Probably too late but here goes.
I can't find on google search a 541 only reference to jaws opening 541mm and models Wm825.
I've had a dual height one since 1970 still going strong. I had a problem similar where the folding clamps/clips did not release completely when depressed, [under the ends of the jaws on the frame] re-adjusted by slightly bending out from frame solved the problem then it started jamming half open that was the pivot screws coming loose. So check all around the unit see it the clamps aren't releasing properly or pivot screws are tight. When they came loose on mine the frame distorted on lifting to work position and wouldn't completely open. I now check mine monthly.
Nobby
February 2009