Troubleshooting Forklift Hydraulics

An old forklift seems to have a little internal bleeding. What's the remedy? January 31, 2012

I've got this old offroad forklift and when I get a heavy load on the front, the cylinders that control the mast tip slowly leak down. Would one assume the cylinders need rebuilding or possibly the hydraulic valve itself? No oil leaks out anywhere. The other thing is that even when empty, trying to tip the mast back dogs the engine terribly, like it's fighting a heavy load, but everything looks factory. Acts like the hoses on one cylinder might be reversed. Does this tell us anything about either the hydraulic spool or the cylinder?

Also took this to show the big ash log I cut today, but now it doesn't look so big. 33" diameter small end (this picture is looking at the small end). Quarter sawed it into 8/4 stock. Have had some luck selling it that way. Found a nice bolt smack dead in the middle. Must have put it into the tree when it was 3" in diameter. Hard to make that cut through the middle when I've only got 28" between the guides. Bunch of fiddling to get it slid over the dogs enough.

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Forum Responses
(Sawing and Drying Forum)
From contributor A:
It sounds to me that the spool valve might have something stuck in it. I had both the tilt cylinders for my forklift with a stroke of 4.25 lengthened out to 8 inches. If memory serves me correct it was about $450.00 per cylinder. Well worth the money. I now have 30 of travel at the tips of my 60 forks. Works great for digging out logs I wish to cut from a pile. I also had a hydraulic leak and took the spool valve to my favorite shop and it turned out to be a 50 cent O ring. It was $100.00 to check out the spool valve and test it.

I have had all the cylinders rebuilt on my forklift and when I asked about bleed down of the main mast cylinder I was told the spool valve would have to be replaced because after 50 years and dirty fluid it slowly wears out the valve body assembly. So always clean hydraulic filters once a year, and you should replace the fluid at that time. I never do that, but when I rebuilt the hydraulic system 4 years ago, I scrubbed out the inside of the tank and flushed out all the lines and pump. It is almost never left outside to keep the moisture out.

From contributor S:
The easy way to fix it would be to give that machine to me. Most hydraulic systems do leak down a little even when fairly new. If it leaks down fast, it needs new O rings somewhere, cylinder or valve.

From contributor G:
I worked on lifts for 20+ years. Simple way to check is to tilt cyls to max tilt forward so mast will not move on you, then find spool valve. Just follow control handles. Each spool section will have two ports. Trace hosing from one port and it will split somewhere, but both will go to either top or bottom ports on cyls. Even new lifts have a drift factor set by manufacturer specs. These are usually x number of inches in a set time. There are other ways to check, but if not done correctly they are very dangerous. If hoses are right, the possibility of seal damage exists. If they are right, I will get with you on how to do other checks.

From the original questioner:
I'll check the hose setup, but I assume they are right or they wouldn't lift it at all. You would think the cylinder seals would be the next suspect, or seals in the spools?

From contributor P:
Cylinder piston O rings would be my guess. The same dirty old gritty hydraulic fluid does a number on those O rings, although I have changed some of those that leaked but I couldn't see anything visibly wrong with them.

From contributor G:
Seals may very well be the cause, but if the pivot pins or bearings are frozen up, that will cause problems. I always try to find the problem before I try to fix it. Any system I have seen has had a filter somewhere, either in tank or in line. Next question is if it is one cyl or both. There are ways to check. Some are safe, some are not. Hyd fluid at 2800 to 3200 PSI is dangerous if not deadly.