Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 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? I'm not sure I understand. Did the vacuum pump on the engine pull the 25", or was that using the HF pump? I think you used the truck-mounted pump, but don't want to assume anything. And on the secondary test, you pumped the brake with no vacuum on the system and then started the engine and the brake pedal didn't move? I wouldn't expect the pedal to move by itself just because vacuum is applied. So I don't think that proved anything. Or, did I misunderstand? Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 I'm not sure I understand. Did the vacuum pump on the engine pull the 25", or was that using the HF pump? I think you used the truck-mounted pump, but don't want to assume anything. And on the secondary test, you pumped the brake with no vacuum on the system and then started the engine and the brake pedal didn't move? I wouldn't expect the pedal to move by itself just because vacuum is applied. So I don't think that proved anything. Or, did I misunderstand? The General Hydraulic Brake System chapter of the 1985 factory shop manual says: 1. With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times. Then push the pedal down as far as it will go, and note the effort required to hold it in this position. If the pedal gradually moves downward, the master cylinder is leaking internally and should be re-built or replaced. 2. With the pedal pushed all the way down, start the engine. If the vacuum system is operating properly, the pedal will move downward. If the pedal position does not change, the vacuum system is not operating properly and should be subjected to a vacuum system test. That means we go to the Brake Booster Vacuum Pump - 6.9L Diesel Engine chapter. That has two tests, the first of which says that the pump should pull 21 inches, minimum, at sea level within 30 seconds of starting the engine and running it at idle - with just the gauge hooked to the pump. (But, there is a reduction due to altitude chart, which shouldn't apply to FL.) So that looks good. The next step says that you reconnect the vacuum system but put the gauge in via a tee. Start the engine and you should have vacuum within 3" of the previous reading. But if the vacuum is more than 3" low you should replace the brake booster. Does that make sense? Link to comment Share on other sites More sharing options...
MagooInFL Posted October 8, 2018 Author Share Posted October 8, 2018 The General Hydraulic Brake System chapter of the 1985 factory shop manual says: 1. With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times. Then push the pedal down as far as it will go, and note the effort required to hold it in this position. If the pedal gradually moves downward, the master cylinder is leaking internally and should be re-built or replaced. 2. With the pedal pushed all the way down, start the engine. If the vacuum system is operating properly, the pedal will move downward. If the pedal position does not change, the vacuum system is not operating properly and should be subjected to a vacuum system test. That means we go to the Brake Booster Vacuum Pump - 6.9L Diesel Engine chapter. That has two tests, the first of which says that the pump should pull 21 inches, minimum, at sea level within 30 seconds of starting the engine and running it at idle - with just the gauge hooked to the pump. (But, there is a reduction due to altitude chart, which shouldn't apply to FL.) So that looks good. The next step says that you reconnect the vacuum system but put the gauge in via a tee. Start the engine and you should have vacuum within 3" of the previous reading. But if the vacuum is more than 3" low you should replace the brake booster. Does that make sense? Yes - that all made sense ( I think) Just to clarify what I previously tested - I hooked the vacuum pump up to the hose that runs to the brake booster and then started the engine - then I read the gauge at 25lbs - so I didn't hook it up and then pump the vacuum by hand so I need to do the same test but put a "T" fitting in between the vehicle vacuum pump and the brake booster and then get a reading Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 Yes - that all made sense ( I think) Just to clarify what I previously tested - I hooked the vacuum pump up to the hose that runs to the brake booster and then started the engine - then I read the gauge at 25lbs - so I didn't hook it up and then pump the vacuum by hand so I need to do the same test but put a "T" fitting in between the vehicle vacuum pump and the brake booster and then get a reading Ok, you did the first step of testing the pump. Now you need to connect the pump to the brake booster and have the gauge teed into that line. I'm guessing that you'll have more than a 3" drop in vacuum. But don't forget to also dump all the vacuum and push the pedal all the way down. Then start the engine and see if the pedal drops like it should. If the booster isn't functioning properly the pedal won't drop. Link to comment Share on other sites More sharing options...
MagooInFL Posted October 8, 2018 Author Share Posted October 8, 2018 Ok, you did the first step of testing the pump. Now you need to connect the pump to the brake booster and have the gauge teed into that line. I'm guessing that you'll have more than a 3" drop in vacuum.But don't forget to also dump all the vacuum and push the pedal all the way down. Then start the engine and see if the pedal drops like it should. If the booster isn't functioning properly the pedal won't drop.Ok - simple question - how do I “dump all the vacuum”? Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 8, 2018 Share Posted October 8, 2018 That's what happens in Step 1. "With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times." Link to comment Share on other sites More sharing options...
MagooInFL Posted October 9, 2018 Author Share Posted October 9, 2018 That's what happens in Step 1. "With the engine stopped, eliminate all vacuum from the system by pumping the brake pedal several times." OK - NOW someone should be able to tell me the answer to the question that is looming. I installed the vacuum gauge - and after dumping the vacuum and holding my foot on the brake peddle and then starting it up the peddle doesn't drop at all. BUT the vacuum does climb even with my foot on the brake, then when I release it climbs to 25 then when I press the brakes - it drops from 25 down to zero. what does that mean? bad vacuum pump or bad brake booster? Thank you so much for all who have contributed! Link to comment Share on other sites More sharing options...
MagooInFL Posted October 9, 2018 Author Share Posted October 9, 2018 OK - NOW someone should be able to tell me the answer to the question that is looming. I installed the vacuum gauge - and after dumping the vacuum and holding my foot on the brake peddle and then starting it up the peddle doesn't drop at all. BUT the vacuum does climb even with my foot on the brake, then when I release it climbs to 25 then when I press the brakes - it drops from 25 down to zero. what does that mean? bad vacuum pump or bad brake booster? Thank you so much for all who have contributed! OOPS - let me change that - when I dump the vacuum and then start it it HOLDS at ZERO, then when I let off the brakes it slowly climbs to 25 then when I push the brakes it drops down and the needle bounces between zero and 3 until I let off again! Link to comment Share on other sites More sharing options...
Ford F834 Posted October 9, 2018 Share Posted October 9, 2018 OOPS - let me change that - when I dump the vacuum and then start it it HOLDS at ZERO, then when I let off the brakes it slowly climbs to 25 then when I push the brakes it drops down and the needle bounces between zero and 3 until I let off again! Sounds like the booster diaphragm is bad... Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 9, 2018 Share Posted October 9, 2018 Sounds like the booster diaphragm is bad... Yes. The diaphragm is killing the vacuum. The maximum drop allowable is 3", and it took it to 0. And, it failed the "pedal should drop when the engine starts" test. Now you know what to fix w/o throwing parts at it. Link to comment Share on other sites More sharing options...
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