Deere 410C sucking air into brakes

I have a 410C that has a weird issue that I've not been able to track down the cause of.

Brakes work perfectly fine, no leakdown, etc. Master cylinder was recently rebuilt and fully tested OK. If the machine sits for 2-3 days, AND the temperature changes significantly during that time, the brakes get air inside of them on both sides.

I know they are sucking air into the brake ring area as just a single pump of bleeding fixes the problem. The longer the machine sits, the more air in the brakes.

I see that the master cylinder is supplied by the main hydraulic return line, which dumps back into the axle case. I don't see any vent to the outside world or check valve on the hydraulic return line on the schematic for the hydraulic system or parts book. My guess is that the return line is developing a vacuum when the temperature drops and this is causing a vacuum inside the brake valve and then sucking air past the brake o-rings inside the axle.

Anyone ever seen this behavior or solved such a problem? My next step is to plumb a port into the return line at the master cylinder and verify that a vacuum is being developed, but if that turns out to be the case, I have no idea where to look for a failure. Given the pressure and flow rate of the return line, I don't see how it could develop any significant blockage.

I would think that if the brake o-rings are leaking, it would be readily apparent during a 5 minute leakdown test of standing on the pedal, but that tests perfect.

HELLO Ferdinand I have a 1989 JD 410 C--sorry that I can't help with your question. Never had that problem. I have a question for you if you don't mind. I had to replace the big front mounted Hyd. pump on mine---not cheap--If you are sitting in the seat then I am looking at the right side of the pump. You have to look at the pump with a light. Closer to the front of the pump on that right side there is a small hyd. hose coming out of the pump. Further back on the pump--towards the seat there is another--fitting or plug on the pump--does your pump have a plug there or something else??? It sure would be much appreciated if you could do that for me. len at clare, michigan

On the right hand side of my pump all I have is the large outlet hose, which is a
code 61 fitting, and below that there is a much smaller hose. Both run back to the transmission area. Small hose runs to the reverser according to the hydraulic schematic. Are you talking about the main pump, or the charge pump that sits in front of it?