You don't say what type of pads you used when replacing them but if it were resin pads, the problem could be green fade. Here's a repost of something I posted on this a year or so ago: We get different types of brake fade. 1) Fluid fade. This is when the fluid in the system boils, creating compressible bubbles. The symptoms are (ask Kevin), a lever that at the beginning of of the descent, felt hard but under hard braking became softer and softer until it eventually bottoms out against the bar. If you survive the incident, you may be puzzled afterwards, because the brake will work perfectly again. 2) Green fade. This only happens with new pads. Pads are made from various things bound together by a form of glue (resin). This resin is like epoxy glue. Even when it is hard, it is not yet fully cured and by heating it, it cures a bit more. Unfortunately it gives of gas in this final cure. This gas escapes from the pad as you brake and forms a gas lubricant which prevents you from stopping. It sits betweeen the pad and disc like a cushion. Green fade is the reason why pad manufacturers ask you to break the brakes in first, but applying 20 or 30 gentle stops. If you don't do this, one big braking event could see you fall thanks to green fade. Alternatively, before the pads are completely outgassed, they can still melt and then glaze the discs, which kinda ruins them forever. 3) Pad fade. Various friction materials perform better (i.e. have more friction) at different temperatures. Some work better when cold, other better when very hot. Pad fade is when you exceed the material's intended temperature and it starts to melt. Sintered metal can melt too. resin pads technically don't melt (they sublimate, which is skipping the molten state and going directly to smoke) but this process still gives you brake fade. Sintered metal vs organic pads, vs resin pads. In bicycles, most of the sintered metal pads are actually combination of resin an bits of metal and, organic pads also have some resin in. The common denominators is resin and most pads thus suffer from the bad effects of resin. Sintered metal is just some metal filings or fine metal wire that was heated to just below the metal's melting point and then crushed into shape under high pressure. Metal pads are made from brass and copper and perhaps zinc - all metals softer than steel. A bit of resin may bind them or they can be pure sintered, containing no resin. I doubt that's the case in bicycle brakes but I stand to be corrected. Sintered pads These pads last a long time but are quite aggressive to the disc. They can work at high temperatures but, require more lever pressure to operate. Some discs may not be made from metal tough enough to work with syntered metal pads and thus have a warning on them. Resin pads. Sometimes called organic pads, these are made of fibres - asbestos (now illegal), kevlar, cellulose and other secret stuff - that's all held together with resin. These pads are kind to the discs, require very little effort to brake and smell like Satan's bottom when overheated. When they overheat, they mess up the disc by coating it in hardened goo. They don't like working in the wet since they just abrade away within a couple of stops, given gritty wet conditions. This is not a pure definition, since most pads are a combination of the above two types. In cars, fancy stuff like ceramic and carbon dust is also added to the pads but that's not for here.