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Spitfire 1980 pressure differential switch


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On 30/07/2021 at 10:00, Pete Lewis said:

no the shuttle should close off the pressure lost line as its moved across by the pressure difference to arrest any more fluid loss 




Hi Pete,

I believe you are wrong in your assumption. It does not function as you believe it does.

When you understand how it functions, please let me know.



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I've been doing a lot of reading on PDWAs due to this thread and never having had one actually fitted to a Triumph myself (I have rebuilt them, though) I can only go by what I've read - it seems there are certainly two types, one with two pistons and one with a single piston, but this post on another forum is interesting:

The PDWA came in a number of different configurations. One of them has a set of sleeves that will slide with the piston during pressure imbalance, and almost completely blind the low pressure line. I have that PDWA on two of my 1980 Spitfires. 

This seems to point to a system that blanks off the side that is losing fluid; 90% of the other posts / forums and articles state that it's a warning system only, and all it does is activate the warning lamp. That one came from


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Well I’ve ordered new seals from simplybearings.co.uk 

As a late car mine has the two sleeves. Be good to get this right rather than bodge it since the entire braking system is being replaced +pretty much every other mech part on the car! 

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