When I was building everything in my boat, I decided I wanted/needed a bigger drive oil reservoir and instead of buying a high dollar one I made one out of stainless pipe with the ends capped and welded in a bung/cap at the top. My question is...I went with a vented cap with no check valve in it and noticed that when running hard it would push oil out of the cap vent. Am I on the right track thinking that I need some kind of check valve in the cap so that it will build pressure in the drive and let air in when needed but not push oil/air out when running? Would like to figure it out before I get it back on the water to prevent a mess. Thanks!

YOu need a bigger expansion tank.
Check valve that isn`t going to allow any expansion? What`s the point in having an "expansion" tank then lol
If the pressure can`t expand it will make it`s way out alright , probably thru a seal somewhere .

Makes sense and that's how I always thought of it but then I remembered the regular bravo bottles had a rubber nipple deal in the cap that I think is supposed to act as a check valve to let air in and let the fluid go in the drive but not the other way....I modeled my tank after the one imco sells for my drive but apparently I didn't go big enough....great something else to redo hahahaha

Regular Bravo drive lube bottles use a cap just like you describe with a one way valve that allows air in, but stops air and gear lube from going out. Pressure does build a little in them, but that is why they only get filled about 2/3 full. The level in my gear lube moves up about 1/2" when the drives are warmed up.

That's about all mine would move when it was a bravo...now with the SCX upper it moves a lot...5" diameter reservoir and it will raise an honest 2-3" if I'm running hard and 1-2" just normal cruising around. If I keep the level barely visible in the sight glass I can usually get away without an over flow but anything higher and it spits fluid out when I'm gettin after it.

I did not know that about Bravos expansion tanks, the drives dont hold much fluid so maybe thats why they get away with it...
Wet 6s will fill up this entire thing after a hard run.. but drives also hold 5 gallons each.If I could check valve em that be fantastic but I`m afraid to .

Us poor bravo guys can't afford all that damn fluid Dan! hahahaha

Like griff mentioned they are designed to build a little pressure which I think is designed to help keep water out if I'm not mistaken so brings me back to my original question if I should get rid of the vent and do a check valve? The way it is now the drive can't build any pressure because it's just vented off and I'm wondering if I'm setting myself up for failure not having it build pressure?

I don't think the not having pressure affects anything negatively. The check valve cap is mainly to keep the lube from escaping.

How do you know how full you reservoir is being that you said it was made from SS pipe??? Did you put an Merc alarm in it??

I welded some stainless fittings in and put clear tuning in as a sight glass....I have to keep it barely in the sight glass so 1-2" of fluid in it or else it pushes it out of the cap which has a small vent hole in the cap. It has a strainer/filter on the underside of the cap that I think is to keep fluid in but it clearly doesn't work hahaha

This is the cap kit I welded in