Okay here's the deal. I am sitting at a stop light in my 84 and I have my foot on the brake. All of a sudden my foot starts slowly sinking to the floor and now i am rolling backwards. I pump the brake once, esentially lift my foot and put it back down, and now the brakes are back, the pressure on the pedle is good--like nothing ever happened. I have had the brakes pressure checked, checked the master cylinder, pads and shoes are good, what the heck is going on here? this has happened more then once, one time as i approached a stop light i put the foot down and there is nothing there, pump up and back then they're fine.

I saw a thread where someone replaced his rotors with these monster 13inchers I believe, he said to get rid of "brake fade," is my problem what this guy was talking about? or is that something else? and the hell do i fix this?

 

This type of problem usually points to a bad master cly. Generally they will bypass internally.
Brake fade is from getting the brakes too hot, and they lose stopping power, but not the pedal feel (most times).
Sounds like you need to replace the master cly.

 

As rogue said, brake fade is super-heating the brakes, which causes a layer of gas between the pad and the rotor. It also boils the brake fluid, and can often cause a spongy brake pedal.

The problem you're describing sounds like either the master cylinder is bleeding by, or the rear wheel cylinders (since you said shoes) are leaking. Take off the brake drums and look at the wheel cylinder. If it's not that, then you're culprit is most likely the master cylinder.

Just to check though, check under the dash behind the carpet to see if any brake fluid is there. If there is, it's the master cylinder. If there isn't, press the brake pedal HARD and see if it creeps to the floor. Check for leaks around the brake lines, wheel cylinders, and calipers. WARNING: DO NOT depress the brake pedal without the brake drums on the car, if you do you WILL blow out the wheel cylinders.

 

I've encountered this situation a few times over the past few years. Everytime it has come down to replacing the Master Cylinder. The only counter indication to this would be if you were visibly loosing brake fluid.

 

man i think i have the same problem with my fb i have to press it all the way down for the car to stop . brake pads new, rotors still good brake system bleeded still has bad braking. wen car is turned off i press on the brakes and brakes get stronger but wen driving feels like no brakes at all? damn i guess i need to overual it or replace it

 

I've had this happen on my car as well, just tossed on a new master cylinder, and the brakes were as good as new.

 

You, Sir, need to bleed your brakes...

 

Yeahm the brakes will always feel harder with the car off. It's because there's no vaccum for the brake booster. Since the brakes are un-assisted at that point you can't put as much force down.

Either the slave cylinders/calipers are bad, or you got air in there...

 

Definintely master cylinder. Had the same problem with my '81 GS.

 

i have bleed the system twice already . it would feel fine for about a day then problem comes back

 

Does air keep coming out every time you bleed?

It sounds like you have a leak that's allowing air into the system.

 

seems that way ??? any suggestions

 

Did you bleed the lines coming out of the master cylinder? Alot of air gets trapped there.

start there and then bleed at the calipers.

 

also check for leaks.... crawl underneath and check everywhere!

 

I'd say calipers or slave cylinders are leaking...

 

Had this problem for about six years. In the last two months it got a lot worst. Had the master cylinder replaced. (new) Cost $300.00. Cheaper if you live out of CA. If you want to do it yourself you can get a rebuild for about $40.00. Besure to bleed before you install it. The fluild just by passes in the cyl, it will net loose any. Time about half and hour be sure to bleed the new one before you take the old one off.
Alfred

 

i will go out and say it, we appreciate you searching but its good to check the dates if you are going to contribute advice (this thread is three yrs old)

welcome to the forum

 

Hey guys im bumping this thread!
So i replaced my leaking brake master cylinder today and now theres no more leaks! I followed Rogue_wulff's method from another thread about bleeding brakes by bleeding the MC installed on the car instead of carrying a bench bled unit over paint and trying to place it in the bolt holes. It started leaking out of the ports like it would so be prepared with paper towels or small cups.

Now my question... I bled my entire brake system with new fluid but when I turn on the engine my brake pedal goes all the way to the floor or pretty close to it before I can feel my brakes. When the engine is off... I can pump the brakes and feel the pressure accumulating. Is something wrong with my brake power unit or brake bias? TIA

Oh yes one more thing I almost forgot... When I took out my old MC I heard a noise of air escaping... not sure if this makes any difference with my problem

P.s. Should I do a write up with pics?

 

ok after more reading, i believe my problem lies in improper bleeding. i think i bled the master cylinder not long enough and sucked in more air and incorrectly. i'll have to bleed a lot more now lol. no wonder most people bench bleed their mc before install although i didnt have any extra tubes to route the outlets back into the reservoir. i'll post my results when after i bleed today

 

I had the "internal bypass" symptom happen twice while bleeding the system after it's long sleep. Intentionally cycled the brakes about 100 times after bleed was compllete, and the problem did not recur, so I suspect it was just some settled crud that let one of the cups mis-seal.

But I'll put a cylinder re-furb on he pending list, just in case. Can't be too careful with brakes, especially in SoCal freeway traffic. With the tight fast traffic out here, by the time your foot tells your brain there's something not right, your car has told the guy in front of you "pow!"

 

Argh... I bled the system twice today. First I started with the RR then RL then FR and FL. Then did it in reverse order. I did notice some more air bubbles. I bled the system per FSM instructions.

Still when I turn on my engine my pedal goes all the way to the floor. Maybe there's still air in the MC or lines. Or maybe my power brake unit (brake booster) failed. Or maybe the proportioning valve failed too. I know I have a crack in a line because I still see some liquid accumulating in the front diver side and it isn't fuel.

I think I'll just have to bite the bullet and take it to my local rotary mech.. with some vaseline lol and fix my brake problem and replace all the old original lines too.

I wish I had more time to diagnose this problem but I have to get it moving to resolve other problems with it too.

 

If you've got even a tiny line leak, especially around the MC or proportioning valves, you'll never get a good bleed. System's got to be tight.

 

Theres a nut on the top of the MC below the reservoir, bleed that as well after you bleed the side bleeder screw, if you get one from AA its like 11/16th. Then have a friend pump the brakes 10 times with the engine off, you release the bleeder on the brakes, just enough to get fluid out. If you go more you'll introduce air through the screw and you'll be sitting there all day wondering why air is still coming out.

 

Well my car is a S2 so I don't believe I have a bleeder screw on the MC itself. I think I know what I did wrong. I didn't bench bleed the MC and used gravity. Also I think I sucked in air into the MC too making it trapped there. So the lines probably have to be disconnected at the MC and bench bleed it installed on the car with rags to catch the brake fluid. Then bleed the rest of the system. Oh well... I'll ask my local rotary mechanic what happened to be sure. Also my brake system will get new rubber hoses too because I know one has a crack in it.

Then after this I think I'll be ready for bushing upgrades... hopefully.

 

yeah i had the same issue i changed the master 3 days ago and has been good sense