Testing a brake booster
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Cross-Country
Joined: Nov 2018
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From: Derbyshire, England
Testing a brake booster
I'm trying to iron out all the vacuum leaks in my '74 F100 390 and I'm trying to figure out if the brake booster is behaving as it should.
I'm testing it with a vacuum pump.
It will pump up to 20" vacuum fine and hold there. The moment I touch the pedal it loses all of that vacuum, the gauge goes straight to 0.
If I try to build vacuum with my foot on the pedal, it won't build any at all.
Should it behave this way or do I potentially have issues?
I'm testing it with a vacuum pump.
It will pump up to 20" vacuum fine and hold there. The moment I touch the pedal it loses all of that vacuum, the gauge goes straight to 0.
If I try to build vacuum with my foot on the pedal, it won't build any at all.
Should it behave this way or do I potentially have issues?
Lead Driver
Joined: Jul 2017
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From: San Jose, CA
That's exactly how it should work.
If you were using the engine as the vacuum source, you would have enough to maintain some vacuum even with the pedal pressed, but a pump does not have the volume to keep a big booster reservoir filled once you open the relief valve.
And that's what the pedal does when you push it, and how the booster works. Vacuum is held only on the engine side of the diaphragm, but pulling against the back side cavity with, what I would guess is zero hg? Or somethign to that effect. In other words it can't pull against it. But as soon as you open the valve with the pedal push, ambient pressure is forced into the rear reservoir/cavity and voilá! Boost!
It's the fact that it's holding vacuum before you push the pedal that says it's acting as it should with no vacuum leak.
Paul
If you were using the engine as the vacuum source, you would have enough to maintain some vacuum even with the pedal pressed, but a pump does not have the volume to keep a big booster reservoir filled once you open the relief valve.
And that's what the pedal does when you push it, and how the booster works. Vacuum is held only on the engine side of the diaphragm, but pulling against the back side cavity with, what I would guess is zero hg? Or somethign to that effect. In other words it can't pull against it. But as soon as you open the valve with the pedal push, ambient pressure is forced into the rear reservoir/cavity and voilá! Boost!
It's the fact that it's holding vacuum before you push the pedal that says it's acting as it should with no vacuum leak.
Paul
Laughing Gas
Joined: Aug 2010
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From: Bothell
Watching this... I have a similar issue. Air fuel gauge reads leaner when my foot is held on the brakes (not just when actuating the brakes). Leads me to believe I have a booster leak only when the brakes are being used.
Lead Driver
Joined: Jul 2017
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From: San Jose, CA
Of course, the quick test of that function is to push the pedal with the engine OFF. A couple of pushes at most and you should have a super hard pedal up high. With your foot holding pressure on that pedal, start the engine. Your foot and pedal should immediately fall at least an inch before you feel the hydraulic part of the system lock up as the brakes are applied. How far that travel actually is will depend on the vehicle and how the brakes are set up. But the action of the pedal moving when the engine starts and vacuum is applied to the engine side of the diaphragm is what you're looking for.
Then to test it for leak-down, turn the engine off, wait for a minute or so, then press the pedal. You should still be able to apply the brakes easily for perhaps one pump, or at the most (with most vehicles) two pumps of the pedal. If it has held vacuum prior to you pushing the pedal, it's not leaking.
How else are you checking for vacuum leaks for the rest of the engine?
Paul
Then to test it for leak-down, turn the engine off, wait for a minute or so, then press the pedal. You should still be able to apply the brakes easily for perhaps one pump, or at the most (with most vehicles) two pumps of the pedal. If it has held vacuum prior to you pushing the pedal, it's not leaking.
How else are you checking for vacuum leaks for the rest of the engine?
Paul
Lead Driver
Joined: Jul 2017
Posts: 9,583
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From: San Jose, CA
How much, and how it effects the mixture will depend on several things. Not the least of which are size of the booster, how much vacuum the engine develops, size of the orifice/bleed valve, relative tune of the engine, and just where the booster is attached to vacuum.
I've seen many vehicles with the PCV valve and booster connections reversed. The booster should always be on one intake runner when available, and the PCV valve should ALWAYS be connected to a common plenum area only. Never on a single runner.
Paul
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Cross-Country
Joined: Nov 2018
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From: Derbyshire, England
Thank you for the replies guys - very helpful. Sorry not to reply sooner.
I think I've been so paranoid about vacuum leaks lately that I'm suspecting everything! The booster behaves well and doesn't cause a bad idle.
I've noticed that at idle, if I hit the brakes hard, it very lightly stumbles for half a second then idles like normal again. I guess that's just the time it takes for the vacuum in the booster to replenish.
I think I've been so paranoid about vacuum leaks lately that I'm suspecting everything! The booster behaves well and doesn't cause a bad idle.
I've noticed that at idle, if I hit the brakes hard, it very lightly stumbles for half a second then idles like normal again. I guess that's just the time it takes for the vacuum in the booster to replenish.
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