[LgWoodenBadger]

Would you be able to reseal the cracked glass and regenerate the vacuum through the other end?

More glass, epoxy, or similar?

[adrian_b]

An accidental crack can be resealed, but if the crack had appeared because inappropriate materials were used, e.g. an unsuitable metal-glass pair, resealing is pointless, because cracks will appear again after the device is turned on and off several times, causing expansion-contraction cycles.

[bluGill]

Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.