Jlg 40f boom leaking down

So my 40f has been sitting for about 9 months ,outside covered, a friend was using it to replace siding on his house . Ran it off and on for a few days ,now the main boom is leaking down ,rather quickly you can see it drop in a matter of a minute or so.
Not sure if it is the check valve on the main ram or the ram it's self. No visible leaks on either ram main or the little one on the back of the boom.
Not sure what to do

Normally on those it was the check valve that caused problems. Before you panic, check and make sure that all the manual decent valves are tightly closed.

Thanks for the response
All valves are closed ,they have never been opened.
I thought it might be the valve ,iam thinking of replacing or if not to difficult repair the valve, just never done it
Thanks again

I've learned to start with the simplest things first. Next step would be to obtain a lift cylinder holding valve. JLG #4640265 Counter Balance Valve.
Should be available still

OFF
Have any good ideas wheee to get a service or parts manual online
Any ideas would be great
Also any ideas on rebuilding the locking brakes and hub drives
Thanks

Yes, I have them. PM me with an email address and I will send them to you in PDF.

The brakes were always a pain.......