Is there a way to modify the safety valve. I put a new valve in and the blade sinks after 4 min. It has 80 /90 w . I was thinking about putting a heavier spring on. The pump is clean and works very well. I spent a lot of time cleaning- scrubbed it with toothbrush. I put in a new relief valve . But I think it came from the land of dam near got her right.

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Never a good idea to modify a relief/safety valve with out all the proper tools, specs and knowledge to change pressures. Not saying you don't posses all these, just that it can be dangerous. This should only be done on adjustable valves, which the 8N's hydraulic pump valve is not. Notice the crimp / stake or even solder holding the valve at it's correct pressure. Non-adjustable relief valves are typically set at the factory for the correct pressure for the application they're being used in.

There can be huge consequences of "winging" something with pressures like this, for safety reasons and mechanical damage.

Do a pressure test on your pump / valve. See if it releases at the required spec. If so, your leak down problem is likely elsewhere. If you can't build enough pressure to kick the relief valve open at spec, you have other problems. Only if the relief valve is found to be not working correctly should it be replaced. This is usually the valve being stuck open. Look for excessive turbulence at the valve. Don't confuse the PTO shaft turning with this. If by chance there was debris in the valve causing a leak, kicking the valve open with a pressure test would probably clear it.

Without knowing more about your capabilities, that's my recommendation.

Won't matter if cylinder and piston are worn and need replacing. I concur with Denny - the likely problem. Did you perform the Leak Down Test? Check oil -murky brown or whitish? Yes? Oil is contaminated with water. Can be from one or two things. Water sat in sump and then froze. When attempt was made to activate, pump may have cracked. Remove Inspection Covers. Start engine, run at idle, and engage pump. See bubbles at base of pump? Tat is the big clue. Leak Test: Attach a load like a rear blade or plow. Start engine, engage hydraulics, raise 3-PT lift to full UP quadrant, leave it UP, shut down engine. Using your Min-Mag flashlight, get on your knees and look up thru inspection covers to the hydraulic cylinder bolted on the bottom of the top cover under the seat. Watch closely and observe if oil is seeping/leaking down out the cylinder and piston. Yes? Cylinder is worn and possibly piston too and both need to replaced with NEW. Now is the time to rebuild the pump too. Clean and inspect all parts. The OEM N piston used three steel rings like your engine piston. When the NAA was issued, the hydraulic system used a new piston with a rubber O-Ring and a leather backup washer/wiper. This was a better piston and will also work on any N-Series. I highly recommend you don't think you can just hone out the cylinder and it'll work. It won't. Once grooves get worn in the cylinder wall, it will never seal. The job is not difficult, just time consuming to do it right and you don't want to have to pull the top cover a second time do you? Be aware that the 8N cylinder has a cast scallop at the 7 O'Clock position for clearance of the control rod. Many new aftermarket cylinders don't have this and you'll have to machine a scallop out yourself.

Tim Daley(MI)

It has new O ring and leather . The lift works so good with heavy snow it will bring the front of the tractor off the ground. Blade weighs close to 350 . And I have 80 lbs barbells on front.

Quoting Removed, click Modern View to seeOn such an old tractor,you should be proud of 4 minutes. What makes you think the safety valve is leaking?

In all probability, the seal in the lift piston needs replacing. If you remove the RH dipstick cover and raise the lift, shut off the pto and look inside for oil coming from the top cover. If so, the seals are leaking. There are new style seals and piston available from this site.