Does it not work in either forward or reverse? Might be a relief valve. Could also most likely be a clogged filter. Use 923-3014 or 723-3014. Best and really the only way to drain the fluid is by removing the rear cover. You will need a new gasket 921-3015 or 721-3015. Clean the differential housing with brake cleaner, and wipe with lint free rags. The Hytran capacity is 7 qts. These fill much easier with a relief valve removed to allow venting of the unit. Otherwise, the fluid will burp often making it a slow and messy refill.