Did my bike shop con me into replacing both sets of Trek 7500 Hybrid disc rotors? ....  I recently purchased a Trek 7500 hybrid via Gumtree.   I was warned in advance, prior to purchasing, that both front &amp; rear brakes were 'squealing' but concluded that this could easily be remedied. 

The bike was purchased over 4 years ago &amp; had rarely been used. Stored in a garage.  

I tried 'de-greasing' &amp; cleaning the disks and pads but this failed miserably ; I then took the bike to a local professional service provider who informed me that both sets of disks were badly 'burned' through excessive braking with damaged pads ?   

I was then advised, that the best option was to replace both F &amp; R Disks.  There was visual evidence of blue / black 'streak' marks on both the F &amp; R disks but my lack of knowledge on this issue was obvious so I have agreed to proceed with the ordering &amp; replacement. 

Cannot help sense that I have been conned into this replacement ( estimated costs with labour circa £70.00 ) ;  any advice would be gratefully received.  

Regards

Keni C
There are at least two questions here: 1) did the rotors need replacing; and, 2) was £70 a reasonable price for that repair.

1) Did the rotors need replacing: Unless you have photos, it's difficult for us on the intertubes to ascertain. Blue-ing is indicative of very high temperatures - likely produced when someone went down a very long, steep incline while keeping the brakes half-depressed. This produces a lot of heat and can damage not only the rotors but the pads and brake housing itself. The heat warps the rotors and also weakens them. 

As for the explanation that "both sets of disks were badly 'burned' through excessive braking with damaged pads" - it would be the excessive heat produced by braking over a long/hard distance that damaged both the pads and the rotors. 

So the tl;dr here is that blued rotors might not have needed replacement if you weren't going to ride them aggressively and they might have been able to have been straightened back into shape. But the bluing weakens them for sure (even if just a bit), and the shop may have felt for liability reasons that it was best to recommend replacement. After all, if the rotors did the extremely unlikely thing by shattering right as you headed towards a lorry truck, you (or your lovely widow/er) would have sued them out of existence. Better to be safe (and make a sale) than be sorry.

1a) Maybe the discs were just scoured? It is possible that the discs were just scoured - especially as we don't have photos. Damaged or badly worn pads could have scoured the rotors - as well as a general lack of cleaning and maintenance. That would explain the black streaks you saw. You should have also been able to feel the scouring with your fingernail.  In fact, given that your bike is a hybrid and not likely to have done aggressive downhilling, scouring is a more likely explanation of what happened even though the words "burned" and "blue" that you used are indicative of blue-ing.

If your rotors were scoured, they'd still need replacing if the noise bothered you since resurfacing bicycle disc rotors isn't worth the money. Even if you decided to put up with the noise, you'd have to replace them when the pads ended their already shortened life as you shouldn't put new pads on a scoured rotor as the new pads will pick up the scouring, shortening their life and scouring the next rotor when/if you ever replace it. 

2) Was £70 (approx USD$85) a reasonable price for replacement? We don't do valuations here, but the base price for a pair of the rotors would be around US$25-50 in 2017 dollars. Bike shop labor is around $70/hr here (and I should note that 'labor' also includes the rent for the shop, purchasing and depreciation of tools and equipment, training, utilities, etc.) and an hour of labor for installing the rotors, truing them, and adjusting the brakes is reasonable.  So, I don't think you were charged an excessive amount. 

p.s. As noted above, I do hope they replaced the pads at the same time as replacing the rotors. This is standard for  both scoured and blued rotors and if they replaced the pads (another US$15-30 cost) then £70 was a really reasonable price.

pps. Scouring is indicative of a bike that wasn't maintained properly, blueing of a bike that was ridden aggressively. In either case, you should perhaps ask the bike shop to do a full inspection to make sure there aren't other hidden treasures.