MG TF brakes binding on - all four corners?

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?

Does you car have anti-lock brakes?

Yes, has ABS.

Might be something there then, could be the ABS pump unit shitting itself

I would eliminate the master cylinder first. There is a valve within it that is supposed to allow brake fluid back into the master after you release the brake pedal. That could have failed.
Another more rare effect is the shaft that connects the master cylinder to the servo can get jammed by heat so that the servo is permanently on.. I believe it is possible to to adjust the length of the shaft. Giggity!

Thanks guys. I had also picked up on the shaft adjustment on some discussion threads, nevertheless, I think I'll just go for the full assembly in the first instance - I'm fed up, just want it fixed!!! - plus the front flexis (upgraded because, why not?), nice grooved and drillled 304mm discs, and Brembo pads. Yes, the kitchen sink is getting thrown at it, but it's all good stuff and peace of mind for the future.
[Any theories why I could get 500miles no problem after giving it the 'kitchen sink' approach on the rears, and then it craps out on a 3mile trip? Or do we just file it under 'one of those things'?]
I'll report back here when done - it'll be 2-3weeks before I get back to it (right knee op today to try and save my running career!). Thanks again - I'm fairly new to the TF - always wanted an F/TF and the time was finally right a few months ago. It's great figuring things out, improving the car as you go along, and these forums are a fantastic resource.

David Aiketgate wrote:

I would eliminate the master cylinder first. There is a valve within it that is supposed to allow brake fluid back into the master after you release the brake pedal. That could have failed.

When the brake peddle is released the piston in the master cylinder should move to a position that opens up the brake lines to the fluid resevoir.
This is to both release excess pressure in the fluid due to brake heat, and to allow fluid to replenish the slave cylinders as the pads wear, and pistons move further out.
Fault does sound very much like this is not happening, either through a faulty return spring in the master cylinder, sticking brake peddle not fully returning, or a damaged seal in the master cylinder blocking the fluid path from resevoir to the brake lines.

Update: new discs, pads, and stainless hoses fitted at the front (rears done previously). I have a pre-owned master cylinder and servo on hand so that'll be the next step if this issue rears its' head again.
I also fitted the servo brace bracket kit from MG-Mania - that's a seperate tale for which I'll start a new post. In brief, the bracket as supplied does not fit correctly and had to be adjusted to make it work.

An update on this - I finally got the chance to swop out the master cylinder and servo assembly at the weekend. Early days, but it is so far = so good. If it does prove out I'll be pretty happy I don't have to go down the route of the ABS - apparently you need to program it on the car, which I don't have the software for, therefore would have to transport it to the nearest Xpart garage (~35miles) which is difficult when your car can't be trusted to go more than a couple of miles. Apologies if this is tedious - I just want to close the loop on this thread.
Related subject - I don't see any "How To's" on the ABS in general - best way to purge it, bleed it, program it ............

The www.pscan.uk does the ABS ECU.

(there is a pscan sub forum within the T Bar forum).