The scenario:

A few days ago, I started up in the morning and went to pull out of the parking lot. Hit the brakes for the first time and the pedal was spongy. Pumped and tried again, and they caught just fine. Had no further problems that day. Same thing happened yesterday... truck sat at work all day, went to leave and the pedal was really spongy again. For the first 30-60 seconds of driving, the pedal went back and forth from scary-spongy to normal. Then, after that, they were fine all the way home (1/2 hour).

I changed out the proportioning valve about 3 months ago and bled the whole system at that time. I don't see any leaks and the fluid reservoir is still full.

Any ideas on what could cause temporary loss-of-pressure after sitting for 10-12 hours? And why might it start happening out of the blue? Could it just be a really tiny leak somewhere? Thanks!

 

A hardened seal within the master cylinder is what I'd guess. Or maybe it's simply wore out.


If you can quickly stab the pedal and it bites.. then I'd be very confident that's what is wrong.

btw, Seals failing a month or two after a fluid change is quite normal. Especially if the fluid was old and had debris in it. When you flush the fluid the larger particals get trapped in the seal areas and then act as an abrasive on the seals over the next few months.. then they fail.

The only way to prevent this is to either flush the fluid every couple years since the car is new.. or to never flush it.

I choose the first method.

Rich

 

My first assumption was air in the lines, but I must agree with Rich on the seal issue.

I have an entire brake system from the Explorer (4WABS) if you need any parts.

 

Okay, thanks. Don't know the service history before I bought it, but I know most people don't put brake-flushing very high on their priority list.