I'll try and make this as brief as possible without missing any relevant details:
A couple of weeks ago I noticed the car (MGTF160 MY2004) was 'dragging' on a short local trip of 2-3miles. Had to stop, found the back discs were red hot. Waited 30-40min, they had free'd up and I limped it home at low speed. Figured I'd throw everything at it since it was on my to-do list anyway - replaced rear discs, shims, pads, calipers, and flexi hoses. Bled it thoroughly, road-tested, and all seemed good. Couple of days later travelled down to Yorkshire (from Glasgow) and back - shout out to Jon and Claire at Rough Luck Racing, new cambelt! - with zero issues. Now the weird bit - couple of days after that trouble-free 500mile round trip, I got the same dragging feeling on a short trip (3-4miles). I jacked up the front and found both fronts were hard to turn by hand and discs were hot. Unfortunately I couldn't verify if the rears were locked up as well, as my trolley jack failed on me, but the rear discs were certainly also hot. Had to park up, get home on the bus, and go retrieve it that evening. Brakes had free'd up by that point but still baby'd it home. Handbrake is not holding on.
Now, I already have new discs, pads, and braided flexi hoses on hand for the fronts (as it was on my to-do list, again), but would you have any other thoughts on this? All four corners - if that is truly the case - makes my mind go to the master cylinder, but without further troubleshooting I don't want to jump to conclusions. The pedal seems to return to it's normal position and doesn't feel harder/softer than normal. I have noticed quite a bit of 'flex' on the master/servo assembly - it moves noticeably as the brake pedal is pressed/released so I have also ordered up the servo bracket stiffener kit from MGMania (link below).
First thing is to replace the front flexis (rears already done) which have been known to collapse internally and hold pressure on the caliper side which slowly releases back to the rest of the system, and fit the additional bracket to the servo. Then, if/when it happens again I plan to verify all four corners are stuck, and then loosen the bleed valves to check if the issue is fluid pressure being maintained (points to system fault, MC etc.) or, alternatively, if the calipers are not still pressurised (points to fault with individual caliper). Any experiences/insight from the community that might help me root cause this intermittent problem?
https://www.ebay.co.uk/itm/303575703745?hash=item46ae85a0c1:g:TqQAAOSwOyJX2bm0