Still a reminder to double check the Hydraulic oil level in the rear end, sight gauges on the left side of the rear housing.

If so, I would test the circuit test pressure.  At the rear of the tractor on the right side....behind the right final drive planetary is the PTO valve, it sits at about a 45 degree angle to the ground.  On the outboard side should be a Tee with the rear capped off.  This is a test port.  Shut the tractor off, and install the gauge.  Start the tractor, it should read at idle or wide open throttle between 280-350 psi. with no other hydraulic activity in the system, like moving remote levers or raising the 3-point. These tractors are known to have issues with the PTO level dropping out when pressing the differential lock, but not so much with the brakes.  If that passes muster, then I would check the PTO circuit for pressure.  On the back of the tractor directly above the PTO, there is a round cover that is partially covered by the PTO shield.  There are two plugs on top of this cover.  The LEFT hand one is the PTO clutch pressure, which also gives a reading of the PTO clutch circuit, when the PTO is engaged.  This should be done with the implement PTO unhooked for safety.  When the PTO is engaged, it should read nearly identical to what you read on the other test port.  While this is engaged, try depressing the brakes and see what happens, there will likely be a minor drop in pressure, but not major, depressing the differential lock would create a larger drop in pressure.  See what happens an report back.

Remember, while this is annoying, this is somewhat normal on these tractors, but mostly only when depressing the diff. lock, so yours could have a leak somewhere, or low standby pressure.