* LLS, Mars weaken 65 cents to $1.05 on narrower arb * WTS up $1.50 at $6.50 under, and at $5.05 under for June * WTI Midland sells for discount of $6, about $1.75 stronger * Analysts cite new pipelines to Gulf Coast * TransCanada plans maintenance of Keystone but no shutdown By Bruce Nichols HOUSTON, April 11 (Reuters) - U.S. Gulf Coast cash crude premiums weakened on Wednesday as the transatlantic spread narrowed an additional $1.38, but West Texas grades bounced back