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Help: Vacuum Issues....

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Old 04-17-2018, 08:29 AM
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Default Help: Vacuum Issues....

I've been chasing a very bad brake issue which I believe to be directly related to lack of vacuum. My boost/vacuum gauge is reading 0-2" of vacuum and I have to press my brakes to the floor to get the car to even stop coming out of the driveway.

Things I have done so far:

- New Brake Booster and reservoir installed
- Brakes Bled and bled and bled again just to be sure
- Check valve on booster checked good
- Checked over and over for any leaks in vacuum lines (smoke test done as well)
- Compression test done on all cylinders (160 on all cylinders accept 1 of them measured 190psi)
- Leak down test done and all cylinders measure 20-27% (gauges verified on my Tahoe that measured >5%
- Dyno'd the car and still made 1k even with these issues.

When I did the leak down test, I found that the air was getting past the rings and all of the air was coming from the oil fill hole. This is a 416 with an procharger F1x. Looks like I need to have the motor looked at, but wanted some opinions on what's going on here. I even went and got HP Tuners Pro to monitor everything, but new to the software so no real help. Any help is appreciated.

-Justin
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Old 04-17-2018, 08:20 PM
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How radical of a cam shaft are you running? A stock GM cam shaft makes plenty of vacuum to power a brake booster. A high performance cam shaft . . . a lot less vacuum.

Rick
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Old 04-18-2018, 07:22 AM
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I would double check that check valve.
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Old 04-19-2018, 11:14 AM
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Originally Posted by B52bombardier1 View Post
How radical of a cam shaft are you running? A stock GM cam shaft makes plenty of vacuum to power a brake booster. A high performance cam shaft . . . a lot less vacuum.

Rick
Cam specs are 242/258 115 and idle set to 800rpm.

Would I gave a hard pedal with no vacuum? Ive been told this is the case but not sure.
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Old 04-19-2018, 11:15 AM
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Originally Posted by farmington View Post
I would double check that check valve.
Easiest way to do this? I just blew into it from both directions to verify.
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Old 04-19-2018, 11:28 AM
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Originally Posted by Hou_tx_ss View Post
Would I gave a hard pedal with no vacuum? Ive been told this is the case but not sure.
Yes a hard pedal and but pedal travel will stay about the same if you push hard enough.
If the pedal is going to the floor then you have other issues.
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Old 04-19-2018, 09:48 PM
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Originally Posted by Hou_tx_ss View Post
Cam specs are 242/258 115 and idle set to 800rpm.

Would I gave a hard pedal with no vacuum? Ive been told this is the case but not sure.
243/251 115 here. Was having the same problem with the hard brake pedal after bleeding the system and it ended up being a bad brake booster. You need 10-12 on the vacuum for the brake booster and your not getting enough vacuum to properly work the brake booster.
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Old 04-19-2018, 09:55 PM
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Your only getting 2 inches of vacuum from the intake manifold?
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Old 04-20-2018, 11:36 AM
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that is a really big camshaft, coupled with not perfect leakdown numbers. You might have to go to a separate vacuum canister to get enough vacuum to operate the brakes. what kind of vacuum are you getting at say, 1500rpm. any better. could be from retarded initial timing too. Using a hole in the throttle plate to get it to idle?
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