1985 Bronco Posted August 13 Share Posted August 13 How much vacuum should a good brake booster hold? I have a booster from a 1986 F350, it's holding 7 inches of mercury, and not dropping at all, but that number seems kinda low. From reading online, it sounds like I need 14-16 inches? My brakes suck, but I though a bad booster holds no vacuum at all? My vacuum pump is holding 22-23 inches of mercury. Link to comment Share on other sites More sharing options...
viven44 Posted August 13 Share Posted August 13 When I go back home I will need to find out for sure. My bronco's brakes are less than stellar and I have doubts around the brake booster. Link to comment Share on other sites More sharing options...
mat in tn Posted August 13 Share Posted August 13 if your vacuum pump is holding 20 plus while connected to the booster then I think you are good. often when a diaphragm deteriorates it will have a hissing sound near the pedal in the cab and usually changes with range of the pedal. this becomes a vacuum leak to the engine and a controlled one at first then progressively worse until brakes get harder to push and idle suffers. it's a lean condition so pinging is also a symptom. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 13 Share Posted August 13 I'm confused. Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"? If that's what you are saying then the booster is good. But the problem with your brakes is that you aren't getting enough vacuum to the booster. So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster. Link to comment Share on other sites More sharing options...
mat in tn Posted August 13 Share Posted August 13 I'm confused. Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"? If that's what you are saying then the booster is good. But the problem with your brakes is that you aren't getting enough vacuum to the booster. So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster. I just noticed the signature. 12v Cummins. good catch Gary. test the vacuum pump. Link to comment Share on other sites More sharing options...
1985 Bronco Posted August 13 Author Share Posted August 13 I'm confused. Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"? If that's what you are saying then the booster is good. But the problem with your brakes is that you aren't getting enough vacuum to the booster. So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster. I disconnected the line going from the vacuum pump to the booster, and inserted a vacuum gauge. With the engine running, I was getting 22-23 inches on the gauge. So, I believe the pump is good. Then, I connected the mityvac to the check valve on the booster used the mityvac to apply vacuum, and got a reading of 7 inches. The engine was not running, and the only vacuum line was the one going to the mityvac. Did I test this correctly? Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 13 Share Posted August 13 I'm confused. Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"? If that's what you are saying then the booster is good. But the problem with your brakes is that you aren't getting enough vacuum to the booster. So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster. I disconnected the line going from the vacuum pump to the booster, and inserted a vacuum gauge. With the engine running, I was getting 22-23 inches on the gauge. So, I believe the pump is good. Then, I connected the mityvac to the check valve on the booster used the mityvac to apply vacuum, and got a reading of 7 inches. The engine was not running, and the only vacuum line was the one going to the mityvac. Did I test this correctly? Ok, it does seem like the engine's vacuum pump is working. But the Mityvac ought to be able to pump up as much as the engine's pump does - unless there's a leak that is killing the vacuum at about the speed you are pumping. Can you tee into the vacuum line and see what vacuum you have with the engine running? Maybe the leak is overwhelming your engine's vacuum pump? Link to comment Share on other sites More sharing options...
mat in tn Posted August 13 Share Posted August 13 I'm confused. Are you saying that you are only seeing 7" of vacuum at the booster when the engine is running? But when you pump vacuum on the booster with your Mityvac it'll hold 22 - 23"? If that's what you are saying then the booster is good. But the problem with your brakes is that you aren't getting enough vacuum to the booster. So the vacuum pump on your Cummins must be bad or you have a leak between it and the booster. I disconnected the line going from the vacuum pump to the booster, and inserted a vacuum gauge. With the engine running, I was getting 22-23 inches on the gauge. So, I believe the pump is good. Then, I connected the mityvac to the check valve on the booster used the mityvac to apply vacuum, and got a reading of 7 inches. The engine was not running, and the only vacuum line was the one going to the mityvac. Did I test this correctly? does you mity vac while connected to the check valve, not go any higher than seven at all or does it sink down and only hold seven? Link to comment Share on other sites More sharing options...
1985 Bronco Posted August 13 Author Share Posted August 13 Ok, it does seem like the engine's vacuum pump is working. But the Mityvac ought to be able to pump up as much as the engine's pump does - unless there's a leak that is killing the vacuum at about the speed you are pumping. Can you tee into the vacuum line and see what vacuum you have with the engine running? Maybe the leak is overwhelming your engine's vacuum pump? The vacuum was holding steady at 7 inches for a good 30 seconds without me pumping, so I don't think that indicate a leak? I guess I could connect it tonight and see if it will hold the same number for 5 minutes. Link to comment Share on other sites More sharing options...
1985 Bronco Posted August 13 Author Share Posted August 13 does you mity vac while connected to the check valve, not go any higher than seven at all or does it sink down and only hold seven? It doesn't go any higher Link to comment Share on other sites More sharing options...
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