Handbrake ratchet has suspect engagement

Anyone else have this problem? 87 3.2 Mondial, when applying the handbrake with more than just minimal holding force it tends to "jump" back off. Can be alarming when parking down a slope!! (I try to always put it in gear also, but I don't like relying on compression alone for any period).

I thought the ratchet teeth must be worn so took the "lever" unit out of the cabin but everything looked pretty good. The only thing I noticed was that both the ratchet movable "pawl" and the fixed "sprocket" teeth have a symmetrical rather than "saw-toothed" profile, if this makes any sense. It looks like under any reasonable force the pawl will just wedge itself back out again! I tried filing the pawl to a more vertical profile on one side (ie make it sawtooth shape), but it hasn't helped. I also slackened the cable so that I would be engaging "unworn" teeth further up the travel but still same problem.

Anyone else have a dicky handbrake?