Thank you everyone for your help.

I don't actually own a Libelle (got an ASW15, no problem with weak
links but I have to have the insides of the spars looked at with a
borescope every year!)I'm just the person in the club who makes and
maintains our towropes and links so I was asked about this by one of
our members who does have a Libelle. I thought his releases were
overhauled just this winter. Now I'm not sure so I'll check with him.
I didn't think it was the release mechanism itself which is a problem
because the immediately observable problem was the binding on the
fibreglass, not in the actual hook. Now I'm going to check on this
more carefully.

We did have some of the older style when I first took on the job of
making the weak links but we seem to be very adept at losing several
weak links a year. The rare older style being the first to be lost
naturally.

I WON'T use chain myself (I'll never make a weak link with anything
but the proper Tost or Schweizer part for the reasons enumerated in
your messages) and all of our club equipment is made with the correct
parts but I have seen gliders towed by a weak link which was two links
of hardware store chain on the end of a rope. All the links I make for
the club are made with the proper Tost or Schweizer rings (bought them
all from Wings and Wheels and have to buy another batch so I'll be
calling soon Tim) depending upon which glider they're intended for.

I'll try to see what I can do about tracking down the service
bulletin about enlarging the aperature to cure the problem.

Thanks guys, I was at a dead end there and you've given me some more
leads to a possible solution.