Bobcat 328 Boom Leaks Down

[Novanut 63]

I have a 2004 bobcat mini excavator that the boom leaks down. We have been trying to bring it back from a hard life. We have had all the cylinders rebuilt, new lines and swivel valve was rebuilt as well. New hydraulic filters and oil. Digs fine but the boom will drop while you are moving, and throttle is at max.
Any help would be appreciated.