intermittent fading brake pedal

[Yanny]

2009 Mercury Mariner has recently begun having an intermittent spongy brake pedal. Has 70k miles.
I'm planning on new fluid and bleeding this weekend. Should I be looking elsewhere?

Thanks!

 

[zfasts03]

Probably high water/moisture content of brake fluid and when hot water boils and makes it spongy. I have seen this before with an old Honda Accord.

New fluid, bleeding at the calipers and wheel cyl. should do the trick. At least 1 qt. or more needed of fluid or 3-4 small containers. Suck the old out of the master cylinder to get most out before you begin.

If that doesn't fix it then it is time for a new master cylinder.

 

[Yanny]

Probably high water/moisture content of brake fluid and when hot water boils and makes it spongy. I have seen this before with an old Honda Accord.

New fluid, bleeding at the calipers and wheel cyl. should do the trick. At least 1 qt. or more needed of fluid or 3-4 small containers. Suck the old out of the master cylinder to get most out before you begin.

If that doesn't fix it then it is time for a new master cylinder.

OK - I'm going to guess that the fluid is original. I just noticed in the manual that it should be changed every 24 months. It may actually be 168 month old!

I appreciate your comments.

 

[D60]

Very different vehicle but I'm doing a GMT400 right now where a leaking wheel cylinder caused the pedal to go to the floor one day, then brakes seemed fine the next day.

Or to answer your question, YES, just look for leaks. I initially thought the leak at rear axle was wheel seal until it occurred to me I could not detect that wonderful smell of gear oil. Obviously you don't have a live rear axle, though....

 

[eljefino]

Could also be a mechanical problem, pad sticking and cocking sideways. Should fix sooner than later as it can drag and overheat your brake fluid.

Diagnosis is super easy, take everything apart and see what moves freely.

 

[mattd]

This generation of ford products have a bad track record with internal ABS leaks in the HCU. If you have no fluid leaks this is your most likely problem.

 

[D60]

This generation of ford products have a bad track record with internal ABS leaks in the HCU. If you have no fluid leaks this is your most likely problem.

Can he tell if that plunger moves on the HCU like on bubble bodies?

 

[mattd]

Can he tell if that plunger moves on the HCU like on bubble bodies?

No not really. The way these issues show up is that the pedal fades with slow gradual braking but will stay more or less firm with a quick, hard brake application.

 

[97prizm]

2009 Mercury Mariner has recently begun having an intermittent spongy brake pedal. Has 70k miles.
I'm planning on new fluid and bleeding this weekend. Should I be looking elsewhere?

Thanks!

Remember to bleed the caliper furthest from the master cylinder then as you go.

 

[Yanny]

OK, Problem seems solved. I vacuumed out all the old fluid which looked kinda milky and dark.
The pedal worked fine in the mountains this morning.
Thank you all for your advice! This is an awesome forum!