How to check Pressure Reducing Valve

Hi All, I have just done about a thousand miles towing a two ton trailer. I decided to check the brake pads afterwards and found the front pads fine and discs shiney but the rear discs were all rusty as if they had not been used. I jacked up the rear of the disco, got the other half to put her foot on the brake and tried to turn the rear wheels by hand but couldn't.
Could this be caused by the Brake Pressure Reducing Valve and how can I check it ?

Cheers Pete

sounds more like stuck caliper either buy a new one or remove free up and give a good clean

Thanks for the reply, but I have already checked the pistons and they are all free and move easily. When jacked up the wheels can be turned by hand, when the footbrake is applied they cannot be turned by hand.
Can the pressure valve be checked, does it require the vehicle to be moving at a certain speed etc.
Thanks