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Old 02-02-2009, 07:33 PM
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Troubleshoot brake vacuum check valve

My '75 GMC pickup (SBC 350) started running rough today at idle. I played with the mixture and found that it was running lean. By disconnecting various hoses I found that it runs fine (idle back to normal) if I disconnect the line from the carb to the power brake booster and plug it at the carb end (no vacuum to the booster).

Does this mean the check valve is bad, or is there something more serious wrong with the booster? I can pull vacuum on the line to the booster and it holds vacuum up to about 20", and then won't hold any higher (should it go any higher?). I can't blow air into the check valve, so I guess its working okay to block air in that direction.

How do you troubleshoot the check valve and how do you remove it?

Thanks,

Bruce

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Old 02-03-2009, 06:36 AM
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If the 3/8" line that goes to the booster is intact, (you said it does not leak).
The check valve will only be an issue if it leaks around the grommet and allows air in past the grommet. 20-25" of vacuum is all it needs to withstand.

You can remove the valve by spraying a little lube around the joint of valve body and grommet, and wiggling the valve out of the grommet.
If it's running lean at an idle, then something is leaking at 15-20"

You can check the check valve by putting about 15psi on the side that goes to the intake, and seeing if it leaks through to the side that points to the booster. That is all the pressure that valve needs to hold.

It sounds like you probably have a bad diaphram or air valve in the booster.

Get inside the truck, and while it's running, take a length of rubber hose and use it to listen for a hissing sound around the area where the pushrod goes into the booster at the firewall.

Also, you should be able to get 2-3 vacuum assisted brake applications after at least 4 or 5 minutes after you shut the motor off...any less than that and the booster has an internal leak.

Have the brakes gotten slightly harder?

Hope this helps,
mikey
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Old 02-03-2009, 09:08 AM
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It is possible for the plastic check valve to crack and develop a leak - been there, done that. It's easy to test. Simply pull the check valve from the booster grommet, leaving it attached to the hose to the intake. Cover the booster end of the check valve with a piece of cardboard, rubber, or your hand and start the engine. If there is no vac leak, the valve is not the problem. If it's not the grommet (and you can check that by spraying carb cleaner around it with the engine running) then the problem is the booster.
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Old 02-03-2009, 11:26 AM
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Thanks,

Bruce
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Old 02-03-2009, 12:10 PM
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simpler first test?:
a good booster check valve will hold the vacuum for 24 hours....
rev the motor once in park before you turn it off to fully charge the booster...

check the pedal before you start the truck the next day
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Old 02-03-2009, 12:35 PM
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Quote:
Originally Posted by red65mustang
simpler first test?:
a good booster check valve will hold the vacuum for 24 hours....
rev the motor once in park before you turn it off to fully charge the booster...

check the pedal before you start the truck the next day
That won't definitavely test the valve if the booster is bad.

Later, mikey
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Old 02-03-2009, 05:06 PM
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Quote:
Originally Posted by red65mustang
simpler first test?:
a good booster check valve will hold the vacuum for 24 hours....
rev the motor once in park before you turn it off to fully charge the booster...

check the pedal before you start the truck the next day
As Mike noted, this just tells you there's a leak (which we already know). A leak anyplace in the booster, grommet, or check valve will cause the booster to bleed down.
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Old 02-03-2009, 06:38 PM
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he disconnected the booster line at the carb and plugged it there on the carb...
so the leak can be the hose or booster....

got brakes in the morning,,,replace the hose and add clamps...
very likely it is the original 1975 vintage booster hose and it is shot...

got no brakes in the morning,,,proceed to the check valve
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