Syonyk

First, my brakes work fine, and lock the wheels up on dry pavement (though that's not saying much - my tires are all season high treadlife crap that don't stick at all).

However, the brake feel seems to leave a bit to be desired. The brake feel I've seen on other cars (both that I own and that I've driven) involves the pedal traveling smoothly all the way down, with the brake force increasing more or less linearly as the pedal descends.

On mine, the top inch or so of travel is mostly dead (though it does hit the master cylinder - it doesn't take much to stop a slight roll on a slope) under normal braking, then the brakes start to dig in. Somewhere around 1/2 travel or so, I feel the brake pedal "make contact" with something. Beyond this point, the brakes still work, and I can still press the pedal down, but it requires a LOT more force. The slight bit of dead at the top doesn't bother me too much, since I have fairly crappy pads on that likely compress a bit much, but the pedal feeling like it's hitting something and then taking more force to move strikes me as somewhat strange.

I have two theories as to why this is: 1. Broken brake booster (or at least not functioning quite right), and 2. Faulty master cylinder (not properly pushing both pistons down).

I also noticed last winter that my master cylinder was leaking in the cold. I went through about a half quart of brake fluid during the winter, and it appeared to be leaking out the back of the master cylinder into the brake booster. Once it got warm, the leak stopped, and it hasn't leaked since. I've heard of master cylinders leaking in the cold as the seals contract, so I wasn't too worried about it, but I'm wondering now if this could be part of the problem (or point to something that would cause the problem).

I suppose I'm not *too* concerned about this whole thing, because I'm planning on doing a 5 lug swap here in the next few months (and getting the master cylinder, brake booster, and adding SS brake lines), but if it's something easy I'll fix it.

Thoughts? Ideas?

-=Russ=-

homebrewer

At best based on what you said, you need a new master cylinder. At worst, a new master cylinder and power brake booster.

Why you would risk your life and not replace a leaking master cylinder when sighted beats me. Please don't drive in back of me until you get your brakes repaired.

Tournapart

iTrader: (4)

sounds kind of like you have a bubble in your brake system, i know i may be wrong but have you tried bleeding the whole system first, seems cheaper to try that than to start buying expensive parts.

Tournapart

iTrader: (4)

oh, and home brewer, not everyone can afford to just go out and start buying parts left and right, first they need to know what the problem is, and secondly someone told him that master cylinders sometimes leak in the cold, which ive never heard myself, but i live in california, doesnt get that cold here.

Syonyk

I've bled the lines going to the calipers (multiple times), but I haven't bled the master cylinder specifically. I suppose I could run another pint or two through the system & see if it changes anything.

As for not replacing the master cylinder, I kept a very close eye on it. The car sat for most of the winter (largely due to my inability to get it out of my parking lot - we had quite a bit of snow), and it just slowly drained brake fluid. I filled it up every two or three weeks, but I didn't ever actually succeed in getting it out of the parking lot. Once the weather warmed up, the leak stopped completely. I haven't had to add any fluid at all. The brakes work fine, and consistently will lock the tires up on dry pavement at speed.

-=Russ=-

WAYNE88N/A

If everything is installed as it should be, if you're bleeding the calipers, then you're effectivelt bleeding the MC too...

The above statement goes out the window if anything's leaking, though, especially at the MC...

I'm thinking you probably saturated the diaphragm inside the booster with fluid, and it's starting to tell you so

Syonyk

Hm. I suppose I'll pull the brake booster off the parts car before I get rid of the chassis, then. Any tips? I understand they can be a pain to remove.

-=Russ=-

Syonyk

Well, bump for updates. It's my thread, and a direct continuation of the topic, so I figured I'd post updates for those searching.

I pulled the master cylinder & brake booster off the parts car.

As it's been getting cooler, the master cylinder has been getting worse. I don't drive the '7 that much right now (motorcycle is running again), so it wasn't something that I was regularly noticing.

I took it out to get something a few nights ago, and noticed the brakes were quite strange. Hitting them sharply or normally worked fine, but if I depressed the pedal slowly, I got zero stopping force, and the pedal would just drop. It came really close to the floor and still wasn't doing anything. Not an enjoyable feeling. Anyway, I got home and proceeded to redtag the car until I fixed it.

The reman master cylinder came in today (O'Rileys), and I didn't have anything going on tonight, so I installed it. I also swapped the brake booster out, because I wasn't so sure of the old one. After pulling the old master cylinder off, I noticed that it was a reman unit as well. Interesting.

Removing the master cylinder isn't too difficult, but removing the brake booster seems to require removing the rear coil & coil mount. Also, there doesn't seem to be a good way of getting the nuts off from inside the car, short of laying on your back half in the car. Painful after a while.

I bench bled the new master cylinder without a bench - I just held it and pushed the back with the end of a socket driver. I'm not sure how far you can safely push it in without causing problems, but I wasn't pushing it more than 3/4" to an inch - I've heard you can have serious problems if you push it in too far.

I did NOT bleed the entire system yet - I just put the lines back on and tested it. Bleeding the 4 corners is tomorrow's project, because it's late tonight, and I also seem to recall breaking one of the bleeder screws at one point (and I don't have a spare laying around). I also didn't really mess with the pushrod adjustment. I figured I'd see how it was, and adjust it if needed.

Even without bleeding the last bit of air out of the system, the brake feel was significantly improved. The pedal no longer has the dead spot, and while I wouldn't call it insanely firm, it will lock all 4 wheels up significantly higher in the travel. Modulation is also significantly improved, though the limit right now is the rubber (cheap all season stuff).

I'm planning to bleed the rest of the system tomorrow, which should hopefully improve feel a bit more. I'm also considering SS brake lines, but there are other things that are needed before those (like a new set of coolant lines).

Also, when I got back, I jacked up one of the front corners and spun the wheel. With the pedal actuating so high, I wanted to make sure that it wasn't staying applied. The wheel spun freely, so I don't think the brakes are hanging at all.

Anyway, it's better now. MUCH better.

keywords: master cylinder brake replace change swap replacement

-=Russ=-