Help me diagnose power brake issue.

Richard Casto · **Posted:** Tue Sep 29, 2009 3:09 pm

Due to various reasons I have been driving Tina’s Civic recently and I have noticed a low pedal at start up with her Civic in the morning that my Civic does not have (both are 1998 LX models). This morning I pull out of the house and at the first stop sign in the sub-division the pedal goes to the floor. I have to pump it a few times to get it to come up. But once up it is fine. This is by far the worst it has ever been. I drove back to the house and the fluid level is good and a quick visual shows no signs of leaks. I start the car again and the pedal is fine. It seems that it is only an issue when left a long time (such as overnight) and as I mentioned above, it has never been anything as bad as this morning.

I am not a power brake guru, but my first thought is that it’s an issue with the master cylinder. However, I am puzzled as to why I only have this issue after the car has been sitting for a long period. I guess I am expecting it to work or not work, but not some type of in-between. Anyone have any answers? I don’t want to start spending money to swapping out parts and not fix the issue.

Some history on the brakes… I rebuilt the front calipers about a year or so ago. At that time I replaced the fluid. I am pretty sure the low pedal did not exist at that time. I noticed this morning that the seal on the top of the reservoir is cracked. So that needs to be replaced (along with new fluid), but I don’t think that is related to the problem. Front pads still have good life on them. I seem to be unable to wear out the rear shoes on Civics.

Patrice Bousquet · **Posted:** Tue Sep 29, 2009 3:36 pm

On Kathy Honda I took the habit of adjusting the rear drum band each time I do a tires rotation.
If I don't do it the brake pedal goes lower and lower. But like you said the band don't seem to really wear off.
I change the front twice but are still on the original band in the rear.
Might not be your problem but might be worth to have look at them.
Try just pull the hand brake a little bit and see if the pedal is better, if yes adjust the rear band.
There seem to be an automatic adjuster on the drum but it doesn't seem to work at all so this is
why I do it at each tires rotation.

Vincent Keene · **Posted:** Tue Sep 29, 2009 4:34 pm

Richard Casto wrote:

I am not a power brake guru, but my first thought is that it’s an issue with the master cylinder.

The 'power' in power brakes has no affect on pedal travel at all. With or without vacuum assisted boost, the pedal travel should remain constant. The vacuum assist or 'power' just helps you apply less force (leg effort).

Since braking is a closed system and there are no leaks...my guess, it's the master cylinder as you already suspected.

Scott Johnson · **Posted:** Tue Sep 29, 2009 5:16 pm

4 wheel disk or disk-drum?

Do you park on a hill? (seriously).

A bad residual pressure valve could be letting the fluid back into the reservoir. For drum brakes the valve holds some line pressure to keep them from retracting fully. Some disk brakes also use the valves if the master cylinder is located fairly low (thus the park on a hill, putting the master cylinder lower than the rear brakes).

Scott

Richard Casto · **Posted:** Tue Sep 29, 2009 6:25 pm

Scott...

disk-drum, not parked on a hill, master cylinder if high on the firewall.

VK...

As you say the closed system part is why I thought MC, but I am curious why it's acting the way it is.

I like the idea that maybe something is going on with the drum brakes. I usually always park the car (on level) with parking brake engaged, but it may have been off last night and I think this might have allowed the pistons in the drums to compress somehow? When I left work this afternoon, brake pedal was fine, but I also had the parking brake on. I am going to leave the parking brake on tonight to see what they are like in the morning. If pedal is fine, then tomorrow night I will leave it parked all night without parking brake set.

Karl Shultz · **Posted:** Tue Sep 29, 2009 8:03 pm

It's the master cylinder. I've replaced them on a lot of Hondas over the years, all of which were behaving exactly as you've described yours.

Richard Casto · **Posted:** Thu Oct 22, 2009 9:18 am

If anyone is curious it was the master cylinder. I replaced it last weekend and symptoms are now gone.

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Help me diagnose power brake issue.

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