Forklift hydraulic connections on the mast.

Error code -51 on a Clark forklift typically indicates a problem with the hydraulic system. This could be caused by a variety of issues, such as a faulty hydraulic pump, a leak in the hydraulic system, or a problem with one of the hydraulic valves.
To troubleshoot this issue, you will need to first check the hydraulic fluid level and make sure it is at the correct level. If the fluid level is low, refill it to the proper level and see if the error code clears.
If the fluid level is fine, you may need to perform further diagnostic tests to identify the root cause of the problem. This could involve checking the hydraulic pump and valves for any visible signs of damage or malfunction, as well as testing the system for leaks.
If you are not comfortable performing these tasks, it may be best to contact a professional mechanic or the manufacturer for assistance. They will be able to diagnose the problem and recommend the appropriate repairs.

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Do you hear the groan of the hydraulic system?

clark forklifts have a safety device on the brake pedal hydraulics that prevents movement when lifting the forks
check the operation of that safety device as it may be jammed in the neutral position

by using mechanical or hydraulic advantage
Forklift truck Wikipedia the free encyclopedia

too much oil will cause froth of the oil and that will expand out of the breather
it will cause excessive heat in the transmission oil

It depends on the make and model of the fork truck, but on average you can tilt the forks between 15-30 degrees both up and down

Leaking valve on return line for tilt cylinder, or dirt.debris is keeping the sequencer valve open. The cylinder should be flushed to dislodge any nore. If the cylinder is showing an external leak, it was overloaded at the fork tips, and a seal or the stack ruptured. The valve and lines should also be flushed.

check the amount of hydrlic fluid in resivor, sounds like you are low. if it is full your bleed back valve is sticking open

The Piston seal on the cylinder may need replacing! Otherwise the valve has an eccessive leak rate. The leak rate is normal due to spool clearances although the cylinder shouldnt go down that soon. You may want to check the lift cylinder. As far as negative effects they are minimal other than danger to surrounding things under the forks.