When braking I get that pumping feeling like I am repeatedly loosening/tightening the lever.

From this I am pretty sure this is due to a warped rotor.

Before I start working on this is there an easy way to check if just one or both front disc rotors are affected and if only one is it OK to replace just one or do they need to be replaced as a pair?

Finally is there any other reason that this might be happening other than a warped disc?

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Bike: 2008 XT660Z Tenere with various bits and bobs added or subtracted

Before you condemn you discs for being warped first check that the disc bobbins aren't seized.

The discs are a 'floating' type and rely on free movement of the bobbins (a sprung clip) allowing the rotor to move a small amount independently from the carrier when the brakes are applied. If one or move of these bobbins are seized the rotor will not be able to run true between the pads giving the sensation of pumping.

It is very common for bobbins to get gummed up and stuck at this time of year in the UK as we come out of winter and the brakes have been subjected to several months of corrosive road salt. Freeing them off should be on the 'must do' list every spring. Freeing them off is a very easy thing to do and it just might fix your problem without the expense of new discs. However, it is worth noting that riding for long periods with seized bobbins on floating discs is the primary cause of warped discs, not heat as many people think.

Anyhow, check your bobbins first and then we can run through how to check a disc rotor is true.

Here's a good video that explains the whole process (not an XT, but the idea applies to any bike with floating discs and sliding calipers)...

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I hadn't even considered that. Excellent. I'll give it a go tomorrow.

Thanks Pleiades

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Bike: 2008 XT660Z Tenere with various bits and bobs added or subtracted

What's he spraying on the bobbins? is it brake cleaner?

Yup its disc brake cleaner. Fortunately I have an almost new can in the cupboard.

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Bike: 2008 XT660Z Tenere with various bits and bobs added or subtracted