Trapped Line Pressure Valve (Post #142)

Now that the original rear wheel brake cylinders have been properly restored and reinstalled thanks to the parts supplied by Franco, it is time to take a hard look at the master cylinder.

Serial numbers for Peter's m/c (top) and The
Alfatross'.

The rubber and metal components inside Peter's
master cylinder.  The mystery valve is second
from the right at top.

Pursuing the theory that perhaps the master cylinder on The Alfatross may not be the original, or may be defective, I purchased a used 1900 cylinder from Peter Marshall's collection and disassembled both to compare them. Outwardly, they appear to be the same, other than having different serial numbers.  Assuming that the numbers reflect serial production, the one Peter sent had an earlier number (311000) than the one on The Alfatross (413340). 

This may or may not be significant, but there were other substantial differences as well: The "pushrod" connecting the bottom of the brake pedal lever to the master cylinder piston is 100 mm long on Peter's m/c but only 66 mm on The Alfatross'. It is also larger in diameter and lacking the nut and shackle needed to connect it with the pedal shaft.  Also, the diameters and threads in the ports connecting to the fluid reservoir and the hard lines out to the wheel cylinders are different from The Alfatross'. 

But the biggest difference is the presence of a mysterious component located inside Peter's m/c at the back of the compression chamber.  It appears to be a valve of some kind but what does it do?  

The mystery valve . . .

. . . fits neatly into the recess with the "button"
end facing the piston . . .

. . .  and the spring pressing against the steel
washer on the valve.

The schematic for the Girling M/C.  No. 6
is the trapped-line pressure valve.

The first clue was a reference to and image of a "trapped-line" pressure valve appearing on a period service kit instruction sheet.

The second clue came from Bob Lembcke as one of a number of possible causes for The Alfatross' brake problems:

"Oh yeah, on all drum systems, often need a pressure check valve to hold them "out" against the springs . . . old Bugeyes had those when all drums . . . without the check valve (usually in bottom of MC at discharge) the springs can return all shoes back, needing more volume (2 pumps or more) to get them into working position."

So now I have the name of the component and some idea of what it is supposed to do, but I still don't know how it is supposed to work or if it is the solution to my problem or totally unnecessary.

At this point my options are: (1) keep researching trapped line pressure valves, (2) reinstall the original M/C as is and be done with it, or (3) reinstall the original M/C after adding Peter's trapped line pressure valve to see what happens. 

I'm leaning toward option (2), but could be persuaded to go with (3) if option (1) produces results in the next few days . . . .