We have a Michigan 125 loader that we had to rebuild the torque coverter in. There was a bearing that went out and a chunk got under one of the gears and broke the case. Anyway, we got her put back together the other day and now the hydraulics will only work if the motor is wrapped up.
Is this right after start up, have you cleared all the air out of the system? Do you have steering and brakes?
Add to those questions is are there any new noises? Missing any shop towels? I would go over all the hose clamps on the suction side of the pump(s) to make sure one did not get missed!
It does it all the time while trying to operate it. Steering works great, no brakes, but didn't have brakes before either. We were thinking air too, how might a person go about getting the air out.
There should be a bleed screw right at the pump inlet. Look at your pipe for a plug or bleed screw. Purge air and retest.
Check your suction strainers they are in the hydraulic tank.