MagooInFL Posted October 8, 2018 Share Posted October 8, 2018 I recently acquired a 1986 F350 with a 6.9 diesel. Ive bled the brakes and I now get a firm peddle, BUT I am not getting brake assist. The brakes are really tough to push down. I pulled the main tube right off the vacuum pump and it has vacuum (I just put my finger over it, and could feel the suction) - but I understand that it may NOT be enough. How do I know if the problem is brake booster or the vacuum pump? Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 Welcome! I don't know how much vacuum the diesel pumps are supposed to give, but I would assume that it would give something similar to the minimum vacuum a gas engine would show at idle, and that would be around 15". Do you have a vacuum gauge? If not, they are inexpensive and you can get one at most parts stores. Then you may be able to put the vacuum gauge's hose inside the hose coming from the vacuum pump and test the output of the pump. I will check the factory shop manual tomorrow to see if they have spec's on the vacuum you should expect. Link to comment Share on other sites More sharing options...
85lebaront2 Posted October 8, 2018 Share Posted October 8, 2018 Welcome! I don't know how much vacuum the diesel pumps are supposed to give, but I would assume that it would give something similar to the minimum vacuum a gas engine would show at idle, and that would be around 15". Do you have a vacuum gauge? If not, they are inexpensive and you can get one at most parts stores. Then you may be able to put the vacuum gauge's hose inside the hose coming from the vacuum pump and test the output of the pump. I will check the factory shop manual tomorrow to see if they have spec's on the vacuum you should expect. Gary, looking quickly at Alldata, it looks like it should pull 20" with no leaks. Does the low vacuum light illuminate? First thing suggested it check system integrity, it works brakes, cruise, HVAC and if a C6 the transmission vacuum modulator (I think). Some used a mechanical system from the injection pump. Link to comment Share on other sites More sharing options...
MagooInFL Posted October 8, 2018 Author Share Posted October 8, 2018 Gary, looking quickly at Alldata, it looks like it should pull 20" with no leaks. Does the low vacuum light illuminate? First thing suggested it check system integrity, it works brakes, cruise, HVAC and if a C6 the transmission vacuum modulator (I think). Some used a mechanical system from the injection pump. No AC on the vehicle but the heater works No cruise control on the vehicle It seems to shift fine and yes it is the C6 I have a harbor freight vacuum pump for bleeding brakes - could I reverse this onto the pump to get a vacuum reading? Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 No AC on the vehicle but the heater works No cruise control on the vehicle It seems to shift fine and yes it is the C6 I have a harbor freight vacuum pump for bleeding brakes - could I reverse this onto the pump to get a vacuum reading? Yes, you should be able to connect it to the pump and just read the gauge. But the base heater doesn't use vacuum, so that doesn't prove you have adequate vacuum. And if your C6 has a mechanical system that doesn't prove it either. You should be able to see a vacuum line going to the tranny if it uses vacuum. Bill - Good info. Thanks. Link to comment Share on other sites More sharing options...
Ford F834 Posted October 8, 2018 Share Posted October 8, 2018 Welcome to the forum 🙂 The most common issues with diesel vacuum supply are: pump diaphragm failure and the coffee can vacuum reservoir on the driver side inner fender likes to rust out. As a side note, if your warning light is not on... and you indeed have no vacuum... get that warning light fixed! On mine the diaphragm failed and I had no idea until I exited the freeway and went to use the brakes. The only thing that saved the guy in front of me at the stoplight was the fact that I was towing my trailer with electric brakes. Fix the warning light and/or install a vacuum gauge in the cab 👍 Link to comment Share on other sites More sharing options...
grumpin Posted October 8, 2018 Share Posted October 8, 2018 Welcome to the forum 🙂 The most common issues with diesel vacuum supply are: pump diaphragm failure and the coffee can vacuum reservoir on the driver side inner fender likes to rust out. As a side note, if your warning light is not on... and you indeed have no vacuum... get that warning light fixed! On mine the diaphragm failed and I had no idea until I exited the freeway and went to use the brakes. The only thing that saved the guy in front of me at the stoplight was the fact that I was towing my trailer with electric brakes. Fix the warning light and/or install a vacuum gauge in the cab 👍 Good info.. My 1988 F250 7.3, I used to own had the vacuum pump seize and throw the belt. Don't remember a warning light after it happened. Link to comment Share on other sites More sharing options...
Ford F834 Posted October 8, 2018 Share Posted October 8, 2018 Good info.. My 1988 F250 7.3, I used to own had the vacuum pump seize and throw the belt. Don't remember a warning light after it happened. The really bad part of this is that when the warning system fails, it seems to fail in a way that indicates everything is good when it’s not. If I were going to continue to run vacuum brakes on my diesel I would install a mechanical vacuum gauge as a back up for the warning light. In my case, I am deleting the problem by switching to hydroboost. If the pump fails, loosing my HVAC controls and cruise control is not that much of an emergency. Link to comment Share on other sites More sharing options...
MagooInFL Posted October 8, 2018 Author Share Posted October 8, 2018 Welcome to the forum 🙂 The most common issues with diesel vacuum supply are: pump diaphragm failure and the coffee can vacuum reservoir on the driver side inner fender likes to rust out. As a side note, if your warning light is not on... and you indeed have no vacuum... get that warning light fixed! On mine the diaphragm failed and I had no idea until I exited the freeway and went to use the brakes. The only thing that saved the guy in front of me at the stoplight was the fact that I was towing my trailer with electric brakes. Fix the warning light and/or install a vacuum gauge in the cab 👍 Good info.. My 1988 F250 7.3, I used to own had the vacuum pump seize and throw the belt. Don't remember a warning light after it happened. Yeah - there is no warning light on. I like the idea of installing the dash gauge! Never thought of that - don't know why. I had the vac pump go out on my '85 F250 - never saw a warning light then either - I had a mechanic diagnose that issue for me. But I am trying to do more and more on my own. Link to comment Share on other sites More sharing options...
MagooInFL Posted October 8, 2018 Author Share Posted October 8, 2018 Gary, looking quickly at Alldata, it looks like it should pull 20" with no leaks. Does the low vacuum light illuminate? First thing suggested it check system integrity, it works brakes, cruise, HVAC and if a C6 the transmission vacuum modulator (I think). Some used a mechanical system from the injection pump. I used my harbor freight vacuum pump for bleeding brakes and hooked it up to where the vacuum tube connects directly to the brake booster - I had 25 lbs there. As a secondary test - I pumped the brakes a couple times - then started the engine and the peddle didn't move at all. Is that the definitive test in my case? bad brake booster? Link to comment Share on other sites More sharing options...
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