600 ford lift

Hey guys, I have a ford 600 with a hiccupping lift. Here is where I'm at. I have replaced all o rings. The lift cylinder oring and backup, lift cylinder to lift cover orings etc. Still corrects or hiccups every 10 to 15 seconds or so. I replaced the check valve seat with a new one and replaced the spring (which was broken)new orings and new ball. Still hiccupped. I forgot to mention I am attaching at least 150 to 200 pound implement for all my tests. Ok the next thing I did was raise lift (loaded) and turn off the tractor with left side inspection plate removed and inspected for leaks with flashlight. I discovered a small weep of oil coming from around the control valve spool. I purchased a used cylinder complete with control valve from a running 600/800 tractor off ebay and installed on mine. Still hiccups. The pump seems to be doing its job and the lift works great ,though it does shutter slightly when I raise the lift. Its just that dadburn hiccupping gosh! About the only thing I haven't touched is the pump relief valve located in the bottom of the sump. Are these available through aftermarket? Anyway I am now turning it over to you guys for your thoughts. I have read some horror stories on hiccupping ford lifts I hope I can trace this down with the help from this forum. This is very depressing. Thanks so much for your time!

As you no doubt know by now, the hiccups are a result of the lift cylinder leaking down until the position control mechanism senses the lower position & then acts to correct it back to its previous position. So, the object is to find where this leak is occurring. In the diagram below, you can see that all in green must be leak free & holding pressure. In addition to the gasket interfaces, other potential leak points are the ram cylinder safety valve, the ram cylinder, the check valve, the control valve at the red circled point. No leaks along that path & no hiccups.

Nice post JMOR, you are right on!

Hey JMOR, thanks for the reply. Ok let me rehash a little. New check valve ball and spring should remedy any leak there. A complete cylinder out or running tractor (the guy I bought from said there were no issues when removed from tractor). I will go back and check for leaks on that cylinder around the control valve. There were no leaks at the cylinder. Now you mentioned as well as outlined in your reply the safety valve could be faulty. It seems strange that the safety valve in the original cylinder and the one I purchased and installed could both have faulty safety valves ,but stranger things have happened. My next question would be do ya just go blindly and replace the safety valve or how would you know if it was leaking? Is that something you can see with the side inspection cover removed? The safety valves are not much money but still. I am a long way from a hydraulics expert. Again thanks in advance and I appreciate your patience in this matter!

the tube marked as cyl exhaust & pump return line also carries the ram cylinder safety valve flow when it relieves (or leaks), but I don't see how you can detect that small leak if present. If you can find something with the same threads as the ram cylinder safety, you might replace the valve temporarily with the plug and see if that stops the hiccups. Won't hurt anything temporarily, as it should only open if you are driving across furrows, at speed, with a 1000 pound disk hanging out back.

Ok thanks! I also now have a confession to make. I
started doubting myself as to the proper installation
sequence of the back pressure valve behind the
check valve. Sooooo during removal of the check
valve seat I broke it. Luckily I still had the old one
and was able to resurface the seat on the lathe
where I work.(I'm off today and tomorrow) There is a
slight chance that I didn't have the seat pressed all
the way in the first time, so I am going to try the lift
again tomorrow.

Those good old ford hiccups! Ok I tried the lift again today after I made sure the back pressure behind the check valve was installed correctly, which it was. As said earlier I resurfaced the original check valve seat and reinstalled it. I then tried the lift again and it still hiccupped only more often now. What a headache! I couldn't find anything so far with 9/16-18 threads (that's #6 sae threads in hydraulic terms, an uncommon thread in the hardware stores) to plug off the safety check cavity so I tried another safety valve from another cylinder. No luck, and no change. I did notice after raising the lift and shutting off the tractor that oil is dripping out of the unloader valve. I think some oil is suppose to bypass the unloader valve while in operation maybe? But not after the engine is shut off, not sure. Anyway I'm lost. according to the I.T. manual there is another relief valve at the bottom of the sump where the oil enters the system. Could that be where the problem lies? I have replacement orings for the unloader valve if need be. LOST!