If you wish to retain your early XK150's original Dunlop round pad cylinders with retractor pins, this page might be of help.

Fortunately, the front cylinders on my car had all eight retractor spring bushes and pins, although one pin was slightly bent, and rust was preventing at least two of the front brake retractors from working properly.

Details of the retractor system

I've included a diagram of the original cylinder and its retractor pin setup.

If you place a retractor spring bush between the measuring jaws of a vernier caliper, and if your springs are the same as mine, you will find one end of the spring is of smaller diameter. I presume this is deliberate, and that the small end is inserted on the pin so the small end is away from the disc.

If you put a retractor spring bush between the measuring faces of a micrometer, you should find that it is possible to compress the spring by about 0.20-0.25mm (0.008-0.010") (yes, using your micrometer as a G-clamp!). It seems to me that this is the way that the retractor system works. The small end has the tightest grip on the pin, and when the brake pressure is removed, the compressed spring bush pulls the pad away from the disc by about 0.20-0.25mm as the spring expands in its hole in the cylinder body.

If your spring bush does not have as much "compression length" as this, then maybe you can stretch your spring a tiny amount, but don't overdo it as it will be irreversible and you'll find the pads retract too far and there will be too much brake pedal travel. And remember, no-one sells the pins and spring bushes any more.