Boe: I can confirm your CAPS gear pump can pull fuel through the FASS-TS pump and filters so your engine will continue to run in the event your FASS pump fails. However, this not the question you asked. I will get to that later, because how you prime and start your engine is another story.
A) If you are starting the engine "cold" then I would think you need a lift pump (any lift pump) to prime the system in order to start it.
This is why leaving your stock lift pump and fuel lines in place may be beneficial. ...But how often, does your FASS or AirDog newer generation pumps fail? ...Probably never.
...So some people think leaving your stock lift pump in place is redundant and just inviting air or other problems... like a stuck check-valve in the lift pump manifold that might cause problems down the road.
Therefore, I would say the reason why you would just leave your stock lift pump fuel lines "as-is"; and the reason to leave your stock lift pump electrical connection to the ECM "as-is" ...is based on the idea that this approach will reduce your installation time by 2+ hours. (Or $250+ in a shop labor.)
And so, if you have 2 lift pumps installed and are functional, I proposed using a Delay-On-Relay (set for 30 seconds) so it will turn your FASS or AirDog pump on after the stock lift pump turns off. (This relay should cost ~$25 and is much cheaper than bypassing and disconnecting the stock lift pump.) This way you are not running both fuel pumps at once.
AS TO YOUR QUESTION: I do not know if the stock lift pump can pull through the FASS pump, but it should.
All if these vacuum pump pull only -3 to -5 PSI... and since there will be fuel in the line from your FASS pump to the lift pump manifold you will not have a air-gap problem in the fuel line to contend with.
...But if you did, and we are talking about B) "How to start your motor with a stock lift pump in the event your FASS pump fails?" I would think you just need to cycle you key 2x or 3x just like you do when you change your fuel filters.
Therefore, you can mount your FASS or AirDog next to your tank if you want.... but I assume you have access to the the top of the fuel tank in your Roadmaster Chassis?
...Allowing you to utilize your existing fuel line from the tank to the lift pump manifold (LPM)?
...And you have another way of adding a "bung" to the tank to accept the FASS fuel return line high flow rate?
Note: AirDog-II-4G pumps push 25psi and return less than 1 gal/hour of fuel to the tank. Now compare this to a FASS-TS pump the pushes 15-18psi and returns 60-90 gal/hour.... and you can see why you should NOT use a "T" fitting with the CAPS fuel return side to connect with the FASS fuel return. ...It's too much fuel flow and that back pressure will cause your FASS pump fuel pressure to increase.
So if you plan on using a "T" fitting (at the tank or at the engine) to plum a aftermarket fuel pump with your CAPS fuel return line, then I would go with AirDog-II-4G (150 model) which I believe is Kit #A3SPBT461.
You might also look at this post regarding an Alpine owners AirDog-4G pump installation next to his generator:
https://www.irv2.com/forums/f104/fas...ne-484540.html
or this installation where you remove the #1 filter and install an AirDog-4G in its place:
https://www.irv2.com/forums/f123/air...on-490140.html
...and of course my FASS-TS installation in a Freightliner Chassis:
https://www.irv2.com/forums/f123/cum...mp-458337.html
Summary:
I like the AirDog if you want to or need to "T" into your existing CAPS fuel return line. However, I prefer the FASS pump if you decided to return fuel to your fuel filler neck, because I like their "fuel polishing" benefits.
My Freightliner chassis does not have access to the top of the tank so returning fuel to the filler neck was the only way to go... in my case.
...And as I said, I believe in the FASS "fuel polishing" benefits of recycling fuel over-and-over. Especially if you have a algae concerns or a dirty fuel tank. And in my mind, every tank 10 years or older has a dirty fuel tank; and it would take AirDog 30,000 miles to clean what FASS pump will clean in 3,000 miles. So I went FASS.
I also prefer to change the filters back by my engine. So really, for me, the decision to install FASS or AirDog has more to do with how you want to plumb your pump fuel return line?
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