580 SE brakes fade at idle

My 580 SE brakes work well with RPMs up, but at idle the brakes fade away. Does anyone have a solution? This machine has two master cylinders, but no equalizer valve. The wheel cylinders do not leak. I have been told it would cost 5000 to go thru the system at the dealers. OUCH! I hope it is something I can do myself. If I remove the hydraulic pump can it be tested reliably? Someone mentioned the hydraulic vent may be plugged. Could that be the problem? I am a fair operator, but a p poor mechanic. Tom

580 SE brakes fade at idle