Contour Enthusiasts Group Archives Forums Modifications Brakes Soft brakes - why?

My brake pedal is a bit on the soft side. This has happened slowly, over the years. I have fresh fluid, SS brake lines, and new pads. The pedal is firm, but does leak down ever so slightly.

Any guesses as to what is going on? Could a cylinder in a caliper be leaking? I replaced the rears with used units recently, and the fronts are not original, and seem to be on good shape. Or, could it be a master cylinder issue? Or something else? It really feels like there's air somewhere, but I've bled and flushed multiple times. Is there someplace air can stick in the system, like the ABS system? It's not horribly soft, and the clips are on the calipers right, so it's not one of the common problems. What else could give the pedal it's soft feel? I drove Doug's SVT the other day, and the pedal seemed to have a slightly better feel (I think he has SS hoses). There was much more grab, which probably comes from better pads, and the pedal didn't travel so much. There seemed to be a similar leakdown, which most cars seem to have, so that doesn't worry me.

Sounds like the master cylinder to me from your description.

I just remembered something. I may have run the master cylinder dry when I installed the SS hoses. I thought there was a little fluid left in the resivour, but it was close. Reading old posts (after searching for master cylinder) I feel that might be my problem. So how do I bleed (prime) the master cylinder?

From the brake bleed how to on the mainsite:

IMPORTANT NOTES:
DO NOT LET THE FLUID RESERVOIR RUN DRY!!!! Doing so requires a trip to the dealer to get your master cylinder primed or a new master cylinder all together.

You may be over reacting.

Yes, make sure you do a very thorough bleeding so you are sure that all air is removed. But be aware that on any hydraulic brake system you can force the pedal to the floor if you try. If you are not aware of that and knew that you had perhaps ran the master cylinder dry, you may be picturing that something is wrong when it is OK.

When you damage a master cylinder from running it dry, the problem is obvious, not subtle.

Also, you will have a slightly softer pedal with new pads until they seat in.

Crap. So what else could cause it to be soft, then? Could it really just be low-torque pads and small rotors? It's certainly not a very bad pedal feel, it's just something I'd expect on a Corolla. My freind who owns an E36 3-series commented on it as soon as he drive my car, and was shocked to hear that I have SS hoses and clean fluid.

If the pedal continues to leak down after you apply it, then there is likely something wrong somewhere.

There are a number of possible issues that could cause this.

1. Master Cylinder rear seal leak. If the rear seal of the master cylinder is leaking, it can allow the pedal to continue to drop. The fluid leaks, but it leaks into the brake booster, so you don't see it until a ton of fluid has been used up.

2. Master Cylinder internal leak. If one of the internal seals is leaking int he master cylinder, it can allow fluid to leak back from the pressure side back into the reservior. The only real way to fix this is to replace the master cylinder. However, don't jump to the assumption that your MC is done just yet.

3. Other Leak. Obviously, a leak somewhere else can cause this too. Inspect the system thouroughly looking for damage (people should be doing this occasionally anyway . . .) and signs of brake fluid. If you see something and aren't sure about it, clean the area up really good w/ brake clean, and drive it around some more, and see if there is any sign of fluid. This likely isn't your issue though.

4. Air in the lines. If there is any possibility of air having gotten into the system, through any one of a hundred different ways, it can cause similar problems. Start here, and do a very thourough bleed of the system and use a good fluid. The Ford HD fluid (from the dealer) is easily the best bang for the buck in terms of performance. If air has entered through the master cylinder, prepare to do an extra thourough bleed job. One thing that helps to get trapped air out of an already installed master cylinder, is a vacuum source applied to a modified reservoir cap. Otherwise, you can bleed the master cylinder (its messy in car) one circuit at a time, by loosening the fittings just like you would a bleeder screw. This usually works for any car that doesn't have extra bleed screws in the master cylinder from the factory.

Okay, maybe leakdown was a bad description. Even if I stand on the pedal all day, it'll never go to the floor.

I just went for a drive and learned this:

There seem to be two distinct zones of pedal travel. First is a couple inches of long, mushy pedal. If I pump hard and fast it'll get a little firmer, but as soon as I press it slowly again it's soft. Then it hits a wall. It actually feels like it's hitting a rubber stop - except it's far short of the travel you get when bleeding. I can still press down into that travel, but we're talking about stand-up-and-pull-on-the-steering-wheel kind of effort. The brakes lock up (or get into the ABS) during the mushy travel.

BUT... the pedal was very firm when I started the car up today. It's been sitting for almost a week. It didn't have much travel, and grabbed quickly. This went away as the pads warmed up. The brakes do grab and stop very well. The new Ford pads seem to work well. But could they suck horribly as far as feel, and give me this squish? The hard pedal never came back, even after sitting for 10-15 minutes, but the brakes never got cold again.

I got some brake fluid while out testing, so I'll flush it out just to be sure. I remembered I wasn't the last person to bleed it, and it's been since December anyway. I'll get messy and bleed the MC while I'm in there, if I can find a helper.

Any idea what causes this two-stage pedal feel? Is it possbile that the pads are soft enough to give that kind of feel? If not, what else has give in the system?

Oh, one more thing. With the engine off, the pedal (when pedal prssure is applied) sinks to, and rests on that transition to "hard feel". Pumping a few times gets it hard, and eliminates that travel: it's very similar feeling to the place it was, but at the very beginning of the travel. If I hold the pedal like that and start the car, it sinks down and feels just like always as soon as vacuum build. Could the play/leak/air be in the brake booster? I'm off to try to learn exactly how a power brake system works, because I don't really know what's happening here.

You have somehow introduced air into your master cylinder. You can bleed it yourself by having someone pump the brake pedal ten times and have them hold it to the floor and then break one of the seals of the hard line using a flare wrench. Retighten the nut and repeat twice. Then do it for the other hard line.

If you still have the two-stage pedal feel, then the MC is toast and will need replacement. CarQuest sells reman-ed MCs for just under a c-note, and includes instructions on bench bleeding it.

Have fun getting the reservoir off if you need a new one. It's a [censored]. I used a cotter pin on reinstallation instead of Ford's stock pin.[pollstop]

Crap. Well, hopefully it'll be fixed by a good bleeding. Howstuffworks.com has some good animations on brakes. Do I have air in one-half of the MC, and when I hit the other side it gets hard? That would make a lot of sense... I'm not just looking for a fix, I want to know exactly why and how this happened, and how the system works.

http://auto.howstuffworks.com/master-brake1.htm

That page has an example of what happens when there is a leak in one half of the sytem. I've had a brake line blow out, and it does not feel anything like that. But air in one half of the cylinder would make a lot of sense as far as the two-stage feel goes. Is that picture accurate, or is it over-simplified?

Sorry to be putting you through your paces, today, Rara.