I'm helping my dad out with his 1953 XK120 OTS. The front brakes were converted to aftermarket disks a number of years ago and the brakes work well. We have noticed that when the brakes are used a lot (ie hot) the brakes start to drag. The theory is that the brake fluid is expanding with the heat.
According to the sources I have seen, the car should have the tandem master cylinder. Instead, it has what appears to be the correct factory single master. Dad has a fluid reservoir for a single MC but it is not installed. The bracket for the bottle is in the car but not used. My plan was to reinstall the reservoir bottle with new fittings, hose, etc to give the brake fluid some space to expand.
The cap; on the MC has \a hole in the middle which has been plugged with a threaded plug. While parts searching, I also saw MCs that appear to have a cap with a hex head bolt with what appears to be a vent hole drilled into the plug/bolt. I don't see this in the parts books but was this done at one time? No fluid reservoir and a vented hex head plug in the MC cap? Maybe I just need the vented plug?
Thanks, Craig

 

All of the hydraulic brake systems I have ever had anything to do with were vented.
I think you are probably correct in going after a vent system.

 

Thanks, Bill.
I'm working on tracking down the parts to recreate the system. The banjo and banjo bolt are not in stock anywhere that I can find. I have a lead on some used parts but it is taking him forever to get back to me to confirm.