y pipe
 

On through prop applications, is there a check valve in the Y pipe? Or does the system rely on enough vertical rise to prevent back flow?

In a way. There is a butterfly assembly in each horn right at the rubber boot. Older years used a single flapper style like what are used in exhaust tips. The replacement flappers are the new style. They just snap in. They are a good thing to have for that very reason you mentioned.

These flappers are no where close to being water tight right? They just reduce the pressure head of the water rushing backwards? Does 1/2 of the flapper cover each branch of the Y?

Yes, not 100% water tight, but not far from.
Yes

There are 2 pair to a set. 1 goes in the top of each leg where the divertor valve is connected with the rubber boot.
Picture a standard home door hinge, with the pin in the middle and each half of the hinge hanging down into the y-pipe, but partially opened. As water comes up the y-pipe from bouncing while stropped, the water will contact the 2 hinge halves(4 total) and force them to spread outward and seal against the y-pipe wall, stopping the water flow.
They just snap in at the top of the pipe and are very cheap. sumtin like $30 a set??? I forget.

Old style below (New style is the "hinge design)pt # 4

US1Fountain is correct. I replaced them on mine a couple years ago. The pivot shaft on the old style flappers had started to egg-shape the holes in the Y-pipe. The new style rubber butterfly wing style is much better.

How do you know when they are bad and require changing?

A lot of times you can raise the drive to the trailer postion and see a broken flapper that fell into the bottom of the y-pipe in the exhaust exit hole. (where the round bellows is attached below the drive shaft belows).

But best is to just remove the elbow, or what ever, that is clamped to the top of the y-pipe and look. If they are the 1 piece valve, I would replace it while it is apart.

There are some that have had the older style flappers break, fall into the y-pipe, and rub a hole thru the side of it allowing water to enter the boat. I would venture to say you DO NOT want that. ;)
I installed a new set of the old style a few yrs ago, and by the end of that same season, the pin wore thru the brass bushing,wore the pin ends almost off and egg shaped the hole as MACDAD said. Only time and they would have fallen inside.

I have a question, rather than start a new thread I will ask here. Do you have to pull the engine to remove the Y-pipe? I am thinking of removing my silent choice system.

i have an '86 sbc w/alpha 1 drive. This spring when I was considering a silent choice my mech said we would have to pull the engine to install the "y" pipe. So taking it off would be the same I imagine. Not much room down there.

Yea, you'd have to pull the engine to get it out.