fat sack fill empty vent????

I am thinking about automating my ballast system. I have a fat sack locker sack, and two fat bricks. I am leaning towards useing aerator pumps. I was thinking, two pumps each sack, one fill one empty. All three fill pumps will be on a manifold of some sort near the motor. The empty pumps will be attached close, and below the bags to prime/drain.
I think my locker sack has three connections, which is perfect fill, drain and vent all have their own lines.
My main question is, with my two fat bricks, there are only two connections, fill and drain. How do I make this work? Check valves and t's? I was thinking the fill line to the top connection, and the empty from the bottom hole. where and how do I vent?
Maybe run fill line to a t, one end into the sack, one end to vent hose out thru hull. In theory the bag SHOULD fill before creating preasure enough to vent?
THanks guys.

Don't worry about the overflow on the fat bricks. They will build up enough pressure to push water back through the drain pumps and out the side of the boat to overflow. If you do get air in them just push it all out before you fill them up. That is a lot of work to get 155 lbs. of water. Can you fit any bigger bags in that location?

I cant really fit any larger there. This instal is kind of a test, I am trying to make a biologicaly safe(no mussels) ballast system, so Im not looking for a ton of ballast. This is going in a 21' i/o wellcraft. IF this works than I will go bigger in my next boat(fingers crossed that I will ever be able to get into an dd or v drive).

also how much do you all think a 5.0 ford weighs? 300lbsish?

A 302 long block should be around 475 lbs, fully dressed for marine use maybe 550 lbs.

thanks dude.

on a similar note.... If I keep my pumps below the sacks, plumbed out of the bottom most connection, and fill from above, will the empty pumps provide enough resistance to keep the sack full? Meaning, if I fill a sack, it over flows by forceing water through the drain, I stop the fill pump. will the empty pump, then with the sack full, pumps all off keep the bag full till I turn on the empty pump?

Originally Posted by epic1

on a similar note.... If I keep my pumps below the sacks, plumbed out of the bottom most connection, and fill from above, will the empty pumps provide enough resistance to keep the sack full? Meaning, if I fill a sack, it over flows by forceing water through the drain, I stop the fill pump. will the empty pump, then with the sack full, pumps all off keep the bag full till I turn on the empty pump?

If you loop the hose up high enough before it goes out the boat then yes that will work.

Basically you want that loop to be higher then the tallest point of the bag, plus a little bit more for piece of mind.

You can also T your vent line into this exit line since air won't bleed out of the bottom of a bag through the drain pump. Once the bag is full you will start to have water come out of the drain pump hose and vent line at the same time.

What he said. Have fun with the project.