A JDCQ member had a problem with his Jag 420 series brakes. The power boost system appeared to have failed and it required incredible pressure to stop the car. We got involved in a diagnosis one morning with the car upon his hoist and to be very honest it took a fair bit of effort to sort out the problem.
What appears to be a dual brake system is not quite what it seems. The master cylinder is in fact a single primary system that then feeds pressure and air information to the remote power booster system where the system becomes dual. Each primary and secondary system has its own hydraulic reservoir.
The power booster system uses vacuum on both sides of the air piston. A remote air valve mounted on the primary master cylinder supplies vacuum on the drive side of the piston but when the master cylinder is depressed introduces ambient air pressure to the drive side of the piston hence assisting the hydraulic brake action. Heath Robinson would have been proud of this arrangement.
The final diagnosis was that the air piston/diaphragm in the booster unit was leaking badly. By serendipity another acquaintance was "parting out" a 420 with a good booster and this was acquired to get the car back on the road while a more extensive overhaul takes place on the original booster.
It should be noted that we discovered that a number of E types from roughly 1965 on share the same braking system.