Horse riding exercise machine?

There is no make or model on it. It still works with no weight on, but just pressure from the hand is enough to stop it moving. It looks like this:
http://www.proactivefitness.co.uk/product.php?productid=16510 but was not purchased from this company.

It must be something to do with a weight bearing mechanism but I can see nothing obvious when taking it apart. I love this machine and want to fix it if I can. Please, any ideas? help!