John Deere 410 Backhoe hydraulics bleed down

I have an early '70's JD 410 backhoe I purchased almost 5 years ago. I use it around the farm and never in a hurry.
I didn't know what to look for before purchase. So the front bucket being really, really slow after warm up went unnoticed. As did the excessive bleed down and lack of power in the rest of the hydraulic systems.
I tested the pressures in the system a couple of years ago. 1500 lbs. cold, 1250 warm. System is supposed to be at 2300-2400 lbs. I don't have a flow meter, but was told the pump was replaced about 5 years before I purchased the machine. He gave me a box containing the old pump.
Since every system was bleeding down and I had a ton of oil coming out of the return line with all the valves in neutral, I guessed it was in the valves. I purchased rebuild kits from John Deere.
Today I pulled the valves. Looking at the spool through the port, I see that "97" has been inscribed on all the spools.
Questions: Does that mean that someone rebuilt the valves in 1997? I wouldn't think they'd need it now. If that is the case, I'd just replace the "O" rings between the valves and put them back.
If it isn't the valves, does that indicate I have to rebuild all the cylinders? Or could there be some other problem to look at?
Thanks,
Brian

I'd put the valves back in and then cap the pressure line at the front pump. Take a pressure reading then. If its got good pressure with the pump output capped, then you have a problem downstream. If its still low with the output port capped, then you have a pump problem.

I capped off the lines going to the backhoe valve block. Pressure at the pump jumped up to 2400 lbs. So I think my problem is in the valve block.
Assuming all the valves aren't bad, how would I go about isolating the valve or valves that are leaking internally?
Thanks,
B.

Anyone know what the cost of a used control valve or the entire block would be?
Does someone have one in their pocket?
Thanks,
Brian

Just my two cents.

If you go to a used set of valves you would have to take them apart, clean and new rubber in them anyways. No way would you want to go through replacing them all and find that the used set is dirty, or has worn rubber in them. Pretty much the same work anyways.

If you have the manual, you should be able to see where to block each valve off. Might be easier then pulling them all.

If no manual, let me know an email and I can send.