tallyho!
Following a strict mediterranean diet...
Mine's in a verbindend (actually the Jeffries copy) as well, to get the tongue room. When engaged, they port, it's a very clever engineering fudge that gets over the no port rule!
I'm asking for collection with half halts, but when his head gets near the vertical, he's cutting off his air supply and starts to gurgle. He's half Clydesdale.
Could I please ask how you're achieving collection (and to what degree) with just checks on the rein? I can't seem to achieve any unless I do at the very least a SI or turn on the hocks off the track to get the best lift/stretch over the poll. If I tried to collect her through half-halts I would get over-bending/twisting immediately.
I can't say that my old boy who was also a clyde x could ever collect that way either. Must be a technique thing. However, I suppose in collected work you would want the lift out of the whither, muscles at the top of the neck to really stretch, poll to be the highest point and the throat angle to be open or else it'll be too closed.
I did use a port with him for a very long time though and indeed he went the best ever in a pelham, but for competition reasons I was determined to crack the snaffle thing!