856 MCV pressure

On this site I have read that some have put extra oil in trans to keep the power steering and brakes working. With the oil level raised then the MVC pump will not draw in air. My question is, were does the vacuum leak occur? The 856 I am working on has no pressure at all. I can remove the pressure sender and leave the hole open and run the tractor at full throttle and not a drop of oil will come out. I have removed the MCV and then when I took the cover off of the filter on the other side of trans I noticed oil then ran out of the port for the MCV sump. The hydraulic filter is old and will be changed. Do both pumps draw through this filter? If the filter is bad, could the larger pump cause a vacuum in the line to the MCV pump? Plastic sump check valve and spring were in place. Thanks,Denny

Both pumps draw oil from the same filter. The hitch pump is located directly opposite the filter and is fed through a short jumper tube between the filter cavity and the pump. This tube is normally below the oil level in the transmission and rarely causes problems. The MCV pump draws oil from near the top of the cavity behind the filter inner support plate. From there it goes up a little to the front of the rear frame. Between the rear frame and the center section is an O-ring with a spacer ring to keep the O-ring from collapsing. This O-ring is above the normal oil level and after a few years it can harden and take a set so that it no longer seals as it should. Adding the additional five gallons of oil raises the oil level above this O-ring.

The other place that can leak air is on the filter end of the oil pickup tube. The inlet end is near the bottom of the rear frame between the axles. The outlet is in the filter cavity above the filter. There is an O-ring to seal between the tube and the rear frame that can also allow air to enter. The extra oil will also insure that this O-ring is covered with oil.

Removing the sending unit is not the best way to bleed the air from a troublesome system since the sender is at the end of the hydraulic circuit. No oil will be available there until the system has developed enough pressure to open the pressure regulator valve. Rather remove the plug below the sender that has a 5/8" hex head. That plug is in a port that is fed directly from the MCV through the 3 gpm orifice. Be aware that when oil starts to come out of this port it will exit at 3 gpm and doesn't take long to make a mess.

Awesome answer, thank you! So if the o ring between the frames goes bad, when you start the tractor does the larger pump right next to the filter start drawing first. And the MCV pump looses prime. I did over fill it some to start with but not five gallons. Thanks,Denny

Yes.The hitch pump is bigger and draws more oil.It tends to starve theMCV pump.Especially if the filter gets restricted at all.Yes,5 gal overfull is the standard 'fix'.Rmark the dipstick at the 5-over line.I've done that on all my tractors.

If one of those two O-rings leak air will enter the passage way until the oil level there matches the level in the transmission.

If the pickup tube seal leaks and air enters the filter cavity it does not normally cause too much trouble since the filter cavity can contain some air due to the cavity extending higher than the top of the pickup tube exit. As long as the filter itself remains completely submerged in the oil no air will enter the pump.

If the O-ring between the rear frame and center section allows air to enter the oil level in the passage on the suction side of the MCV pump will fall to the level of the transmission oil. This results in an air bubble that enters the MCV pump just after the engine starts to crank. The other thing that happens when this O-rings leaks is the the hitch pump will also draw oil back towards the filter as air enters. Since the MCV pump does not pump air very well it doesn't take a lot of air entering the circuit to keep the pump from priming.