MF-135 hydraulic pressure control system

[Bart CA]

My MF-135 with pressure control....
Hydraulic arms drop in 1 minute when engine is turned off. There is a light jerking when engine is running. If I place the diverter valve (on top cover near the seat) to aux position the arms take one hour to drop...then there is no problem with the cylinder.

The stand pipe has a "T"...I changed the top end and the lower end "O" rings with teflon spacers on the stand pipe. The side tube from the "T" goes (I think) to the pressure control valve. What type of seal or "O" ring is used on the tube that connect to the pressure control valve I suspect that there is a light leak from there.

Thanks in advance

Bart

 

[DavidP, South Wales]

Hi Bart,
You have completed the first stage of investigation by being able to isolate the cylinder. In general the Pressure Control system gives very few problems. The components of the PC system are shown in the photo. The square aluminium unit houses the damper. Diaphragm failure here is unlikely to cause such a rapid drop. A possibility is the small connecting pipe between the damper and PC valve body...loose or broken...rare! The valve body itself is unlikely to fail unless something has happened in the linkage and the spring pressure has been greatly reduced. There is a small O ring between the stand pipe block and swan-neck pipe and another where the swan-neck pipe is attached to the PC body. It is not uncommon for the pipe nut here to work loose and possibly have the sealing ring broken.
Draining the oil to the level of the bottom side cover bolt and observing what is happening as the linkage falls may reveal the source. There is little that can be done with the top cover in place but you need to find the source of the leak first if you can.
DavidP, South Wales

 

[Masseynut(Australia)]

If you are unlikely to ever use the pressure control unit for it's original purpose, or as a variable relief valve, it is possible to remove these parts that David has outlined.
If you procure a straight thru, standard stack-pipe, less the "t" junction, and fit a 2950 P.S.I. relief valve into the back of the hydraulic pump, the hydraulic position control lever can be used as per normal. The only difference is that you will not have the ability to vary the relief pressure of the pump, in the front portion of the quadrant. But leave the response control module in place, (the part w/ rubber band fitted).
By your description, I would be looking towards a sticking exhaust valve or worn control valve, with-in the hydraulic pump. Both can leak excessively under load conditions, if worn.

My two Bobs worth,
Evan.