XK150 front brake calipers
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I noticed a short while after purchasing the car that on each front disc, a pad was not retracting fully. I could see that the retractor pins and bushes were not evenly protruding, and there were signs of rust affecting the retractor spring bushes in the holes in the cylinder body. It was clear I needed to dismantle the front calipers at some stage.

In conjunction with concluding that I need to refurbish the front cylinders, I reflected on the fact that on a warm dry day, on good smooth hot-mix bitumen, I was not able to lock the front wheels, no matter how hard I pushed the brake pedal. As a result, later in 2014, I replaced the Girling booster unit I had repaired with a Lockheed-copy diaphragm unit. While the Lockheed-copy unit was supposed to result in improved braking performance, I still could not lock the front wheels under test conditions.

Eventually, I concluded mid-2016 that I should replace the calipers and cylinders on all four wheels with modern calipers/cylinders. Later, in 2017, I replaced the Lockheed-copy with a genuine PBR VH-40 booster. But that was not the end of the story! When in 2019 I removed the head for a full rebuild, I discovered the rubber hose installed in c.2000 to connect the inlet manifold and the booster was not a petrol-resistant product. The rubber material had deteriorated, broken down, and had been preventing the check valve within the inlet manifold hose connection seating properly. I ordered a replacement hose made of silicone, cleaned out all of the degraded rubber, and connected the silicon hose. Later, in the very first test drive after rebuilding the engine, the braking performance of the car was markedly improved. I could now lock the front wheels!

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