I Have A Mercedes Motorhome 307d . Brakes Failed. Replaced Master Cylinder, They Worked For A While. Brakes Failed Again. Pumped The Brakes. Replaced Load Dispensing Valve. When I Put Pressure On The Foot Peddle It Gradually Moves Down To The Floor Under Pressure Could It Be The Servo Or Something Else?

Firstly, check that you aren't losing fluid from the braking system. Look at the bottom of the backplates on the rear axle.. if there is any sign of dampness this would suggest you have a lealing wheel cylinder. Otherwise, you are describing the classic symptoms of a failed brake master cylinder. It sounds as if the fluid is leaking past the seals. It might be worth checking where the rod from the brake pedal goes into the rubber seal on the master cylinder.. again look for signs of brake fluid.
The only other thing that might give a similar symptom is a badly failed front wheel bearing that has to be 'straightened up' by the front brake pads before the brakes are effective. When driving normally, the wheel leans on the pads and pushes the fluid out of the caliper causing the need for a pump on the brakes to fill it all up again. It might be worth jacking up the front wheels and giving them a waggle holding top and bottom of the wheel.