Cannot get a constant vac on cooling system before putting coolant in it. Radiator and heater core test fine. New pump impeller. Can only get vac to around negative 8.
Should i just fill with water and see what happens or any suggestions before that step.
Thanks

I have never heard of pulling a vac on the cooling system.
since it is meant to be under 12 to 16 pounds of pressure, I have to ask what was the point?

Some items may not hold pressure /vacuum if they were designed for the opposite.

if you put it under 10 pounds of pressure, does it hold?

Hello Bill. You dont want to fill it with water cuz if the leaking is internal then it will start rusting & must be torn down ASAP but if it is inside & we find it with another method at least we have some time to get after it if we confirm it being on the inside by dry checking. I would get a "tank valve" from O'reillys for less than $3 & with your air chuck & a tire pressure gauge pump the system up to 10 lbs & see if you hear where the leak is coming from & you can squirt some soapy water around the pump & intake manifold and head/block interfaces to see if it bubbles. it is a schrader valve with a 1/8"PT fitting & screw it into the cooling temp sender port on the water pump. EDIT pic added. MORE EDIT if it wont hold vac it wont hold psi as it is a simple sealed system ex for the possible (not sure) complication of the type of rad cap with the loose thin brass "recovery" piece dangling on the bottom. with a "regular" cap if it wont hold vac then there is a problem. I had a weak cap so I put a piece of rubber under it to seal the system when I was dealing with my issue.

That's how the factory filled them and the reason for the lower radiator hose spring.

Very common to pull a vacuum on a cooling system before filling. Especially on the newer stuff, in some cases the cooling/heating tubes are so small in the heater core for insistence it is impossible to purge all the air out of the heater core without a vacuum.

I would try to pump it up and see if you can see/hear where it is leaking next.
Good luck.

It has to be used to fill big truck diesel engines.

If a person does do a wet test, use 100% green prestone antifreeze to keep the ferrous metal from rusting till it does come apart.

interesting. learned something new.

it will be interesting to hear where the leak was. Please keep us posted.

Water pump shaft seal probably won't take a vacuum, and its not meant to. Pressurize it instead, maybe 10 psi.

There are two valves on a radiator cap, pressure and vacuum.

The vacuum valve is to refill the radiator from the overflow tank when the motor cools and creates a vacuum.

It only takes 2 lbs of vacuum to open the return valve.

I would add the $3 tank valve. pump it up to 10 lbs. See if it drops. If you have a compression tester you could take the hose & remove the schrader valve from it & screw it into each cyl with a balloon sealed on the quick disconnect end & see if it expands as you maintain the 10lbs. spray some soapy water around the freeze plugs & the drain plugs & sparinly on top of the intake manifold to head face(s), both sides and around the pump and pump housing & see if any area bubbles show. holler how it turns out. EDIT If you pump the cyls & if this is a new cam breakin I would undo the rocker arms so the valves are closed/cyl(s) sealed so you dont have to turn the dampener to get each cyl on TDC which will wipe the goop off of the lobes.