The parking brake issue seems to be a problem for the enterprise, I wonder if the control panel on the 201 is the common issue here rather than the coaches themselves?
The 201 doesn't really have to interact much with the MK3 coaches, it just pulls them and applies air brakes. It's not very complex
The relationship with the CDE or Enterprise sets is a bit more complex in so far as they've parking brakes and other systems on board.
While the failures on the MK3 weren't generally bad enough to stop the train I've seen plenty:
1) Air con / heating - very very regularly fails or is set up incorrectly so that you've a very uncomfortable journey.
2) Internal doors, invariably broken down or locked out making the a/c problems worse + adding a lot of noise.
3) I had a light fitting fall on my head in the doorway area of a MK3 recently. Not heavy enough to do any damage though, just a plastic light shade type thing.
4) The lights all went out when we left platform 11 at heuston as the train took a sharp turn. Seemed like a loose connector between coaches.
5) Intercom systems rarely work properly
6) I've seen doors stick and have to be reclosed quite a few times. Apparently the MK3 doors were highly problematic when they were first introduced. They've calmed down though since.
7) I have been on a MK3 where the suspension seemed dodgy. Ended up with a very bumpy ride like as if a damper had failed or something.
8) I wittnessed a member of staff being hit in the head by a falling door in the dining car one day. It looked extremely painful. A hinged door that closes up the bar area seemed to fall open on his head. (Poor design / poor modification of the original design.. either way it was dangerous)
Just to add a bit of balance to the argument
The MK3s ain't perfect either !