One problem related to brake fluid leak has been reported for the 2004 Honda Odyssey. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Honda Odyssey based on all problems reported for the 2004 Odyssey.
The brake warning lamp can illuminate, indicating a low brake fluid condition, although no leaking brake fluid is visible throughout the system. A worn rear master cylinder seal can cause fluid to leak into the brake booster, which will cause diminished brake performance over time. There is already a recall issued for this same issue on 2005-2007 Odysseys, but I believe it also applies to 2004 Odysseys.