Rear brake pad install woes

[volks28]

Just put a pair of new rear brake pads. this is a 850 GS 1983 , rear discs. Had to compress the pistons a bit

Went for a ride, they brake alright, but they rub way too much

Since I know a little car mecanic and done brakes many times before, I figured it's probably because the brake fluid needs to be pushed back, so I opened the reservoir to let the "air out"

Now seems to work fine. Does this makes sense? Do I need to open the brake fluid reservoir when servicing brakes?

[argonsagas]

In a word, yes.

You did not have to let the fluid be pushed back, but you did need to remove the compressed air in the master cylinder that was creating the pressure that was pushing the fluid forward, and thus pushing the pads against the brake disk.

In every case, as brakes wear, there is a tiny vacuum created in the main cylinder as fluid moves from the cylinder into the lines. This happens due to the fact that space is left behind when the fluid is moved into the lines. If there is a vent, or a leak in the seal, then air is drawn into the cylinder to fill up the space formerly occupied by the brake fluid.

In a car or truck, there is no seal, and this is what is supposed to occur every time.

On a bike, you may have a tight seal in the main cylinder, or one may form over time, that prohibits the exit of air that needs to escape when you push back the pads. The compression of the brake cylinder pushes back the fluid, and it has only one place to go...into the main cylinder, and that means any air above the fluid becomes compressed.

If there is no means of escape for the air then, once you complete your brake pad replacement, you still have the compressed air in the main cylinder. It is not a potent force, but it is persistent and it will slowly exert itself against the fluid, and that pressure will transfer to your brake cylinders, and push the pads against the disk with sufficient thrust as to restrict rotation of the disk, and thus the wheel.

[volks28]

This board is great!! Thanks for the info, it makes perfect sense.

does bleeding work the same as cars? pump, pump, pump, open valve, repeat?

In a word, yes.

You did not have to let the fluid be pushed back, but you did need to remove the compressed air in the master cylinder that was creating the pressure that was pushing the fluid forward, and thus pushing the pads against the brake disk.

In every case, as brakes wear, there is a tiny vacuum created in the main cylinder as fluid moves from the cylinder into the lines. This happens due to the fact that space is left behind when the fluid is moved into the lines. If there is a vent, or a leak in the seal, then air is drawn into the cylinder to fill up the space formerly occupied by the brake fluid.

In a car or truck, there is no seal, and this is what is supposed to occur every time.

On a bike, you may have a tight seal in the main cylinder, or one may form over time, that prohibits the exit of air that needs to escape when you push back the pads. The compression of the brake cylinder pushes back the fluid, and it has only one place to go...into the main cylinder, and that means any air above the fluid becomes compressed.

If there is no means of escape for the air then, once you complete your brake pad replacement, you still have the compressed air in the main cylinder. It is not a potent force, but it is persistent and it will slowly exert itself against the fluid, and that pressure will transfer to your brake cylinders, and push the pads against the disk with sufficient thrust as to restrict rotation of the disk, and thus the wheel.

[gremlin]

does bleeding work the same as cars?

yes just the same as a car. befor you start bleeding your brakes when was the last time you replaced the brake fluid. if its been a few years you mite want to pull the caliper and clean them. brake dust creeps in and starts making the cylenders harder to push and pull.

Ps you need to do the same thing to cars and trucks about every two or three years.