Sorry I had no idea of your set up only you know it best. It is always hard to second guess up here what people really have. Anyways the cap on the gear lube monitor from mercruiser plastic bottle / stock app is persay a one way valve as it lets air in at the brownish / red rubber underside and the nipple just sticking out.

It is somewhat hard to put into words to understand fully - the cap and the o-ring (underside of the cap) hold the pressure in the drive system. As the drive is running it builds internal pressure, the one way valving in the cap allows air to enter into the drive reservoir to equalize those pressures, with out the o-ring or the cap or the vent clogged in the cap for proper sealing pressure, the gear oil will rise / bubble out the gear lube reservoir as it builds its internally pressure. Hope this makes sense.

Post 10 is correct you need to put it back in - I promise this is your problem like I tried to explain in my first post but not really knowing what you had for reservoirs. It was basically along those same exact lines.

I have speedmaster 4's also on my boat. Except I don't have boxes and use the original inner transom plate with reservoir/expansion chamber. Whats weird is, when I Bought my boat, it had been sitting for a couple years. I went to drain the drive oil, and when I loosened the lower drain plug, oil squirted out as is was under pressure. Ran it for that season. Changed oil again, and same thing, loosen plug, oil shot out at me.

I then noticed on the inner transom plate, there was a NPT pipe plug, where the little vent plug should be, like the vent on top of a borg warner. I looked up on mercruiser parts, and saw they listed the vent to go in that hole, so I ordered the vents and installed them. I no longer get oil spraying at me when loosening the plug. But, now you have me thinking, did something know something about plugging that vent, in order to keep pressure in the drive, so that it helps keep water out or something? I was worried it was building too much pressure and maybe blowing a seal out or something.

Hmmm. See I thought it was an expansion bottle to allow for expansion but I see I was wrong.
Im` pretty sure I threw the parts for the check valve away.. dammit.

Dan I wonder if that's why the acrylic in the reservoirs was cracking. It doesn't take much pressure to do that in a tank like that. I had those tanks on my formula with trs. The acrylic cracked on those too. Removed them and used the stock trs reservoir and no issues

yeah ok.. thats why all of mine were cracked

Too much pressure can cause seal leaks. In the case of the cracked bottles IMO you are building too much pressure. Internal heat can cause too much pressure build up. Maybe check your drive for bearing wear or any form of wear. Also like I said drives that have been set up with too high rolling torque specs can do this as well.

It does allow for expansion. There is air on top of the oil in the drive, when the drive oil and air get warm, they expand a bit. Just the heat of the day will put a bit of pressure in the drive if the night before was fairly cool. The drive should not get real hot with use though, shows up as way too much expansion/foaming.
A bit of pressure in the drive helps keep water out also.

Both drives are fresh with lots of new parts. Colabella rebuilt them last summer. The cracks are just a crappy brittle acrylic. Everything was fine last year.

Not SSM's, but a buddy of mine has been having some air bubbles in the gear lube reservoir of one of his Bravo I drives. I told him to switch caps this weekend and see if the problem moved with the cap. It did.