Brake pedal likes to go soft randomly?

[Loghead]

So I just got done putting in a new fuel pump, fuel filter and new lines and when I went to take the car out for a drive to make sure it was all good to go, I noticed my brake pedal sunk to the floor. After pumping it again after that it was fine. I put it back on the rack and made sure to triple check that I didn't some how mess up one of the brake lines and didn't notice anything wrong. I drove home and on the way home (which thankfully is only 3 minutes away and all country roads) my pedal decided to sink down again, now I obviously don't want to drive it anywhere until i figure this out. I'm thinking this might be the master cylinder on its way out because I've replaced all the brakes and made sure to bleed them, but when the cars off I don't lose pressure at all. I'm just curious if there is anything I'm missing and if not what are some pointers for putting a new master cylinder in, I've read the chilton manual on what to do but any other tips would be awesome.

[Leung]

if you have read the manual, you ought to be good to go. Just make sure you bench bleed the MC first before putting it in and then go around starting at the farthest corner and start bleeding.

[deevergote]

It's most likely the master cylinder. Replace it, bleed the system properly once again, and all should be good. Bench bleeding the master cylinder would be helpful, if you're able to do it. If not, bleeding will just take a bit longer.

[Loghead]

alright i figured as much, luckily the new master cylinder isnt going to kill my wallet like the new fuel pump lines and all that fun stuff. bench bleeding is just running lines from the cylinder back into the resivior right?

[Ely585]

alright i figured as much, luckily the new master cylinder isnt going to kill my wallet like the new fuel pump lines and all that fun stuff. bench bleeding is just running lines from the cylinder back into the resivior right?

Yes it is.

[Nicefeet]

I would check your front wheel bearings. If the wheel bearing is shot the extra slop allow the rotor to push the brake pad back in making the pedal feel randomly soft at times.

[deevergote]

The caliper clamps to the rotor, and the caliper is fixed. I don't see how a bearing would affect brake travel at all.

[oyajicool]

If the axle bearings are worn, wheel will wobble as rotate. This wobble will push pads apart. With pads pushed apart and you are to brake, it will require extra travel of brake pedal to take up that much of space. If you pump the pedal the second time, that much extra travel is not needed since the first application of brake did that job.

I am not sure how Accords' axle bearings are set up. But I had similar situation like above on other form of transportation I was working on many years ago. At that time, other techs and the boss (back then) didn't understand what was going on and I got in the heated discussion which ended in 4 letter words, from my own boss!!

I quit that place and opened up my own shop after that,,,,,

[quaidum]

The caliper clamps to the rotor, and the caliper is fixed. I don't see how a bearing would affect brake travel at all.

actually deev the calipers are floating not fixed but i see what you mean, the bearings should play no role in this what so ever

[wildBill83]

Sounds like air in the lines, or MC failure ... do you have ABS? Have you also bled it properly and checked its operation? My 93 has two reserviors to bleed as well as extra points introduce air into the lines if they have failing caps or seals. How old is the fluid? Be sure to never use an open bottle of DOT4 fluid and change your fluid every 3 months (more in humid areas), it collects water from the point the bottle is open and steam or water in your lines can cause this issue as well.

[oyajicool]

I don't know which statement is true and which is doubtful.

[quaidum]

i can almost guarantee it is a hydraulic problem not a bearing problem

the experience i have had with wheel bearings being bad is that, the rotor would move slightly and rub against the caliper bracket it never affected brake pedal feel though. it would vibrate slightly when braking but not terribly, nothing to affect pedal pressure.

[oyajicool]

I, too, think the OP's issue is due to faulty brake master cylinder.

The statement of affect of loose/worn bearing to brake pedal travel is real for all forms of transportation if you ask me. It is more evident on certain brake system than the other. On cars, one master cylinder is working on four slave cylinders. You may not be able to detect this happening just by the feel of pedal. But, it does not mean it is not happening.

[Loghead]

I'm sure there is no air in the lines because i have bled them more than enough to get any air out from when i redid the brakes all around. I picked up a master cylinder at work today so I'm just waiting for some time to get it bled and put it in and then once again bleed the brakes all around. At this rate i should have some amazing stopping power by the time im done haha aside from the booster and new lines everythings going to be brand new

[wildBill83]

Do you have ABS and have you verified it is working correctly? I asked earlier and never heard back.