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Old 06-26-2012, 04:18 PM
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Master Cylinder Vacuum Leak???

72 Chevelle with moreate engine mods to a 454 I just rebuilt. Also, converted rear drums to disks.

Now this one really has me baffled. Due to a lack of vacuum to the booster/brakes I installed an auxillary Summit vac pump. Per the instructions, I ran a vac line from the pump directly to the break booster. When I turned it on there was a vac leak around the master cylinder. Upon further evaluation I was surprised to find at the bottom of the master cylinder just before it bolts to the booster, a through hole which intersects a vertical hole in the master cylinder housing. If I place my fingers over the hole I eliminate the vac leak. The hole looks like it was machined at the factory.

Would anyone know the purpose of this hole and if I can block it up so I have complete vac?
Thank you in advance for any input you may add.


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Old 06-26-2012, 06:07 PM
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If you installed you aux vacuum line into the normal booster hose location with the check valve, I might think the booster bladder was bad. That hole your referring to is probably a vent hole for the movement of the bladder.
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