Spongy brakes after brake pad swap - moisture or bubbles?

Question

I changed the rear brake pads on my BMW last week, following the steps outlined in this video.

Unfortunately the brake pedal now feels spongy. The pedal feel was firm prior to the brake swap, so it is definitely a sudden change and not a gradual degradation of brake fluid.

One thing I forgot to do was undo the brake fluid reservoir cap before pushing the piston calipers back with a C-clamp, so I suspect that the the sponginess is due to trapped air in the system since the reservoir was never exposed to outside air.

Or is it possible that the extra pressurization of the system via the C-clamp resulted in moisture absorption? The only issue I see with this reasoning is that the system was never exposed to outside air.

Answer 1

Two things to look for.

1) air in the brake fluid.

2) incorrectly assembled brake pads, especially the anti noise shims.

Air in the brake fluid is the most common cause of low, spongy brake pedal feel. Moisture in the fluid will not substantially change the pedal feel until the fluid temperature exceeds the boiling point of any water in the fluid, then the pedal feel gets very soft. Brake fluid absorbs water if exposed, so the systems are to remain sealed. Brake fluid older than three years or any color other than that of tea should be flushed.

Improper assembly of the brakes can cause the pistons to be pushed back too far into their bores when the brakes are released. this cause a longer than normal stroke of the brake pedal before resistance is felt.

Also new brake pads installed without surfacing the rotors can result in more pedal effort required for normal braking.

Answer 2

As stated prior, if the rotors weren't machined or lightly surfaced when the pads were changed, that will give you a spongy feel as you are not stopping as effectively and takes more effort to slow down.

With new rotors and pads, the machine marks on the rotors help to break in the rotors and pads together. Once broken in, they will work well as they wear together.

If my rotors have no runout (warping) when I change pads, I will use an abrasive disk on the rotor to scuff and break the shine on both sides. Do not take any material off. The rough surface will help mate and break in the pads. After changing them, I take the car out on a short run and run it hard, then stop fully hard at least a half dozen or more times. This helps the break in... while you may initially smell the pads when heated, that will go away.

Answer 3

If the brakes are always spongy, it is air bubbles.

If the brakes are only spongy when hot, it is moisture.

Answer 4

Pedal feel is always softer until the pads have bedded into the rotors properly.

Make sure that you follow a proper bed-in procedure before deciding that you have a problem.

Answer 5

I have a older Cherokee that some how pulls in air when the caliper pistons are pushed in for a pad replacement. It does not make sense but I always have to bleed them after a pad replacement. I have tried opening up the bleeder while pushing it in and it makes no difference. Also, Promasters do the same thing or the factory ships them with tons of air in the system.

I'm inclined to think that while the piston is being moved in the cylinder,it is also forcing another piston in the system with it which has some inertia so it keeps on moving after the caliper piston stops. This would suck air in past the caliper piston.