Mc Bypassing? Near Miss

Had a hairy road experience on Saturday. I was driving the jeep for a few local errands into town. On the way back, doing about 63 on a slight down grade on a two-lane desert highway I approach a signaled intersection. Just about at the stop or go point the light turns yellow and I step on the brakes. Pedal goes all the way to the floor with zero resistance and not even a hint of slowing. By now I'm almost at the panic stop range of the intersection. Don't know why I didn’t pump, but I didn't. Downshifting like a madman, chirping the rear tires, at what must have been about 15 MPH I grabbed the parking brake and pulled real hard. Stopped a few feet into the intersection realizing I had just missed what could have been a horrible experience.

For the back story. With added weight and larger tires the brakes were inadequate. A couple of years ago I converted to power disc/drum, hanging pedals and hydro clutch. Those 11” rear drums with parking brakes each side and lever handle between the seats saved my butt this weekend. Life’s priorities got in the way and the jeep sat. An occasional run around the block exhibited a soft pedal now and them. A mechanic friend drove it and said the master cylinder was bypassing. After I moved I ordered another MC and installed it. Same issue. So now I’m wondering what it could be. A second new/reman MC is bad or is it something else? What could mimic a bypass?

 

If there is no fluid leaking out anywhere, a leaky MC can really be the only culprit. New parts aren’t guaranteed to be good parts. Glad you’re ok.

If it’s the single circuit system, it might be a good time to upgrade. If it’s a double circuit, something is definitely faulty to go flat to the floor with zero braking.

 

That kind of thing was more or less normal when I was young. Why we called them "emergency" brakes - not "parking brake." (Harumph...)

Will the pedal "come up" with rapid pumping? If so, it may be air in the system, or a maladjustment at one or more wheels. Lost pad at one caliper, or a stuck wheel cylinder suddenly released, or...

The m/c should have a double set of seals and also might pump up if just one is leaking.

 

Sounds scary. I lost steering in an older k5 blazer on the highway and ended up heading into oncoming traffic. Luckily it was an emergency search and rescue vehicle and I had lights on but the minivan I almost hit as I went into the ditch on opposite side of highway was to close for comfort

 

I remember on the 66 Wagoneer, my dad going through a stop light like a dream. Holding down the horn wide open with no slowing down and the pedal to the floor. I was about 12. Fortunately, everyone stopped as we flew through the intersection.

I tell my son he doesn't know how good he has it with the new brakes. I always leave more room to stop than he does, because I have been in older vehicles with brake failure.

11in brakes will be going on the 64 CJ5, and I will look at doing the dual MC also.

Glad you got it stopped safely.




Dave

 

It is a dual circuit system. 1" bore Corvette style MC w/ 8" booster. The MC (both times) was bench bled and installed. Put a level on it and made sure every last bubble burped out the compensation ports then bled all four corners. I have speed bleeders and usually go well beyond what appears good.

The booster works so well there is no effort. To the point I have considered plugging the vacuum and give it a try. With engine off pedal is rock hard and no sinking over time. When running the lack of braking effort bothers me, but it's still a firm pedal. I like some feel. While not using it I have made a point of "driving around the block" monthly to keep things lubricated. That is when I first noticed the problem on a rare occasion and it prompted me to change the MC, but it continues.

It will stop the jeep on pavement so quickly it feels like it'll rip the axles off. It's just that 1 in a 1000 that it doesn't and that's 1 time too many.

I'll likely bleed it again just to eliminate that possibility (I've screwed stuff up before). I still have the first MC and just may buy a kit for it and swap them out. I know how questionable parts quality can be these days, but gotta have good brakes.

 

Just curious… Have you mysteriously lost brake fluid? I.e., have you checked your fluid level since this happened?

A leaky booster can suck fluid into the canister through the plunger, and you’ll lose fluid without realizing it because there’ll be no puddles.

 

I still have the ingrained habit of lightly pumping the pedal, "just to check", as I get off the highway or approach a stop sign.

 

x2

 

That's where my money is.

 

Fluid level is normal and I probed the booster and it is dry.

What I don't understand is if there is air in the system the characteristics would be consistent and not intermittent as I'm experiencing, right?

 

Yes, unless you have a brake line that is too close to an exhaust component. That section of fluid inside the line will boil and create a vapor pocket. It will force some fluid to the path of least resistance (back into the reservoir), and the next time you hit the brakes, you’ll be compressing that air bubble rather than applying brakes. Check to make sure your clearances around the exhaust are adequate.

 

Intermittant failure has been my experience when a m/c seal is failing.

 

Tha

That's your destiny, man

 

It happened to me with, long ago, with a Norton "model sport" ... A good way to test one's luck.

 

Two things come to mind. Ime front calipers need pulled off, a block of wood inserted and the bleeders pointed straight up. You can bleed them multiple times on the bracket, get straight fluid and still have air. Second the reproduction fixed proportioning valves are notorious poor parts and leak.