LL - There are a couple of factors here and I'm not certain that I really understand what you are asking. Let me take a cut at it -

1. BOTH types of ballast tanks ["main" and "fuel"] have hydraulically operated main vent valves.

2. In the event of a hydraulic failure, the main vent valves are capable of manual operation.

3. BOTH types of ballast tanks have manually operated  emergency vent valves except for MBTs #1 and #7. I think this is the crux of your question.

There are two basic configrations for MBTS #1 and #7 vent risers. In GATO and BALAO classes, the vent risers pass inside of the pressure hull (Forward & After Torpedo Rooms) while in the TENCH class, they remain outside the pressure hull.

The GATO/BALAO configuration was always viewed as a serious vulnerability, but the tank arrangement below the FTR and ATR and superstructure configuration simply wouldn't permit a non-penetrating external route. In addition, there is a lengthy discussion involving maintaining trim under the full operational range the boats could have that drives much of the MBT #1 configuration.     

In the TENCH design as well as GUPPY conversions of BALAOs, MBT #1 was relocated to surround the torpedo tubes. That automatically placed the entire MBT #1 and vent risers outside of the pressure hull.

Since MBT #7 was inside of the pressure hull in all 3 classes, it was typically converted to a VFO or a storeroom, depending on the particular boat and the extent of the GUPPY conversion. 

So the standing question is why didn't MBT #1 have emergency vent valves once the risers were outside of the pressure hull? I have a short answer for that -- BESOM!