Dirk,
I appreciate you efforts to fix your lift. Let's assume that you are loosing TRANSMISSION oil. That could explain your leak and why you can climb a hill half way. The transmission dip stick is between your drive tires. You will actually have to bend it to get it pulled out easy. Insert a small funnel in the tube and add Dextron until it is to the fullmark NOT running. This should cover your transmission.
On the CY40B (big tires) the master cylinder is below the starter. Probably the same here. You will have a heck of a tome getting the cap off. Chisel it off if you need to- you need to get the system bled to determine if the wheels lock up. Use a 1/4" six point wrench if you have one to crack both wheel bleeders at the diff housing. Use brakefluid in a new oilsquirt can with 8" of vacuum hose attached and hook it to the left bleeder. Pump until you get clean fluid out of the right bleeder. Close the right bleeder and loosen the inching bleeder (if still equipped) on the top of the transmission. Pump again, then close. Now pump one last time while watching the master cylinder. It should start to fill. Close the bleeder and you may have brakes.
I am pretty sure your lifting problem is an adjustment. Loosen the 2 Allen head bolts that at the bottom of your hydraulic bracket on the lower portion of your steering column slightly. Gently tap them upward about a 1/4" and retighten. This should get her lifting just great.
Steve

yea i agree with duo
this machine may be old but since sooo many were built and are still in service to this day, most of the parts should be readily available still

theyre still money makers (mostly for clarks parts sales though lol)
back then, they used a lot of common parts so aftermarket should have good parts coverage too

I disagree with Submariner. This truck was built when Clark was king and had about a 40% market share. I am quite confident that you can get any part that you need for this truck although in some instances you may have to settle for a used one.
This trucks is very basic and any backyard mechanic can probably fix it.

The lift problem is probably the age of the machine. You have an alternate path through the control/spool valves and or the tilt cylinders. The previous advice about hiring an experienced professional is solid advice. Understand Clark has never been a stable company and of sound engineering practices. By that I mean they utilized very strange technology; whether converting a diesel engine to propane or using wet brakes inside the transmission. Clark has been sold, gone out of business, etc. many ti mes. You will need a very good mechanic for this unit. The average forklift mechanic will have his lunch eaten by an odd Clark design. You need to evaluate the mechanic you choose carefully and understand it may cost a lot to repair. Also you may not be able to get parts.

thats just an old machine "marking its spot"
you should be checking all the fluids as part of your normal operation

but you do have an issue
the mast should raise regardless of the tilt angle
thats very wrong id get that looked at and see if the guy can spot the leaks while hes there, may just be loose fittings or clamps
or
every seal and gasket in the machine is leaking lol

The only reason I thought the fluid might be low was that it leaves puddles whenever it sits for awhile

if the fluid was low, as you raised the mast, it would start to shake and shutter as air is being pumped into the cylinder
if you dont see that, its a good bet the hyd fluid level is ok

as far as not lifting unless its tilted fully forward or backwards
it could be something is getting pinched as the mast moves
or maybe the opening of the relief valve as you deadhead the tilt has something to do with it in the valve

either way DO NOT ATTEMPT TO FIX THIS YOURSELF
is see youre a short time clark "user"
working on and testing the mast is very dangerous and should only be done by trained and experienced techs

please call in a reputable repair shop to diagnose this issue
i can make it but the 3 hour travel fee (eachway + tolls) might not be worth it lol

about 35$ in tolls roundtrip

o whoops lol
i read the brake/ incline post and must have had that on the brain

sorry bout that

tilt is a double acting cylinder in almost all forklifts these days, and as such, the same amount of oil is returned to the reservoir tanks as is taken out, only the lift cylinder is single acting, taking oil from the reservoir to fill the lift cylinder.

The forks won't raise unless the mast is tilted all the way back or all the way forward. If the mast is straight the forks don't raise, does that mean it's low on fluid?

I think Justin was referring to the brake fluid-DOT3. The hydraulic fluid is AW32. The tank has enough hydraulic fluid in it when the mast lifts all the way up and tilts back and forth without cavitating. There is no dipstick because all the tanks are the same capacity and size. It takes alot more fluid to raise the forks 20' than it does to raise them 10'. Make sure the forks are on the ground when you add fluid.

Albany. Thanks for the info. One more question, I don't see a way to check how much to put in. There is no dipstick' etc.

it should be DOT3
where in NY are you im on LI
email me (profile) if youre in the area and need help

What type of hydralic fluid should be put in and how much?

Thanks Again