Piping file to expressjs and res as json

Quick question guys. I think i got the logic down but I am messed up because express doesnt have a pipe method?

anyways, I have this for my express endpoint

app.get('/:report/top', function(req, res) {
    readDataTop(global[req.params.report], function(err, obj) {
    res.header("Cache-Control", "max-age=3600");
    res.json(obj);
    });
});

and currently i am doing this to get data for it.

function readDataTop (x, callback) {
    console.log("Read "+x[6]+" and Sent Cached Top Half");
    jf.readFile( "loadedreports/top"+x[6], 'utf8', callback);
};

(jf just makes writing json.parse and with try/catch blocks easier) anyways I want to instead of just using fs.readfile i want to open a stream because these files are getting huge.

How would I pipe stuff to express but still use json? Something like this?

function readDataTop (x, callback) {
    console.log("Read "+x[6]+" and Sent Cached Top Half");
    var rstream = fs.createReadStream("loadedreports/top"+req.params.report+".json");
        rstream.pipe(callback); 
};

Any ideas? Req is undefined but I thought im passing it through the pipe correctly?