XK150 single circuit brake system
Can a dual circuit system be fitted to an XK150?
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My car has a single circuit braking system. So does every other standard XK140 or XK150 on the road today.

Do I fret about the single circuit braking system? Yes(*).

Can I fit a dual-circuit braking system to my XK150? Yes.

One XK parts supplier in the UK now offers a tandem master cylinder for the XK150. It is of necessity longer than the standard master cylinder, but it looks like it will fit in the place of the standard unit. To complete the conversion, the brake piping needs to be split into two separate circuits.

I've decided that this is one safety-related modification that I will not tackle. The best plan is to make sure that all cylinder seals and hoses are in first-class condition, and that all three hoses are replaced at the first sign of any degradation. Also, my standard procedure is, as I take the car onto the road, I stop in the driveway, and push the brake pedal as hard as humanly possible, way harder than I ever would apply the brakes on the road. That way, I ensure any thin wall part of the brake pipe system will 'pop' before I am on the road. 

Now I'm a bit more relaxed about the single circuit brake system on my XK150.

(*)  Why do I fret about the XK150's single circuit baking system? Because a long time ago, in 1968, in my Mk V 3-1/2 litre saloon, I experienced a complete brake failure.

I was approaching a cross-road intersection in the country on the outskirts of a town. I braked for the intersection, intending to turn left. The brake pedal went to the floor. Pumping the pedal did nothing. I survived, without hitting anything. There was no other car entering the intersection, which was good for me because all I could do was go straight on and roll to a stop with the help of the pistol-grip style handbrake.

Later, I found the cause. On the left side rear, the connecting arm on the lever-type shock absorber had rubbed against the hydraulic pipe which was routed along the rear axle tube, but not properly clamped in the correct position to the axle. The rubbing had gone on long enough for the pipe to be worn away until it burst when I applied the brakes as I approached that intersection.

Shortly after I bought my XK150 in 2013, I went right through the car looking for any point in the hydraulic system where rubbing could abrade a pipe or hose. I found a couple of points where the rubbing might occur, and made sure that no rubbing could occur. Later, in 2018, when I replaced the crown wheel and pinion in the rear axle, I found that the right side rear shock absorber had been rubbing against the brake pipe because - wait for it - the person who had rebuilt the rear axle for the prior owner had routed a new brake pipe along the rear side of the axle tube, not along the top of the tube. So fifty years on from my MkV experience, I went close to having the same thing happen again!

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