[giff1]

I have a 3500 Express. I've read of problems with the holding tank collapsing, due to the high suction during pump out. I have also read, if you open the toilet valve during pump out, it shall let plenty of air in the tank and keep it from collapsing. My thought is to stick a foam noodle float in the toilet to keep the valve open during pump out. Any thoughts if this would work.

[hiccup]

The tank has a vent. It is almost certainly clogged if the tank is collapsing. The vent will typically have an in-line charcoal filter to prevent the smell from venting out but allow air to pass freely. When the filters get old, and especially if they get wet, they no longer allow air to pass through. Start by replacing the filter. If that doesn't work, you will have to dig deeper to figure out where the blockage in the vent line is. The vent line allows plenty of air movement to prevent suction from building in the tank during pump-outs when operating correctly.

[giff1]

My tank and vent/filter is good. The concern is, I have read the 3500 Express has a history of their holding tanks collapsing because of poor venting. I just wonder if placing a pool noodle (which would not hurt the toilet valve) in the bowl to keep the valve open would give the system more air during pump out.

[hiccup]

If it's collapsing, your vent is not good, period.

[Sans Peur]

If you have a vacuum system you will have to shut that off first. I do agree about the vent being enough to allow for air flow and you should check that first. They are not to expensive and I keep.a spare on my boat. I change it out every 6 months to a year depending how much I use the boat.

[egstasea]

I like your pool noodle idea, and I do think that will work.  On my 40 Express, I added a tee before the charcoal filter and ran a new hose zip tied a to the engine room ladder with a quarter turn valve at the end.  When I go to pump out I open the valve for an unrestricted supply air path for the tank.  I used a full port valve.