I have a 1966 Mark X, 4.2L.
I believe I have a bad brake booster, and I wouldnt mind having a new master cylinder as well.

Someone once mentioned that it was possible to retrofit a booster from an early XJ6 which would be ideal since they are cheaper and more plentiful.

Anyone have any tips or has anyone done this before? If not an XJ6, do any of the early sedans, or an xke, have a compatible system?

 

The XJ6 was in development when your car was built, and as Jaguar's Sir William Lyons was notoriously stingy with development money, you may well be right about the XJ booster, although later series may not be.So check out a Series 1 or 2 booster. I think the XJ Series 3 was a bit different, though.

 

Had a 420G that i changed the booster/master cylinder to an early XJ6 set up. from memory it went straight in but mine was RHD. I think I had to space the booster from the mount about a quarter inch or so just to set the pedal height, long time ago though. There may have been an adaptor to match the lines to the master cylinder as well.

 

Dave I have a 420G and two years ago was in your position. I suggest you look at a web site for "Booster Dewey" in WA state I think. He rebuilt my Dunlop booster which I run with the later Girling M/C . I have the original Dunlop M/C and the setting tool but have yet to find a complete rebuild kit. Could you post a pic of what you have so a bit of better advise might be given. The factory did issue a bulletin and with the XJ6 the 420 G repair was to install the "Super Vac" from the XJ6 which is why the original Dunlop system is a very rare sight.

 

Hi Davis,

do you solve your booster problem or always need help?

 

Finally solved and back to running! I bought a new master cylinder for a 1980 xj6, under $80 new. I had to enlarge the mounting holes with I think a 9/16" bit, but it bolted right up to the original brake booster using the original counter suck bolts and spaces. The new master cylinder had both 10mm and 12mm ports, the original I think they are both 10mm, so I had to do was change the front fitting. As for the master cylinder work, that was pretty much all that was needed; very simple.

Unfortunately, I had a lot of other brake work. The original master cylinder's rubber seals/etc disintegrated and polluted the brake lines leaving them badly clogged. This took a good deal of work to take everything apart, clean, and reinstall.