Just how important is the modular valve on a transmission?

[Chieftain27]

I have a th 400 transmission on a chevy chassis. My line to the modular valve hasnt been connected for 4 whole years of driving this rig.

[Meames1]

How does it shift ? Does it feel like it always shifts late and harsh ?
The vacuum modular is how the transmission senses engine load and knows when to shift (as you may know, can't tell from your post). So with no vacuum signal it would be strictly relying on the governor to shift once it gets to a high enough governor speed. Plus the modulator not being hooked to vacuum will mimic the engine always being at high load.. Usually that will create a late and harsh shift.
There are some "shift improver" kits that could have been installed that would bypass the modulator signal, BUT "to my knowledge" the only ones that bypass it are set to manual shift mode only.
I would hook it up.

[GypsyR]

I bet it will shift better at part throttle when you put a modulator hose on. But if your coach is at all like mine you don't really know what part throttle means when accelerating.

[darnold9999]

Without the vacuum module the transmission is shifting as if you had your foot on the floor and the engine is under heavy load - all the time. Late and harsh shifting. Hard on fuel if nothing else.

[D Gardiner]

You might see some better gas mileage.