3pt doesn't stay raised after engine is shut off

[J_J]

Are you really saying that bypass fluid from a worn piston seal can not leak into the crankcase?

I sure thought that must all 3pt internal cyl's backsides were open to the sump.

If a piston seal is perfect, the only way fluid can get back to the sump is through the descent valve, or a bad valve spool, and yes, it will block the fluid if you want for transport.

It is also used as a variable descent device.

If you close off the descent valve, you close off the 3pt valve as being the problem.

 

[vtsnowedin]

Are we considering that there is a relief valve in there that may have failed plus a number of small O rings that if failed let oil drain back to the sump?

 

[ovrszd]

You are right it would be the valve. If the cylinder is bypassing and the valve is good their is no place for the oil to go so it wont move.

Bingo. Took me a long time to understand that simple fact.

 

[Leejohn]

I'll just say one thing, there is no oil on the rod side of the piston.

 

[ovrszd]

Are you really saying that bypass fluid from a worn piston seal can not leak into the crankcase?

I sure thought that must all 3pt internal cyl's backsides were open to the sump.

If a piston seal is perfect, the only way fluid can get back to the sump is through the descent valve, or a bad valve spool, and yes, it will block the fluid if you want for transport.

It is also used as a variable descent device.

If you close off the descent valve, you close off the 3pt valve as being the problem.

I want to stand corrected here. In my earlier post I was thinking a 2-way cylinder. 3pt systems use a 1-way cylinder. The rod end of the barrel dumps to atmosphere. In that regard it is truly possible for the cylinder to leak down caused by cylinder piston bypass. In a 2-way cylinder situation that is not possible if the valve holds.

Good catch JJ

 

[ovrszd]

I'll just say one thing, there is no oil on the rod side of the piston.

In a 3pt system or 1-way cylinder system, yes that is correct. As stated above, I was thinking of a 2-way system.

 

[J_J]

There are some tractors with external hyd 3pt lift cyl, but I don't know if they are SA or DA.

Anybody know?

 

[flusher]

There are some tractors with external hyd 3pt lift cyl, but I don't know if they are SA or DA.

Anybody know?

My 1951 Minneapolis Moline BF tractor has a DA hyd cylinder on the 3pt. Also my 1951 Farmall Super A which has been retrofitted with a 3pt that has a DA hyd cylinder.

 

[ovrszd]

My M9540 is external but they are SA. They have a soft hose on a fitting at the rod end to let them breathe clean air from the rear axle housing.

 

[J_J]

So, if your cyl seals ever leak, where do you think the fluid will go?