The boom on my ht4 traxcavator leaks down and when I try to slowly control the boom up it drops, it also leaks out of the valve bank. Does anyone have a blown up image of the pump/valve bank? I believe the pump is the same as the no.44 hydraulic unit one, but not sure.
Thanks
Josh

Hi Josh,
first thought would be that a load check valve was faulty, BUT, these do not have them, so next thought is that there maybe something amiss with the Make Up Valve, No 15, situated in the main Relief Valve assembly.

This valve is not shown, but is part of the schematic as No 7, but its action is described--it essentially allows oil to Reservoir oil to flow to the cylinders when negative pressure is active in the circuits and the pump cannot make up the required flow via the spool control valves--hope I described it correctly.
Scan shows the circuit and a description is part of this scan for the Make Up Valve.
the Make Up Valve is shown in the picture style scan and is marked No 7 (No 15 in description below)

Also I doubt it but check the check balls, No 6, for the Surge Tank oil system--NOTE the Note on the left page-- as described it was later upgraded to a larger surge tank to negate the use of these checks.
On the opposite right hand page is a picture of the Makeup Valve No15.

Taken from The Servicemen's Reference Book for Hydraulic Controls and Equipment, Form No :- 91294-1

Rushed here, need to be elsewhere as finding and scanning take time--hope you can follow this disjointed waffle.
Cheers,
Eddie B.