Here are some pics.
I replaced the block and filled it with a compound to make it more solid. I was hoping that would stop the front brace from sliding back and forth.
This did not solve the problem either. With a new mast receiver, new block and everything tightened the brace will slide during tacks.
The real problem here, IMO, is a bad design. The only thing keeping that brace from sliding is 2 bolts through the brace to the mast receiver. If any of the parts start to wear, which in my case the holes in the brace, can contribute to the slippage. The clamps hold the brace but do not stop it from sliding. I might add some rubber gaskets to help stop it from sliding.
I probably need a new front brace.
Short term, I am going to try to putting some clamps around the brace near the block to keep the brace from sliding.
At first I thought I could solve this problem via stuff underneath.
P1040712 by
PhotoByMark, on Flickr
Mast receiver removed. Note there is a rubber gasket underneath the plate.
P1040708 by
PhotoByMark, on Flickr
The old one did show some wear and potentially was a possible reason for the brace sliding.
P1040698 by
PhotoByMark, on Flickr
P1040694 by
PhotoByMark, on Flickr
P1040696 by
PhotoByMark, on Flickr
P1040690 by
PhotoByMark, on Flickr
Filled the block with a rubbery compound.
P1040718 by
PhotoByMark, on Flickr
P1040726 by
PhotoByMark, on Flickr