lines.
Ford is using basically fuel line, and in my case, the 2005 factory lines became hardened, and no longer hold pressure.
Another possibility --- the vacuum booster on mine had a 90 degree angle -- with a filter at the end going into the brake booster.
The fitting was not properly installed and leaked vacuum.
Replace the plastic fitting there and also the length of hose is critical.
On a truck as old as the one in the OP, replace the line to the vacuum pump with new hose is also very important.
In addition to checking for a bad vacuum switch.
Is there a differential pressure (hydraulic) sensor in there somewhere as well?
It boiled down to bad vacuum lines.
Ford is using basically fuel line, and in my case, the 2005 factory lines became hardened, and no longer hold pressure.
Another possibility --- the vacuum booster on mine had a 90 degree angle -- with a filter at the end going into the brake booster.
The fitting was not properly installed and leaked vacuum.
Replace the plastic fitting there and also the length of hose is critical.
On a truck as old as the one in the OP, replace the line to the vacuum pump with new hose is also very important.
In addition to checking for a bad vacuum switch.
Is there a differential pressure (hydraulic) sensor in there somewhere as well?
New User
Joined: Apr 2010
Posts: 3
Likes: 0
The lines seem to be ok and the pump is producing plenty of vacuum and the brake pedal is not hard at all. It is definitely the switch as I can make the light go on and off but tapping on it so it must be shorting out. Thanks for the advice though, food for thought down the road if I continue to have issues.
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