You can it like that. In fact that's a lot less back pressure than if your exhaust boots were hooked up.

On the front of the air compressor it's self is pressure relief valve. Unscrew that from the pump and take something with a small point on it and push open the relief valve. While holding it open have someone scrub it with dawn soap and a tooth brush. When it looses air pressure is when the exhaust opens. It only takes a little leak to cause trouble keeping the silent choice open for all four pipes. This will fix your problem.

Lucky check the date on the post.

Glenn, Bought a 28 labbed bravo from you a few years ago, Runs good on my 29 fountain. Hate to see you leaving the fountain family, But good luck on your sale.
P.A. Pirate

Thank you. Glen