newb here JD400

I wish I had found this forum years ago I have an older JD400 backhoe/loader that Ive had for over 30 yrs. Im always fixing something on it as it had a hard life before I got it and has never really had the chance to recover. Latest problem that has me completely stumped(hence searching the net for a suitable forum) is the front loader spool valves(Cessna). It started many years ago with the bucket dump slowly creeping down under pressure to the point it would roll the bucket and lift the wheels off ground if I left it running. So I took the unit off and had a local shop "rebuild" it. I supplied all the O rings via Deere to John and thought all was well. I guess my money was thrown away because the bucket dump still creeped,if nothing even worse. A leaked then developed in the spool valves,which when operated would squirt out at the operator. I had enough and pulled it apart to redo myself. Since many O rings purchased were in quantities of 10 or more,I had plenty of fresh ones to redo. I found that the spool valves were leaking due to years of harsh conditions and wear at the sealing o rings. My fix was to silver solder the worn area and have it machined to spec.This fixed the spool problem but after a couple hours of leak free operation a new leak developed. The pressed in check valves at top of housing is pouring out fluid. These 2 check valves are retained by an apex thru bolt and have an O ring at top and bottom. I pulled the check valves out and found out the shop that did rebuilt didnt even touch these,as 45 yr old o rings and backing rings were hard and one backing ring was in pieces. I replaced the O rings with fresh ones did what I could to fabricate a new backing ring and put it all back together. I got about 30 minutes of leak free operation and both check valves began to leak again. I have no idea what to do to improve the seal so it wont leak again.One thing that troubled me with the broken backing ring was I had to fabricate a new one thats is split. But the other valve that still had a good one piece backing ring is leaking also,just not as bad. I did fix the bucket dump problem so I must have replaced something that was causing that. Any ideas and thanks in advance.