Brake issues

I'm still having trouble with my brakes after replacing the calipers I have been bleeding the system for 4 hours and is running clear with no bubbles the brake master cylinder has now been replaced but that has not fixed the issue what else could it be???

I had an identical problem with a Mini Cooper S. Finally tracked it down to a leak in the brake servo, which had cleared some gunk out when I replaced the brake fluid.

When you replaced the MC on the Servo, you tightened the screws down equally a few turns on the left a few turns on the right.
Otherwise you can cause the plunger to skew itself and then you don't get full travel on the pedal and screw up your servo and the center pin on the servo can pop loose of the fingers holding it to the diaphragm.

Are you losing brake fluid if you work the pedal with all bleed screws tight ?

I had a hole worn in the brake vacuum pump diaphragm do this to me, it triggered all sorts of chasing of tails as replacing the diaphragm didn't fully solve my issues (although was much better). I'd already serviced the callipers, new pads, new discs, new drum shoes and valves, springs, etc to try and sort the issue.

Then I changed the vacuum pump model to a later version (no diaphragm) and still wasn't resolved. Tested the vacuum and was working perfectly - so moved on to brake servo.

After failing to find a new mk1 servo, I opted to call in to Crazy Quiffs for a new MK2 brake servo / master cylinder who then fitted it - but still not fully resolved.

Full front upgrade to Mk1 GTI callipers and discs, with lots of trial and error later - it was resolved. Never fully understood the root cause, but changing EVERYTHING for new did solve it, and other than the cost of wasting the first set of discs/pads by upgrading, has left me with an infinitely superior driving and stopping experience! I would recommend.

My car is 1.6D so some differences with how the vacuum creates suction off the engine if yours is a petrol engine, but all in all quite similar.