Hello, I'm new to this forum but not Vette's. I have a 69 BB and I just found I have a vacuum leak at the intake. My first step is to replace the carburetor mounting gaskets and see if this helps. My question would be, is it a possibilty that the brake booster could be loosing vacuum? I have a 'T' on the backside of the intake and this is were I took the vacuum measurment. One side is the brake booster and I left this connected and the other side is for the main vacuum system, and this is the port I took the reading from. I had about 15", but it did fluctuate quite a bit.

Thanks in advance...

 

If your brake booster diaphragm has a leak, I'm pretty sure you'll hear it when you put the brake pedal down. Get yer' noggin' down there, and put the brake on with your hand, and give a listen...

 

Remove the "T" and take a test from that port. This would tell ya if the leak is in the brake vac or the other system with the common connection to the "T". I think you're on track with the carb gasket though, but try the test I recommended, and take it from there.[/align]

 

Remove the "T" and take a test from that port. This would tell ya if the leak is in the brake vac or the other system with the common connection to the "T". I think you're on track with the carb gasket though, but try the test I recommended, and take it from there.[/align]

 

You can also use either an unlit propane torch or wd-40 and point it at any area you think might be leaking. You'll notice a change in idle speed if you hit a leaky gasket area.

 

Block off all vacuum ports (including the vacuum advance) and run your vacuum test again. The distributor vacuum advance canister could be bad. Once you have disconnected everything and you still get the fluctuations, you can spray WD-40 around the edges of the manifold and the base of the carb. Do it slowly and listen for the idle to increase. If you don't find anything at that point, you may have a leak ounder the manifold in the lifter valley. How do I know that you ask? Been there, done that. A quick check for that leak is to keep all but one of your vacuum ports sealed, remove you oil fill cap and PCV valve or breathers and plug the holes with clean rags. Put a piece of plastic over the carb and secure it with a rubber band. Pull your dipstick out and use your air compressor to pressurize the block. If you have a leak you will know at the vacuum port. Place a small piece of tissue over the port and see if it moves. You should only need about 40 PSI to see if there is a leak.