is there a gauge with an air fitting that fits into the radiator cap opening. Or do I have to use a heater hose bung? Maybe an inner tube on the water inlet and outlet? I want to use compressed air to check for leaks. How much pressure is needed?
I don't want to pull this motor if I don't have to.

It's a '77 B-200 with a 318. I added a new radiator and a couple new freeze plugs last fall. It was still leaking from (?) so I drained the coolant, before parking it.
I want to find out just where the trouble is before I continue.
Thanx for all responses in advance.

get a "tank valve" schrader valve fitting from O'reillys, less than 3 bucks & it has a valve stem with the black tire valve stem cover just like on your tires & several 1/8 NPT regular pipe fittings at your hardware store & plumb it into the water temp sender port in the intake manifold. get a triple female T along with a double male "nipple" 3 or 4" in length so you can plumb a psi gauge AND the tank valve in & get it hot & blow it up to 15 lbs which is pretty much the max for a cooling system & if it holds that much you can go further if needed & remove the rad hoses & cap the pump and stat (1&3/4/1&1/2) nipples with hardware store plumbing caps/hose clamps & remove the heater hoses & cap those nipples also. I think at that point it should be able to hold whatever your air compressor can put out, say 100 lbs. EDIT I reread your post, no need to remove anything, just add the valve & start to pressurize it to 5 or 10 for a start & (highly) you'll spot the leak. MORE EDIT it ain't like airing up a tire which is where your mind will be at when using an air chuck, it will reach 10 or so FAST as in seconds so pay attention to the gauge.

Right-On Rapid Robert.
That is just what I was looking for.
So I can just plug that into the intake manifold, charge up the system, and listen for air, right?

Should I take the spark plugs out, or fill it with water first?

Take the spark plugs out in case it is leaking into a cylinder.It's easier to see which one that way.

OK thanks.
If I hear anything from a spark plug hole it's time to pull it anyway, right? Head gasket or crack, right?

I mean, I have other motors. If I have to start adding gaskets, it would be better to get a spare ready for install.

Yes but it'll be peeing (hopefully just a minor seep instead that is easily fixed) antifreeze from the leak. I'd start with 6 lbs or so & work up as needed till it shows. EDIT Easier yet just plumb the schrader valve by itself into the mani port (no extra plumbing needed) & add some psi & carefully keep tabs on the amt with your tire psi gauge, the leak will show itself down below in short order. MORE EDIT I reread & saw that it has been drained. Minor air leaks can be hard to ID & if you dont hear anything then I would add some antifreeze & pump it again then you WILL see the leak point

Thanks again Robert.
If I find any leak. I will probably be swapping out the motor. I was simply looking for the easiest way to find out if it's still holding pressure. I.E. Gaskets or cracks. Hoses or more freeze plugs I would get after.
It would just be easier to prep a motor in the shop, then swap. I was hoping to avoid that for a season or two.

If it is leaking in the cylinder you may hear it,yes.It could be the gasket,the head or a crack in the cylinder itself,but at least you will know which one.Look at the plugs as you pull them out,a clean/green one will be a clue!Don't forget a small block has freeze plugs on the back of the block in the bellhousing area too.

Yeah, just changed the plugs last summer.
I went a tad hotter as oil was what I saw.
No sign of coolant then. That was before the trouble though.
Full disclosure, I did do a couple 1st gear full rpm blasts after I installed the new Heart Throb exhaust.
Neighbors were impressed.
Leak was an embarrassment.

Didn't even consider the back of the block.
Will be keeping an eye out for that.
If it's leaking behind the bell.
Motor and trans swap.

In a B-van I'd just slide the trans back or remove it to fix a plug on the back of the block.

Easy enough.
Once the trans is free though, might as well go all the way and be done with it for what's hopefully to be the rest of my life.
I have other motors and transmissions.
I just have to go through them before an install.
This test is what will decide which way I go next.
A few other toys to get moving again.
Here's hoping to just get it running again.

when the time comes, post where the leak is coming from.

$

And to repeat what Fast Bob said about being CAREFUL when you pressurize the system! Liquids are not compressible, and the amount of air in your cooling system will be nil. Please set your compressor regulator to 15psi!
If you hit that valve with say 80 psi you will over pressure the system in a fraction of a second.

You could easily blow your rad

Robert,
I'll be sure to post the results.

RS23U1G,
The above pick is what I originally had in mind, but posting the question here has saved me a bunch of cash, again.

Pacnorthcuda,
Currently no liquid in it.
Still, thanx for that caution.
Didn't even consider that.
Radiator is brand new.

Thanx, to all for the help.

set the regulator to zero, then adjust upward carefully to the pressure you want to achieve. i believe that is the only way you can do this safely.

Borrow one for free at the local parts store. Why add a permanent air fitting for a one-time test?

If you can borrow one for free at an O'reillys or similar that would work fine as with one of them you can easily monitor the psi you are pumping up, otherwise the tank valve is $2.03 out the door (here) & you can unscrew the coolant sender/screw in the tank valve/air it up/check for leaks/unscrew it/reinstall the coolant sender

I have a hand held tire pump for the grand-kids bikes, so sneaking up on 14#s will be no issue.

I'll most likely use the opening for the temp sender for this test.
The valve will not be a permanent addition to the motor, but to my tool box.

Will be going into town for work tomorrow, so probably get back to this next week-end.

thanx again to all.

318, water leaks, 90% of the time, front cover to block at the coolant hole. Very common problem. Aluminum corrodes over the years, could be a form of electrosis, hard to see sometimes with the accessories in the way.