Hi everyone, recently, I noticed that while driving that the brakes felt a little "off", and what I noticed was that it seemed that more travel than usual was required to slow down the same amount. The problem occurred a couple times, but wasn't going on all of the time. Before I had a chance to get a good feel for what is going on, my radiator went bad, so after I replaced the radiator, I figured I might as well bleed my brakes (done with Techstream). Once I was back on the road, I haven't felt like the brakes have needed excessive travel to stop effectively. Now, I have noticed that if I am at a stop (such as a red light), after say 30 seconds, the brake pedal slowly starts sinking and requiring further travel to hold the vehicle at a stop. If I take my foot off the pedal and pump it, it will keep pressure for a while, but eventually loose it again at a stop.

I'm now trying to figure out what might be causing this, it doesn't seem like air since there is a fairly significant delay between initial pedal action and the change in pedal feel. According to the service manual, the suspected areas for "Low pedal or spongy pedal" are:

1. Fluid leaks in brake system
2. Air in brake system
3. Piston seals (Worn or damaged) (Front)
4. Piston seals (Worn or damaged) (Rear)
5. Master cylinder

For #1, I have not visually located any leaks around the brake area, and I have been regularly checking the brake fluid level, which has not dropped. #2 is possible but it seems strange that would cause this behavior, plus I bled the brakes very recently. For #3, #4, or #5, I am not sure how to troubleshoot these. I can't seem to find a good procedure to determine if the issue is related to a specific caliper or the master cylinder. Just curious if anyone might have any suggestions of what to check or any ideas. Thanks!

 

A sinking brake pedal that can be pumped up usually means you need a new master cylinder. A leaking caliper would not feel that way, and would probably leak at the same time. You have not lost fluid.
You may want to bleed the brakes again to absolutely rule out air in the system before throwing a master cylinder on it.
A less likely failure would be the vacuum assist or a leaking vacuum hose on it.
How did you bleed them? Did you use a one man vacuum or pressure bleeder? Or those fast bleed brake bleeder valves? Or the old way with a helper?

 

I'll check the vacuum hoses on the brake booster and try bleeding the brakes again.
I bled the brakes the old way with a helper using Techstream to go through the process that includes the ABS system. I do have a vacuum bleeder that I may use to do it a second time unless there's some problem with that.

 

Well, vacuum hose looks good as far as I can tell, rebled with the vacuum bleeder, no difference, put a new master cylinder on it and rebled with Techstream, it's maybe a little better, but the brake pedal is still sinking. Not really sure what to try next at this point. Might just need to have a professional look at it.

 

Did you use an OEM master cylinder?

 

Yup, OEM from the dealer.

 

Interesting. If it was me, I would change out the booster vacuum hose (44773-06090) and the check valve/grommet as a final measure before taking it to a shop.

 

Sounds rudimentary but double check all of your pads for wear..

 

A vacuum leak anywhere on the engine could result in this issue, not just a leak in the vacuum hose going to brake booster. Too, the booster itself has a life span, and yours may be up! In other words, the vacuum leak may be happening inside the the booster.

 

Hello Xane,
Were you able to find the root cause to the brake pedal slowly sinking when holding it at a stop light?

thanks

 

Sorry to thread-jack here but I don't feel it's worth a new one.

Recently when I initially start my car after it's been sitting overnight the brake peddle is unusually hard to depress. The car still starts as usual and the pedal seems normal from then on but it reverts back to that same state daily.

TIA!

 

Nothing out of the ordinary for me, personally my car has always been like this. If it starts to feel too stiff during normal driving then the brake booster could be going out.

 

^ Find a video on how to test the brake booster. My car has a brake booster leak, but not enough where it's a major concern. It was hard to tell but I think mine has been replaced before. I couldn't really find any threads on anyone who had this issue when I discovered it like 1.5-2 years ago