i pulled out the master cylinder after it developed another leaking issue, similar to the one I had five years ago.  I replaced the seals then, but decided to replace the master cylinder this time.  The part I have came without the seal that looks like a piece of sponge, fitted around the end that is bolted to the brake booster.  My old part is completely disintegrated and not transferable.  

What is the purpose of this ‘seal’ and does anyone have experience of not fitting one, or fitting an alternative?

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This is not a seal, it's an air filter.  The master cylinder mating bore is the exhaust for the vacuum when you release the brake, to refill the diaphragm.    There is another one inside the cabin on the pedal side.    
 Be wary that if air can't enter there, the brakes will lock up after you use the brakes a certain number of times.  I had that problem with a caddy once, where there was a moraine booster used to replace a bendix, and the wrong breather (or both, i don't remember) was installed.    The vacuum had no way to release, and the brakes would start smelling after about 20 min of driving.

Thanks for the info Mark.  I have not been able to find one of these on the usual sites, do you know if this needs to be something specific, or would an open pore foam (fashioned into the correct form/size) suffice?

Hi Mark,

i endedup making a filter out of foam (I hope it works). But, when I was comparing the two parts (old and new) although I can see the air hole, it seemed to be covered over with a dust seal or boot.  I can’t see this part in the shop manual.  Fitting this boot will stop any air exhausting, as it’s a tight Durand in my case, it seems to have been siliconed in, to ensure a tight fit.  Any info on this part?

ive uploaded photos




the silicone is unnecessary, maybe even would cause problems.   

your setup looks to be as it was, with the difference that the seal installed at the factory was felt and not sponge.   It should work.   If you smell brake shoes burning after a medium length drive, you will need to change it.    The filter is only there to keep dirt from being sucked into the power valve when the pedal is released.  the brakes can work without.

That said, it looks like you've done great to even look into it, most people toss them, so they are rare these days.  

Thanks Mark.

greatly appreciate your feedback.  


Any chance we can get a visual ( photo ) of this air filter for the MC as this is the very first I have heard of this air filter 

As always — Thanks Mark 


Tony, There is an illustration in your shop manual of both filters. The Bendix had a different setup. They vented from the cap as I recall, but had the incab filter like the DM.

Thank you Jason 




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