What was found
DURING ANNUAL INSP, ORIG OIL BREATHER LINE FOUND WEATHER CHECKED AND DETERMINED UNAIRWORTHY. NEW LINE WAS CUT OFF OF BULK LENGTH STOCK AND REPLACED ORIG LINE IAW STD PRACTICES, AFTER LINE REPLACED, ACFT TEST RUN, ENG RUN-UP ON GROUND AT TAKEOFF POWER, CRUISE POWER, AND POST-MAINT CHK IAW CESSNA MM, CESSNA 172M. PILOTS OPER HANDBOOK WITH NO DISCREPANCIES OR OIL LEAKS NOTED, ACFT RELEASED FROM MAINT. FIVE MINUTES AFTER TAKEOFF, PILOT NOTED OIL SMOKE ENTERING COCKPIT, DECLARED EMERG, RETURNED TO AIRPORT. DURING RETURN, FRONT CRANKSHAFT SEAL BLEW OUT CAUSING LOSS OF OIL PRESS, DUMPING OIL OUT OF FRONT SEAL. ACFT LANDED SAFELY. INSP FND NEW BREATHER LINE BLOCKED. PLASTIC CAP SHOVED 4 INCHES UP INSIDE HOSE.