Brake servo in XK120

[Perseverant]

As usual around the end of February I checked over my XK120 ots and spotted an ominous puddle under it, which turned out to be brake fluid. The underneath of the glass reservoir seemed to be the source so I took it off and redid the connection to the master cylinder. So that was dry, but the puddle continued and this time the true source of the obtrusive leak was the brake servo - an aftermarket Lockheed on brackets mounted on the chassis under the steering column, giving assistance to the front discs, again an aftermarket modification. I knew that XKs, other than some 150s didn't have servos anyway so I spoke to a couple of specialists who suggested removing or bypassing the servo. To cut to the chase, I removed said servo and discovered that it was full of brake fluid - yet more mess- and made a plug to eliminate the vacuum take off on the manifold. The local garage went beyond my expectations in sourcing the brake pipe fittings for a bypass and made a pipe for free! Anyhow, all fitted up and around 80 miles later, the brakes feel very positive and don't leak, so I'm very pleased. I assume that the servo hadn't been doing much for ages other than sucking in brake fluid. I wonder what anyone else thinks?

[a8hex]

IIRC the servo was fitted to allow a larger master cylinder to be fitted as a means of reducing the effect of the knock off from the rear discs. There is always a bit of play in the live axel and this caused them no end of problems when they were developing the disc brake systems firstly for the race cars where they ended up using a pump driven off the gearbox and then for the XK150 where they used a servo.
If you've just fitted a front disc conversion then there may well be no real need for the servo.
If the car is reasonably easy to stop without the servo then you're probably alright driving without one. Only your right leg will know for sure

[Perseverant]

Thanks! The car still has drums on the back - the comment about rear discs was interesting. As I said, the brakes feel fine, indeed rather firmer, probably because I think the servo was just along for the ride.

[a8hex]

Perseverant said:

Thanks! The car still has drums on the back - the comment about rear discs was interesting.

The live rear axle tends rattle around a bit and each time it does it knocks the pads away from the discs. When Stirling Moss first tested them on the Mille Miglia in 1952 he found that he had to pump the brakes several times at the end of a straight to get the pads back near the discs so they could actually work. This just about works for a racing driver but was clearly a no hoper for a road car being driven by mere mortals. Even when I had my 150 and I took it on track days it sometimes required a quick pump to bring the rear pads back in. I guess modern bearings have a lot finer tolerances.

[lowdrag]

I had a hell of a trouble finding replacement Lockheed servos for my Lynx XKSS and luckily found a company with two lfft on the shelf. The said that it was unlikely that they would be able to source any more, They are Lockheed 4257-818/8. I suppose standard XK ones are available but just a heads up.

[Allan L]

a8hex said:

The live rear axle tends rattle around a bit and each time it does it knocks the pads away from the discs. . . I guess modern bearings have a lot finer tolerances.

Some ENV axles were designed with a single taper-roller bearing at each end, with the half-shafts butting together to make it work as a single unit. Shims were used to set it up with a suitable small clearance, but a bit too much haste could result in too much clearance, so knock-off resulted.