Vacuum / Brake Problem
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Senior User
Joined: Sep 2003
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From: Cleveland TN
Vacuum / Brake 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 vacuum pump is making 25 inches of vacuum. If I run a vacuum line straight from the pump to the booster it works fine. My real question is what is the device that one of the vacuum 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 other suggestions would be greatly appreciated. This problem has me totally dumb founded any suggestions would be more than welcome.
Post Fiend
Joined: Apr 2004
Posts: 12,835
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From: Silver Lake Dunes, MI
you may have 2 problems here. you may have one vacum problem and then a mechanical problem in the brakes. my guess is that you have a vacum leak in the heater controls some where that only leaks with the key on(i don't know if that is possible). you dropping pedal i first thought master cylinder but you said you already change that out so my next instinct is the rear ABS valve. they have a habit of having same symtoms at a bad master cylinder. it difficult if you don't have all the stuff on hand but you can make a bypass brake line to bypass the valve and see what happens then. i have a set up made a work so i will try to give you a list of what you need to bypass it.
hope that helps
tim
hope that helps
tim
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Senior User
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From: Cleveland TN
I have a good amount of supplies on hand and would be more than happy if you could make me a list of what I would need to bypass it and see if that fixed it. after several days of it being out of service I am not opposed to even spending money on parts to see if it will fix it because I will keep this truck for a long time. I am assuming the ABS valve you are talking about bypassing is the one on the rear axle.
Also talked the the diesel guy at my local ford dealership today he said that the thing on the fender I was talking about with the three electrical connections is a low vacuum indicator sensor but with that said I don't have a warning light in my dash for low vacuum? When my vacuum pump went out about two months ago I know I didn't have any warning light and don't see one when you first turn on the key. But maybe I have a burnt out bulb can you tell me if there is a indicator light.
Thanks for the reply any more suggestion will be appreciated.
Also talked the the diesel guy at my local ford dealership today he said that the thing on the fender I was talking about with the three electrical connections is a low vacuum indicator sensor but with that said I don't have a warning light in my dash for low vacuum? When my vacuum pump went out about two months ago I know I didn't have any warning light and don't see one when you first turn on the key. But maybe I have a burnt out bulb can you tell me if there is a indicator light.
Thanks for the reply any more suggestion will be appreciated.
Post Fiend
Joined: Apr 2004
Posts: 12,835
Likes: 3
From: Silver Lake Dunes, MI
low vacum will only turn on the brake light.
ok i just reread your first post and you said that if you go from the vacum pump to the booster straight that everything works fine? meaning the pedal acts normal, doesn't drop. if that is true then i need to think about this a few more minutes.
if thats not true then here is some info on the abs valve. post back and i will try to have an answer tonight yet.
i could not find the line i used to bypass the abs valve. the valve is on the driver side frame rail right under the front floor board. if you have a good eye you could pull the lines, size them up then get the stuff you need. napa offer line that adapt from one size nut to another. you will need one that matched the two nuts on the line going into and out of the valve then 2 couplers that match the two nuts. then install and bleed the system and check you problem. if your napa is nice and they have a vlave in stock they could pull it out and get you the right plumbing right there. i am sure other parts stores carry similar items too.
ok i just reread your first post and you said that if you go from the vacum pump to the booster straight that everything works fine? meaning the pedal acts normal, doesn't drop. if that is true then i need to think about this a few more minutes.
if thats not true then here is some info on the abs valve. post back and i will try to have an answer tonight yet.
i could not find the line i used to bypass the abs valve. the valve is on the driver side frame rail right under the front floor board. if you have a good eye you could pull the lines, size them up then get the stuff you need. napa offer line that adapt from one size nut to another. you will need one that matched the two nuts on the line going into and out of the valve then 2 couplers that match the two nuts. then install and bleed the system and check you problem. if your napa is nice and they have a vlave in stock they could pull it out and get you the right plumbing right there. i am sure other parts stores carry similar items too.
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From: Cleveland TN
I Greatly appreciate your help after seven very frustrating nights 
I have figured it out. I thought I had a vacuum problem. So I somehow after pestering the hell out of the dealership got him to get me one of those low vacuum indicator switches at his cost of $46.00 not the retail of 97.00 so I put it on about ten minutes ago and hooked all the vacuum lines back up and it works. So my curiousity made me wonder what went wrong in there and I took the old one apart I think the problem is that the diaphram that was in there was cracked bad. All I can say is I am over joyed I restore cars for a living and have never been this frustrated with anything. It was beginning to make think I knew nothing. I am used to working on sixty and back vehichles thought. The new ones with all the electronices get me bewildered. Dang Sensors!!!!!! Thanks though you have been a great help. 
I have figured it out. I thought I had a vacuum problem. So I somehow after pestering the hell out of the dealership got him to get me one of those low vacuum indicator switches at his cost of $46.00 not the retail of 97.00 so I put it on about ten minutes ago and hooked all the vacuum lines back up and it works. So my curiousity made me wonder what went wrong in there and I took the old one apart I think the problem is that the diaphram that was in there was cracked bad. All I can say is I am over joyed I restore cars for a living and have never been this frustrated with anything. It was beginning to make think I knew nothing. I am used to working on sixty and back vehichles thought. The new ones with all the electronices get me bewildered. Dang Sensors!!!!!! Thanks though you have been a great help.
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