No power in loader bucket - works, but won't work under load

I have a '67 Farmall 706 that was rebuilt a couple years ago. It has worked great, but leaks hydraulic oil. I was using the loader with bucket forks to lift a box blind and platform today. I was able to raise the platform, drive it into the field, and raise it about 9 feet up. I then lost the ability to tilt the bucket up. I was lowering the bucket when it suddenly dropped. I was then unable to raise the bucket or tilt the bucket with the blind/platform on it. I shut it off and checked the hydraulic fluid. It was just below the add level. I added a gallon and a half or so of fluid and was able to raise and lower the bucket as well as tilt up and down (without the blind on the bucket forks - i.e. without load). I tried to lift the platform and blind and was unable to lift or tilt the blind.

The tractor has been leaking hydraulic fluid out of the rear directly under the seat as well as from a square housing ( assume pump) that is on the left side of the tractor below the sediment bowl and that a rod goes to the attaches to the clutch.

I know enough to be dangerous, but am far from being an exert. I don't know the exact terminology, but can get pictures if needed.

I don't know if I have air in the system or if I have a bad pump somewhere. The steering works fine, I was able to drive it out of the field and back to my shop. The PTO worked fine the last time I used my rotary mower two weeks ago. I know I need to get the seals or o-rings looked at since they leak. Can they be rebuilt or what do I need to look at?

Thanks for your help.

Sounds like you might have more load than it can lift/tilt on the bucket cylinders. Yes it can happen. I have had our loader bucket to far over and would not pull the load back up.
For the pump you need to look at the pentagon looking casting behine the left step. Ther is a priming plug on a raised looking rib I would suppose you could do a pressure test there also. Though it would be easier to just plug a coupler into the outlet in back with a gage to check for pressure. Could also be the filter needs changed if not done recently. could be the screen at the filter is plugged up from trash.

If you have an external control valve, that sounds like it went bad.

Thanks for the quick replies. I've lifted much heavier things, so I figured it wasn't at the limit. It wouldn't lift the blind off the ground after having carried it into the field, so there is something wrong. I have not changed the hydraulic filter, so I'll start there.

What does the external relief valve look like?

Can it be replaced, or must I order the entire unit?

Well not being there to have a good look at it myself here we are going to GUESS what is leaking here. More then likely on your remotes it is a latching valve but here again it is a guess , now the leak under the sediment bowl where there is a rod i am guessing YOUR TALKING THE t/a SPOOL VALVE IS LEAKING . pICTURES / vIEDO OF AFFECTED AREA'S HELP ME A BUNCH , UNLESS YOUR FIFTY MILES OR LESS FROM ME WHERE I CAN DO A SERVICE CALL . If it is the T/A selector valve leaking then that is just a couple o/rings , not a real big deal but can be a pain in the drain at times to fix . If it is the unlatching valves if you do not have a fair amount of mechanical skills they can be a problem and that repair calls for a couple o/rings and the unlatching valve . Now also when usen a 706 on up thru 1586 for loader operation one should dump at least and extra five gallon of hyd oil in over full as the loader cylinders will take more oil to lift then returns from the down pressure side then you add in dual cylinders to the bucket that takes more oil away from the sump and now your rear end is low and you point the nose of the tractor down hill and what is left runs forward and now your sucking air .

If you are asking about the ...external control valve...not relief valve...mentioned by David G he is
referring to an external control valve. That means your loader would have a valve that takes
hydraulic pressure from the tractor to operate the loader. So the separate valve which has its own
control levers is plumbed into the tractor hydraulics either with a permanent connection or through
one of the remote outlets. What they are suggesting is if yours is set up like this those types of
valves may have there own relief valve and this could have a problem causing the type of problem
you have.

Thanks for all the replies fellas, I appreciate the info! Going off memory (the tractor is in my pole building on my farm 25 miles away from home), there are 4 hoses that run off the right side of the loader (2 pistons for the lift on the main loader arms and 2 for the tilt on the bucket). The 2 lines that work the loader arms go to Pioneer fittings attached to hydraulic lines (permanently affixed metal lines, not hoses) just in front of the brake pedals and floor. Those lines run under the floor, up the pedestal the seat is attached to, and attach under the seat. The other 2 hoses for tilt run over the floor and attach to Pioneer fittings behind the tractor. Those fittings are attached to short sections of hose that attach to the tractor behind and under the seat. I have not removed any panels to see what that pump or whatever under the seat looks like.

Also, I had no idea I needed to run more hydraulic oil in the tractor due to the loader, but that makes perfect sense! I appreciate that info very much.

PROBLEM FIXED! I replaced the relief valve and everything works great again. I appreciate all your help and suggestions! I love this
forum!