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Spitfire 1980 pressure differential switch


AlanT
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Morning Experts,

do you know if rebuild kits are available for these? I’m in the final stages of returning a car from 20 years of dereliction. All other brake parts are new but a new one of these is another £200 I’m keen to avoid.

thanks

 

Alan

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there are two sizes of pdwa valves   the stag kit is cheapest but didnt fit the valve off the spitty even tho they are often listed as the same base valve part number   realy you only need the small 0 rings there is nothing expensive in these valves a small shuttle piston with 0 rings on 

eg  https://www.jamespaddock.co.uk/brake-bias-valve-pdwa-kit-2

nothing to stop you strip and clean what you have and examine seals for wear 

Pete

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considering there is sod all insde why the price hikes 

doesnt make much sense  other than rip off

two 0 rings and crappy switch most are old and seized so knowone knows iif it would work or not

I had a spitty to fix lost all rear fluid and the switch   had been disconnected probably for years was a hammer it out to repair 

its idea is any line failure the piston pops across lights the warning and closes the leaked line off   so no more leakage 

well 45 years ago they were good  , 

and in reality you could remove the piston , who would know , the warning lamp would still have a bulb test from the low oil pressure switch 

thats all a MOT can test for this looses the split line braking but if the valve has stuck you may only have single line and be totally unaware one end of the car has been shut off  for ages till the dyno says theres a low efficiency somewhere 

pete

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Alan, you will feel the pressure differential with your foot (brake pedal sinks further), along with the Spit not loosing speed as quickly as expected! A Posteriori. Far better to put a low fluid switch cap on the master cylinder and wire that to the dash warning light.

Cheers, Richard

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DVD3500, it's a master cylinder cap with a float switch built in. The fluid level drops and the switch is activated. If it's a slow leak it gives you advanced warning. I see Car Builder Solutions have a switch you fit in your existing cap (drill a hole). Or there are period metal caps with switches such as fitted to Rovers.

Cheers, Richard

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Right.... The 1500 I hope to pick up soon has a PWDA I was planning on just eliminating it... I figured the lack of braking was as good an indicator as anything... I doubt those caps work on tandem systems...? Or do they?

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that all depends on the access to the reservoir on  a tandem master

if you remove it and the MOT man knows his stuff that would fail... Where;s you valve mate ???  

keep it fitted but without any inards and knowone knows 

pete

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1 hour ago, Pete Lewis said:

keep it fitted but without any inards and knowone knows 

Ah... did not know that was an option. 😉

Well I am likely going to have to go for a "prototype" approval for my Spitfire.

As I mentioned before I have making one car out of 2 and since a lot of the car is from a US car I might as well build the car I want and get approval rather than getting "exceptions" for everything that is not "original" or "standard".

The German MOT aka TÜV basically allows 3 permission types:

1. H for Historical: The car most be original or "in period" or EVERYTHING down to the radio. Pain to achieve but has allows for cheaper insurance and let's you drive where there are smog restrictions.

2. "Regular" Permission. Easier to achieve but anything that is not standard gets scrutinized and must be added to paper work (at a cost usually). Changed even to the size of tire or wheel are technically not allowed and Higher Power forbid you make "upgrades" like stainless steel brake lines.

3 "Prototype" literally "one-time" approval. Expensive (I have read from 60 to 600€), but essentially the car gets a once over and if you can convince the engineer it is road worthy he allows it.  The best way is to get the engineer on board BEFORE making the changes and then he lets you know if he will allow it before you put it on. Given I am taking the best out of a MKIV (looks) and 1500 (US doors, rear bumper etc.) I reckon this is easiest...

I wasn't planning on anything too crazy for the Spitfire brakes. I just wanted tandem and I don't want a dummy light on the dash. I don't think I need GT6 brakes or anything crazy like that. Mark Field set me up with some nice pads and shoes (that he forgot to send me in December ! 😄 ) and I think I will be fine. 

Great tip about hollowing out the PDWA though!

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On 28/07/2021 at 16:58, DVD3500 said:

Great tip about hollowing out the PDWA though!

Not completely hollow it out, or that would connect the dual circuits and negate the safety of the system! ** I would leave everything in place but, if the PDWA gave problems, add the special cap to the master cylinder. Then you disconnect the wire somewhere it won't be noticed (or maybe you declare it to the Prototype Inspector and hope he understands) and connect the dash light wire to the cap switch instead.

Cheers, Richard

** I had the dual circuit braking grafted on to my '77 Spit when it was converted to six cylinders. Unseen by me or the MOT chap a brake pipe was rubbing against the armoured PI fuel hose. Some years later the brake pipe burst while braking for a roundabout. This is how I know that the PWDA is useless when you get a catastrophic failure. You don't spend any time at all looking at the dash wondering why the light has come on. You immediately feel the brake pedal is different and so is the car's ability to loose speed. Anyway, I was jolly glad of the dual circuit system that day. Not sure I could have stopped on the handbrake in time.

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removing the inner shuttle piston is a "on your own "fix , making it back to single line is better than having the valve shut one circuit off and you  dont know 

dont know why these simple valves cost silly money everyone trys to rebuild a master cyl and wheel cyls  but jib at the PWDA

i dont advocate a bodge but something that works is an advantage over something that doesnt 

is twin line that good  triumph dumped it on the last 2000 saloons ...cost driven 

pete

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Interesting thing about Dual circuit systems, some are split front/rear and some are diagonal split?.

Advantage of one or the other?. My first thought, is that the maximum braking effort is best on the front wheels? But with a diagonal split IF the front circuit is the one that fails, at least one front wheel is being braked.?

Pete.

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On 26/07/2021 at 13:20, Pete Lewis said:

SNIP******r 

its idea is any line failure the piston pops across lights the warning and closes the leaked line off   so no more leakage 

SNIP********

Hi Pete,

My understanding of the PDWA is different.

The PDWA activates a contact & that is it. It's a Warning Actuator only.
Cheers,

Iain.

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Sizes no good on from James Paddock I’m afraid - the original is much bigger. About 14mm diameter. 
The piston only had one seal on it but two circlips. I guess one had disintegrated.

Any idea where I might find the seals? Leaving the piston out would put the warning light on since a metal sleeve sits around the piston and inside the seal and circlips and keeps the switch depressed. 
thanks

Alan

 


 

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http://www.zen11896.zen.co.uk/vixen/parts/pdwa.html

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d019238b9af3e000109c65f/1560384058350/Overhauling+Master+Cylinder+%26+PDWA.pdf

Two articles that may be of use. The first claims that there are two different sizes for the Spitfire; could this be the problem with the Paddock kit?

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