Problem with log space

I make a database bakup every night at 3:00 and a transaction log backup at 12:00, but my available log space never shrink.
When I try to see the Transaction Log space available with the Enterprise Manager or DBCC SQLPERF(logspace), both show that my log space available is reducing.
When I run dbcc checktable(syslogs), this correct the indexes on syslogs and the DBCC SQLPERF shows to me the current space alocated (near 0 %). The Enterprise Manager still shows to me the incorret log space.

The question is:
1 - I always need to run DBCC CHECKTABLE(syslogs) to get a correct space available from log?
2 - If I never run dbcc, when the "virtual" available log space terminate, can the SQL Server send me the error?

Tanks
Ésio