How do I replace a shuttle valve in my 1991 auto Mercedes?

My garage has lowered the auto gearbox sump on my 1991 Mercedes 230CE to change the filter and investigate why the car will only travel in reverse. A shuttle valve dropped out which apparently is there to regulate the pressure of the fluid in forward gears. Has anyone had experience of this please and its ease of replacement - the garage says it has to send the car to a specialist which obviously means specialist prices!