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September 11, 2022

PDWA & Plumbing

By '69, all new cars, at least in the US, had a dual-circuit hydraulic braking system.  The master cylinder actually had two pistons, each drawing fluid from its own fluid reservoir.  One piston fed the brakes at two wheels, while the other fed the other two.  Typically, it was front brakes/back brakes.  This redundancy was a boon to safety, but by itself, it had a down side.  If one circuit failed, the brakes would still stop the car, but with reduced effectiveness, especially in a panic situation.  Thus the provision of a device to detect and report the failure of one of the circuits.

This device, on our cars called a "Pressure Differential Warning Actuator" or PDWA is a mechanical gadget that both brake hydraulic circuits pass through.  The PDWA has no effect on the operation of the brake system.  There is a shuttle piston inside that sees hydraulic pressure of one circuit on one end, and the pressure of the other circuit on the other end.  In normal operation, these two pressures should be close to equal, so the shuttle is unaffected.  If one circuit fails, the difference in pressure forces the shuttle to one side.  This actuates an electrical switch that turns on a warning light on the dashboard.

Triumphs used a number of PDWAs over models and model years.  Mine was an aluminum body type.




There are quite a few documents and videos on the web that show how to rebuild a PDWA, so I guess I'll just add one more. 

The plastic switch screws out of a hole in the center of the device.  Once the two end plugs come off, with their copper sealing washers, the two-piece shuttle can be drawn out.  There is also a small ball bearing which at this point was still stuck in the body.




Each part of the shuttle has a square section O ring seal.  They are a common industrial size -010, but you have to be mindful of the material they are made from.  These new seals are Viton.  Viton is compatible with DOT5 silicone brake fluid, which is what I use in my old cars.




Everything else is cleaned up.  I typically re-use copper sealing washers, but I flatten them and anneal them first.  The end plugs were pretty rusty, so they got derusted and zinc plated.  The switch didn't work at first, but some exercise and some contact cleaner brought it back to life.  The little ball bearing is what rides in the groove  in the middle of the two part shuttle.  When the shuttle shifts, it is forced up into a hole in the body and pushes the snout of the switch.




One part of the shuttle is inserted into each end of the body.  The shuttle is in two parts so that neither of the seals have to be pushed across the switch opening in the bore, which could damage the seal.  Looking down the switch hole, it is obvious when the shuttle groove is centered.  The ball bearing is dropped in, then the switch installed.




Both the brake master cylinder and the PDWA have two different threads on their hydraulic ports:  3/8-24 for the rear circuit, and 7/16-20 for the front.  This makes it harder for the system to be plumbed incorrectly.  I was able so save most of the fittings from the original system by stripping and re-plating them.  The slightly shinier fittings are new, the rest are just re-furbed.




I made new lines to connect the PDWA to the front and rear circuits.  The old lines were a pretty good guide for shape. 




To finally button up the braking system, I drug out the master cylinder, along with its assorted bracketry...




...and got 'er installed with her eight bolts.




The last two hydraulic lines.  These are the originals, which made decent patterns.




The new replacements.  I use a copper-nickel-iron brake line.  It doesn't corrode, and is pretty easy to form by hand.

 


This finishes off the brake system, and it feels good to put a big fat check mark in that box.  Cost for this part was maybe $10 worth of tubing and a few bucks for the seals and new fittings.

Comments to Ed at elhollin1@yahoo.com

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