3551 - Brake unable to release. ~~~~~ 3551 - Brake unable to release.  ER D/L - Done. Found 3551 intermittent stay at high pressure. Replaced 3551 ACU & function test/brake pressure test - OK. 3551 ACU S/N : OLD : 1016-001 NEW : 1306-053  Lee PT/Alcus 44090