It's that servo again

[Raistlin]

Having only just got The Princess back from having her sills sorted, the infamous S-Type hard brake pedal showed up again.

Now, I had this problem a year ago and had the servo replaced.

What happens is that the brakes operate normally in the short term but a prolonged application completely exhausts the vacuum, resulting in a sudden and scary hard brake pedal which requires many times the normal force to control the car. If you can imagine braking gently on the slip road having left a motorway and then, just as you expect the car to stop at the entrance to the roundabout, all braking effort seems to cease.

Having experienced it before I was immediately able to understand what was required and apply much greater force to the brake pedal. If you'd had no experience you'd assume total brake failure.

So, last time this happened to me the servo was replaced and the failed item clearly contained a small amount of liquid which shouldn't have been there, which is responsible for destroying the seals / diaphragm.  Whether this is brake fluid from the master cylinder or Oil from the vacuum pump was never established but conventional wisdom suggests the former. This is a well known fault with the Diesel engined variant of the S-Type and, indeed, examination of today's failed item confirmed the presence of fluid inside.

A replacement servo will set you back £500 just for the part from Jaguar parts suppliers. Even second hand items can cost up to £200... IF you can find one.

There was some good news however, in my case. Again, conventional wisdom has it that there is only one brake servo that will fit the diesel S-Type and indeed, last time this happened to me, I tried three different part numbers before bowing to the inevitable and getting a direct replacement.

Denis spotted some information today on the 'net though which led us to believe that the 3.0L diesel variant of the Jaguar XF is equipped with the same servo. We located a suitable XF donor at a local breaker's yard and the guy at reception was so sceptical that he told us we could have the whole servo and master cylinder for £40, but with the caveat that no refund would be given when it (inevitably) didn't fit. indeed, when we asked the question of the tech people at the local Jaguar dealership, the response was, to say the least, negative.

The donor car was, I believe, a 2011 vintage and the servo and master cylinder assembly looked for all the world as if it had just been provided brand new from Jaguar. Unfortunately for Denis, the car was in the second storey, as it were, of a three storey pile of cars and the vehicle below only had a roof, floor and one side, meaning a very precarious situation for the man with the spanners.

We had applied some thought and took along the item removed from my car. After stripping the assembly from the XF, Denis successfully retro-fitted mine to the car, proving that it would fit. The mounting holes brake pipes and electrical connections were spot on and, sure enough, about an hour later, the servo and master cylinder were safely fitted to the Princess, which then required only minimal bleeding to ensure a firm brake pedal.

Now, if the recurrent servo problems ARE caused by leaking brake fluid, which seems so, bearing in mind my car's recent history, then the new master cylinder and servo combined should solve the problem permanently.

In fact, I'll be raising a thread elsewhere to see if this sort of brake problem is common on XF models. Hopefully not.

For anybody interested, the part number for the servo is 8X23-2B195-BB and is found on the later XF models with the 3.0L diesel engine, not, counterintuitively, the 2.7L as fitted to the S-Type.

I'm not saying that you'd get the same sort of bargain I got today but at least there are now two alternative and proven part numbers to choose from with the added bonus that the XF based servos will be that much newer.

Once again, I am indebted to Denis, for his hard work and sheer knowledge of how these cars are put together, and also to his good lady, Stacey, who, once again had her day's plans turned upside down by suddenly losing her car to the cause as the only viable transport.

In closing, if I might add a subjective observation, the new servo seems much more progressive and less harsh / sharp than the item it replaced, making The Princess even more regal in her progress

[Peter Clatworthy]

What was the part number of the servo you replaced?( as there are several) Does the replacement part  fit the original master cylinder or do I have to replace both items?