Hello, i’ve been having problems with my brakes for a while now. When I first bought the bike used off of someone almost 2 years ago the brakes were fine, I left my bike out during the summer with my dog and she bit the hydraulics so I ended up having to replace them. After replacing the brake hose and bleeding the brakes I realized that the braking was very weak from both front and rear, I later went to the bike shop and had them bleed the brakes for me since they are better with bikes than I am. I go ride my bike and the brakes are still very weak, after a while of leaving it out in the heat and humidity the disc was rusty so I later ended up replacing the front and rear disc with a 180 mm shimano disc and I also bought new metal pads, at first when I tried it out the brakes were very strong, I would be able to brake so hard that I would launch myself over the bars, it was insane. Now this previous summer I went outside of the country and never got a chance to take my bike with me because the airline didn’t let me, my bike stayed in a box all summer and now that i’m back and assembled it, I realized that the brakes were very weak and I also noted that the brakes were very dirty somehow. I cleaned the brakes with alcohol and I thought that would do the trick but the brakes are still weak. I don’t know whether it’s because there is air in the system or what so I came on here looking for help since I don’t want to have to replace my whole brake system. The caliper is a shimano deore.
Brakes that sit for long periods of times (especially when stored in the elements and with big temperature swings) need to get cleaned and bled. Using your brakes just once with corroded rotors can ruin the pads and the rotors as well.
I just started mtb’ing but I’ve been a mechanic all my life. I can’t speak for mineral oil brakes, I’ve never worked on them. Brakes are a sealed system and if it’s a dot4 or dot5 system it doesn’t matter how old it is or if it sits in snow or the desert sun. They are silicone fluids in a sealed system. They are designed to withstand huge temperature swings because they are used in race car brake systems. My guess is as long as it didn’t get over 400 degrees, you should be good.
If your dog bit through the lines. Then then insides were exposed to the elements. Especially bad if he bit right close to the caliper. My guess is you have a seized caliper piston.
I recommend removing the front caliper and hitting the brake lever. This should pop the piston out. Once out inspect the sides, there should be very smooth. Any rust, pit mark, lip or edge will hinder piston movement. Also peel out the o-ring that seals around the piston and check that out. Inside the piston bore should be spotless clean. Usually you can find rebuild kits for calipers.
You can do one piston at a time simply by holding the other piston in with a clamp or some channel lock plier and hitting the lever again. I’m sure you can find YouTube videos about caliper rebuilds. It’s not difficult at all. Just tedious.
I think you took my comment to mean temperature affecting the brake fluid. I was talking about the brake system itself. It is a “sealed system” because of seals which are effected by long storage periods and temperature. Bicycle brake systems aren’t nearly as robust as automotive. A stuck piston on a bicycle brake should be easy to spot since the rotor and caliper are not floating; it would push the rotor visibly to one side.