Thruxton R Brake Lever X Stiff

Exactly how does it feel stiff, that is, is the lever hard to operate, has the feel gone out of the lever, does the brake drag?

 

Is the lever pivot dry and so it's not a brake caliper operation issue but a lever movement problem?
Remove lever, inspect and grease up the pivot to remove this possibility.

 

Has someone topped up the system with a silicone brake fluid (DOT 5.0)?
This could cause the seals to swell and give similar symptoms to what you describe.

 

Dot 5 is silicone based so incompatible with all the others.
A silly numbering system, 5.1 can run with 3.0 or 4.0 BUT 5.0 can only run on its own.

Not sure how you could test without sending a sample to a lab: I'm sure the oil company labs could do it with specialist sampling machines like they do for engine oils.

Worth a little research on your behalf so that a knowledgeable challenge could be offered to the main dealer!

 

Ref the DOT5 contamination idea...if it was that, I don't see why would have happened midway through a ride. What year is your bike? Some of the earlier "R"'s (including mine) had an issue with mushy front brakes, very like air in the system and would "pump up" after 2 or 3 squeezes of the lever.....very disconcerting if you grab a handful of brake and there's nothing there... I have noticed my brakes improve during a longer ride (only about 5mm movement before a good firm bite) but they are usually back to squishy next ride out. There's a very long discussion about this problem on a Speed Triple forum but it references Thruxtons as it's the same hardware:
https://www.triumphrat.net/threads/16-speed-triple-front-brake-issue-brembo.953404/
I wonder if the rough road has dislodged some trapped air in the circuit, it's gone up into the reservoir and the brakes are now operating as they were designed to... about 10mm free movement at the end of the lever. How much movement did you have, how much now? Is the wheel binding?