I have cleaned my tank and replaced the petcock, cap, and float. I filled it and have it sitting off the bike while I wait for Z1E to send my airbox hoses.

I am getting a slow leak from the petcock. It is not from the gasket around the unit, it is through the fuel pipe into the fuel hose.

Since I don't have the vacuum hose hooked up, can I expect to have this leakage occur when I do hook up the tank to the carbs?

This is a 81 KZ100 M1 CSR.

Maybe not seep when the carbs are hooked up, depends how well the vacuum valve cuts the fuel off. If the floats/needles in the carbs are good, they may stop the gas from over flowing. A lot of petcocks seep a little gas.

I love the vacuum activated petcok when they are working but they can be a PITA when they malfunction. My carbs will overflow if the petcock leaks. In theory float valves should stop the gas but in practice it doesnt happen. What can I expect with 30 year old carbs

I have had the same problem and would suggest taking the petcock apart and taking a look at the spring that pushes on the little rubber diaphragm.
It should look like this NNNNNN. Stretch it out a little so it looks like this NNNNNNNN and put it back. I hope this helps.

Only when bike is running would produce vacuum to open the petcock. In time, they may sip a little, but I wouldn't expect a new petcok leaking at all, if position is not in prime.

switch,

As you have gathered from the great advise above, a properly installed and properly working petcock with good gas lines and clamps should never leak. If it does leak then I would repair or replace before riding season.

On an unrelated subject, did you add a premium quality clear inline Fuel Filter between the petcock and the carbs? IMHO, this is not optional in 30+ year old fuel tanks... don't ask.

Best of success.
Bill