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Triumph Stag brake master cylinder silicone brake fluid fail




I have had a major headache with my newly finished Stag project, the new master brake cylinder I purchased from a reputable manufacturer failed after 900 miles in less than a year.


The master brake fluid reservoir is split into two compartments, both were nearly empty of silicone brake fluid which indicated that the master cylinder was at fault. A quick check on the bulkhead under the servo unit confirmed that the master brake unit had indeed failed.


I contacted the supplier that I had purchased the unit from and I was informed that the master brake cylinder was not recommended to be used with DOT 5 silicone brake fluid, and that the manufacturer would not accept returns if it was used (even though there was no warning on or in the packaging).


I then contacted the manufacturer myself who confirmed that the unit was built as the original specification and therefore only suitable for using DOT 3, & 5.1 glycol -based brake fluids.


After some research I came to the conclusion that the base material for the rubber components inside the newly manufactured unit were not compatible with silicone DOT5 and start breaking down.


I was informed by the supplier that there was an opportunity to have the rubber components made in a suitable comparable material: EPDM rubber (ethylene propylene diene monomer) which is recommended by the manufacturer to be used with silicone brake fluid.


After trying to find someone to invest in getting the new master brake cylinder kit re-manufactured I finally decided to get it done myself.


I am now selling them on eBay if there is anybody out there like me that wants the advantage of using silicone instead of glycol based brake fluid in their Triumph Stag.


I have also stated that it is not recommended using silicone brake fluid if you intend to race there Stag, this is because it has half of the compression rate of glycol.

I'm sure everyone has there own opinion on this mine is that I do not intend to push my Stag to the limits and as there is no ABS system on the Stag, brake lock is as much as much a danger than the brake pedal being a bit spongy.


I have had the same Automec DOT 5 silicone brake fluid in my Triumph Vitesse for over 26 years and it still passes its MOT and stops.


I also wish to put to bed some brake fluid myths that are floating out there, after a discussion I had with Automec I did some experimenting with my old stock of DOT 3 & 4 that I had in the shed.

I found that you can mix both with silicone DOT 5 as they remain seperate and inert (No thickening as mentioned in some blogs). What I did find is that the DOT 5 silicone fluid sat on top of the DOT3 & 4, therefore you can theoretically replace DOT 5 silicone with DOT 3 & 4, but not the other way around as it would simply float on the top and not flush through.

I checked both my 25 year old and brand new DOT 5 fluid, the only difference was that when shaken the 25 year old stuff took longer to separate.


If your sad like me, have both brake fluids in the shed and have an old jam jar in the cupboard check it out.



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