Brake switch leaking

[Guest]

'76 TR leaking brake switch. The question is, is the proportioning valve bad, or the switch? It would seem to me that the switch wouldn't be designed to take that kind of pressure. That's the brake bias switch, not the brake light switch, right?

 

[DNK]

Get seals all over for it. In fact someone hear boughta bunch and was passing them around a while ago.
Check the good peoples site who DOCUMENT everything.
Without them ,live would be hockey sticks

 

[TR3driver]

You're correct, it's the valve leaking, not the switch.

But it's not a proportioning valve, and in fact it has no effect on brake operation at all. It's sole function is to compare the pressure between front and rear brake circuits, and light the dash lamp if the pressure is different (indicating a leak). Hence it's name : Pressure Differential Warning Actuator or PDWA for short.

 

[Guest]

Where do I get seals for it?

 

[DNK]

They are pretty standard size.
Pull yours apart and measure them.
I think even the Boxy stores had em.

You could do a search...I know... to easy but yet too futile

 

[TR3driver]

Depends on which PDWA you have. The later ones take standard O-rings as Don says (but be sure to get the right material, all O-rings are not equal).

Check the Buckeye Triumph site, they have a good article on the subject (including veiled references on what to do if you have the earlier PDWA with the unobtanium seals).

 

[DNK]

Hey Randall, forgot that little tidbit thanks.
Lucky for you Billy,you got a late car

 

[TR3driver]

I mentioned it only because someone might have installed the earlier unit.

 

[DNK]

Point taken and for others who don't know and those that their memory has left the building

 

[hondo402000]

NAPA has them, ask to look in their box of hydrolic O rings they have metric and SAE sizes

Hondo

 

[Guest]

I might just bypass it. I do have some a/c o rings here and will try those first.

 

[DNK]

Billy , you prolly have what you need at the shop

 

[dklawson]

Bill, as mentioned above there are at least two styles of PDWA used on Triumphs from the 1970s. Succinctly, does your have "one" end bolt/cap, or "two"? The PDWA's with a single end cap use size AS568-110 o-rings. HOWEVER, for compatibility with brake fluid (DOT-3 & -4) only use seals made from EPDM. Do NOT use "standards" Buna-N o-rings.

If your PDWA uses two end caps, they use a "non standard" size o-ring. You can machine new pistons for it that will use the standard seals. This is described on some of the Buckeye Triumph web pages.

 

[Guest]

Will green a/c o rings work?

 

[TR3driver]

Will green a/c o rings work?

Near as I can make out, the answer is a solid "maybe". "Green A/C" O-rings are usually HNBR, which according to https://www.efunda.com/glossary/design/oring/design--oring--chemical--brake_fluid.cfm are "usually suitable" for <span style="font-weight: bold">static</span> seals in brake fluid. The PDWA is pretty nearly a static application, but personally I'd stick with EPDM as it's both cheaper and the right material for the job.

 

[Guest]

What about dot5?

 

[dklawson]

If you are running DOT-5 you should be OK with virtually any rubber compound for the seals as long as it is "new" rubber that has not already been exposed to DOT-3 or DOT-4.

From memory, the HNBR material mentioned by Randall is a hydrogenated Buna-N material. Think of it as a "standard" Buna-N o-ring on steroids.

 

[mrv8q]

From our friend, Paul:
https://www.britishcarforum.com/bcforum/ubbthreads.php/topics/359406/1