Twin Cam Brake System Removing Pressure Difference Warning Valve

[Certified Lotus]

I am modifying my 72 Europa TC brake system by removing the two (2) brake servo's and changing the brake master cylinder to a .7 bore to accommodate the removal of the servo's.  This modification also removes a number of brake lines used for the servo system allowing for a much cleaner brake line installation.

My question is; now that the servos are removed, what do I do with the Pressure Difference Warning Valve (part number 7 in the drawing)?  It is not needed due to the servo delete, but the valve has a sensor that is wired to a dash light to warn of brake pressure loss.  Do I just eliminate the entire valve and pull the bulb in the dash?  OR do I use the value and plug the ports no longer used, wiring the sensor to the dash light?

I know there is also a Stop Lamp Switch (part number 11 in the drawing) built into the brake system for actuating the brake lights (this must be installed).

What have others done when removing the servos?  I welcome all comments and insights.

[surfguitar58]

I think the diff switch is still relevant as long as you still have a tandem master cylinder. Your front or rear brakes should still work if you break a line. It assume it would be immediately obvious, but the warning light would confirm what’s happening.

[surfguitar58]

Also, I assume you are changing to a SMALLER bore M/C, not a larger bore?

[surfguitar58]

...and (sorry if I am monopolizing your thread) I am following with great interest your simplification of the brake line tangle. I’m thinking of doing this myself. My schematic has the diff switch right up front, as close as possible to the M/C. Let us know how this all works out.

[Certified Lotus]

Also, I assume you are changing to a SMALLER bore M/C, not a larger bore?

Yes, absolutely correct. I changed to .7 bore in my original post. 

[Certified Lotus]

...and (sorry if I am monopolizing your thread) I am following with great interest your simplification of the brake line tangle. I’m thinking of doing this myself. My schematic has the diff switch right up front, as close as possible to the M/C. Let us know how this all works out.

I'm not sure the entire PDWV is needed, just the sensor (I am assuming the PDWV is designed for interface with the 2 servos).  So if that assumption is correct, I wonder if a suitable pressure sensor can be sourced and just screwed into a four way junction? 

Note that I am installing an adjustable pressure valve for the rear brakes for later disc conversion.

[BDA]

The PDWV is used to detect when there is a volume displacement difference between the front and rear brake circuits, the idea being that if there is a leak in one system (front or rear), you will be warned. It is connected to a "BRAKE" warning light in the instrument binnacle. It really has nothing to do with the servos themselves.

I would keep it. It is plumbed by running the front brakes  through one side and the rear through the other. If you wanted to simplify your plumbing, you could mount the PDWV in the front.

Edit:
In thinking about it a bit more, it actually works on a pressure difference, as the name implies, but I thought referring to volumes might make it a bit easier to explain.

Edit again:
If you choose to mount the PDWV in the front of the car, remember that it will have to be adjusted so you'll need to have reasonable access to the adjustment side of it.

[jbcollier]

You are obviously not restoring to stock so I would remove the PDW valve.  They tend to develop internal leaks which then seep out the threads of the plastic switch.  For a long time rebuild kits were not available but I hear they are now.  Still a pain to have to recenter the spool every time you bleed the brakes.  However, you have the light in the dash so why not use a master cylinder cap with a float switch in it.  That way you still have warning of any brake system failure.  Better still, you will have warning if your brake fluid is low due to pad wear.  Useful option.

[BDA]

JB is right that recentering the spool is a pain. I think he really has the better idea.

[Arizona]

I removed mine 40 years ago. Sure makes bleeding the system easier.

[Certified Lotus]

Thanks for everyone's responses and suggestions. I'm removing the PDWV......which what I was planning to do anyway but figured I should get input before I made a final decision.

JB, I happen to have a master cylinder reservoir float cap left over from an Elan S4 I restored.  That should work. The only issue is that I don't have the canister that screws onto the brake fluid reservoir that the float cap screws into. Those are hard to find.

[jbcollier]

Wilwood sells a couple of different ones that might work:

http://www.wilwood.com/MasterCylinders/MasterCylinderAccessories.aspx

[Bainford]

Thanks for everyone's responses and suggestions. I'm removing the PDWV......which what I was planning to do anyway but figured I should get input before I made a final decision.

Thanks for doing the leg work on this. I have been contemplating eliminating my PDWV as well.

[Grumblebuns]

With the Spitfire reservoir being sloped, I don't know how effective the warning light will be.

FYI, the SOVY canister and level float from the S2 will fit on the Spitfire reservoir, see picture. Pegasus sells a Girling reservoir extension that may work for what you are doing. The only unknown is the thread pattern for the newer Girling units. I do know that the small Girling reservoir plastic caps also sold by Pegasus has a different thread pattern from the original Europa caps. The threads are just slightly different that the new cap will screw in about two turns then stop. Being plastic you can force the cap on and it will seal. This may be the same situation with the new reservoir extension.

https://www.pegasusautoracing.com/productdetails.asp?RecID=1114

If you really want a low level warning setup, your best bet is to find is to locate a S2 SOVY canister.

[2483R]

I tried the Girling extension many years ago on my Spit master. It does not work as the threads are different. Of course this assumes the Girling extension has not changed over the years.