Brake Booster Stops Working When Vacuum is Pulled
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2nd Gear
Joined: Oct 2020
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Brake Booster Stops Working When Vacuum is Pulled
So I've looked around quite a few forums talking about brake booster problems but I haven't been able to find any talking about this. Basically when my engine is off the brakes feel firm and stop way before bottoming out, but when it's turned on, they immediately go to the floor and lose all firmness. I've found that any vacuum pulled on the brake booster, even just me sucking some air out of it through the vacuum line causes it to do this. Knowing how a brake booster works, this seems like almost normal behavior except for the fact that it provides nearly no resistance and barely supplies the brakes with any power. As it is, using the brake booster has the opposite effect that it should, causing the brakes to become much weaker and harder to activate, any answers on why this is happening? I'm assuming I'll need to just replace the brake booster but that's a pain I'd really like to avoid if it's not the actual issue.
Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
The booster when vacuum is applied, applies more force to the booster. When there is no vacuum everything is hard and stuff because it is like manual brakes. It sounds like you still have air in the lines and the extra force the booster applies is bringing it out. I know these cars are very picky to get bleed properly.
what I would do to check the booster is hook up a vacuum pump like a might vac and see if it holds vacuum.
what I would do to check the booster is hook up a vacuum pump like a might vac and see if it holds vacuum.
Last edited by Sigforty; Nov 24, 2020 at 11:12 PM.
Safety Car
Joined: Sep 2005
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From: Orange County Ca
The booster when vacuum is applied, applies more force to the booster. When there is no vacuum everything is hard and stuff because it is like manual brakes. It sounds like you still have air in the lines and the extra force the booster applies is bringing it out. I know these cars are very picky to get bleed properly.
what I would do to check the booster is hook up a vacuum pump like a might vac and see if it holds vacuum.
what I would do to check the booster is hook up a vacuum pump like a might vac and see if it holds vacuum.
I just went through a nightmare brake problem, so I have a bit of in site here.
The only tests that I know of for a booster rat home-
With the engine off, pump the brakes till you get a hard pedal. Hold pressure and start the car. The pedal should drop a bit if the booster is working correctly.
As above, apply vac to the booster to see if it holds. If it does, it is a indication that the booster is probably good. If not, definitely bad.
With the car running, if the booster is good and the pedal is soft or goes way down, you probably have air in the system that the extra power exacerbates.
After a good bleeding, i would check the master cylinder next.
On a correctly operating PB system, if the booster is bad, the brakes would still work, but with significantly harder pedal and significantly (30-40%) less braking power. PB cars have a larger mc bore which changes the dynamics a lot.
Last edited by Sayfoo; Nov 25, 2020 at 10:48 AM.
Had a 1976 L-82, 4-sp
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Another way to check your booster. Start the car and let it run for a minute or so. Shut it off. Go have a beer. Come back and pull the check valve from the booster. If you hear a hiss for a second or less, then your check valve and booster is holding vacuum. If you don't hear a hiss, then check your check valve by sucking on the booster side of the valve. If you can't suck air through, the check valve, then the valve is ok but your booster is worn out. If you can suck air, then the check valve is worn out. Replace. Then hook up a vacuum source to the booster as others have said and see if it holds vacuum. If it doesn't, enjoy the removal/replacement, lol. Or convert to manual brakes.
Team Owner
Joined: Jan 2006
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From: Crossville TN
When the booster has no vacuum, it also has NO FUNCTION. You feel a 'hard pedal' because it CAN"T do anything with an non-functional booster.
You know the booster is working properly because the pedal becomes functional. The fact that the pedal goes nearly to the floor and is 'soft' indicates that you have air in the brakes fluid system....somewhere. The brakes need to be bled to remove all air from the lines. Then you will have a good firm pedal that doesn't go to the floor.
You know the booster is working properly because the pedal becomes functional. The fact that the pedal goes nearly to the floor and is 'soft' indicates that you have air in the brakes fluid system....somewhere. The brakes need to be bled to remove all air from the lines. Then you will have a good firm pedal that doesn't go to the floor.
Safety Car
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From: Spring, Texas
Another troubleshooting tip: Purchase plugs for the ports for the two lines from the master cylinder to the distribution block. Disconnect those 2 lines and plug the master cylinder ports. Start the car, give it a minute to build vacuum, then press the pedal. If firm with minimal to no travel, you have eliminated the booster and the MC, and there is apparently air somewhere in your lines. If the pedal is soft and has excessive travel, you could have a bad MC or a bad booster.
