Hi, i have a 721 and installed new seals at the lift and tilt cylinder but they go down rapidly when there is load on the fork (1ft per 15 sec). Does anyone know what material i need to renew the valves? or could it be anything else than the valves? Best Regards Matthias
721 Hydraulic sinks rapidly instead of hold
- Thread starter MS1080
- Start date
I did. i bought a complete seal kit and replaced all sealings on all 4 cylinders. But the problem is still there...
Are you talking about the load checks in your control valve?
I have a 731 that had bad seals making the arms drop pretty fast. They will be behind what look like plugs in the control block.
I think it only can be the control box with its Ring sealings, the cylinders are sealed new, the hoses are ok
SkidRoe
Well-known member
- Joined
- Dec 10, 2009
- Messages
- 1,887
I think Tazza's right, if it is not the cylinders, it is likely the load checks. If the spool seals were bad, you would have oil spewing out of the control. The only other thing it could be is an internal crack, but that is highly unlikely. HTH - SR
Skidroe sorry i didnt get it, what do you mean with the load check? is it the controlblock where the pedals are connected?
SkidRoe
Well-known member
- Joined
- Dec 10, 2009
- Messages
- 1,887
On the control valve for the boom/bucket (connected to the foot pedals), you will see some large, typically hex-headed plugs just above each spool, sometimes between the ports that the lines connect to. Behind these plugs are the load holding check valves. These are what are supposed to block any flow in and out of the cylinder while the valve is in the center position.
There is typically a spring and a check-ball behind the plug. Most likely the ball is not sealing in its seat properly, causing your cylinders to drift.
HTH - SR
Skidroe is right, some even had sort of a relief valve style load check that had Orings and backup rings. My 732 had this setup and the Orings were shredded. Replaced them and it was great.
From what i heard, the load checks also prevent the arms from dropping as you lift them when under load, it makes the operation smoother.
Thanks for your help, i did a test at the weekend if the cylinder does this problem on both ways, up and down. when i lift the bobcat with the bucket that the front wheels are 1 feet off the groud, i measuered it the day after its still the same. there it will hold the pressure in the cylinder. But when i lift something it its a mess, i cant drive 5 meters until i have to lift again and again...
Anoother test i did was to run the machine lift the arms, shut the engine off. Watch the arms lower on their own then hit the pedal to lift, see if the arms stop moving, if they do, text book load check issue.
