GetReactionForce().y does not show 0 even it has no weight

Question

I have built a simple structure with static and dynamic bodies and revolute joints. None of the bodies have weight, I've set the density to 0. Here's the structure:

In screen update method I do this:

b2Vec2 vec1=_revJoint1->GetReactionForce();
b2Vec2 vec4=_revJoint4->GetReactionForce();
NSLog("Joint1 y: %f , Joint4 y: %f",vec1.y,vec4.y);

I would expect the y for both revJoints to be 0, but to my surprise, it's a value like -0.263565, not 0.0000

What is more interesting is that if I drop a body with a certain weight onto this structure, both x and y of the reaction force increases, but they do not decrease back to their initial value even if I remove that body off the structure. Is this a known bug in Box2D?