Hi, and thanks in advance.

I have rookie question: Any thoughts regarding the best approach to detaching the main vacuum hose from the plastic (?) tee hoses with ringed male ends?

Heat?

I just don't want to take them apart diagnostically and damage / ruin them.

Bradley

I don't know or have read of a reliable way of to remove the Plastic Main Vacuum Line and get it back on.
I suppose if you heated it with something like a Heat Gun that would stretch it enough for the line to be pulled off but I don't know of a way to shrink it back after your re-install it.
It might be possible degrease the Metal Barb and ID of the Hose with Brake Cleaner and coat the Barb with Silicon Sealant and let that cure. That would likely seal and glue the expanded Plastic Line back in place.

I cut the Plastic Hose off of the Main Vacuum Line and replaced it with Rubber Vacuum/Emission Hose. That allowed Me to make the line Longer and route it over the Top of the Fender Well where it is out of they way. That and the fact that the line is Rubber decreases the chance of snapping off one of the Small Plastic Nipples.

Once you have the Rubber Hose it is easier to hook up a Fitting for Diagnostic purposes.
I also replaced the Plastic Fuel Lines at the Fuel Injection Pump with Rubber Hose.

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84 300D, 82 Volvo 244Gl Diesel