Mechanics

HAD ONGOING THREAD, RE: BRAKES FADING AFTER STEEP CLIMB & RELEASE ACCEL. PEDAL.

1990 Geo Tracker • 200,000 miles

Can't find it, in e mail. Suggestion was master cylinder. Did pedal pressure test. No fade. Next step was press pedal, as I reached crest of hill, b-4 releasing accel. No fade. Next step, going back to vacuum leak, will be spray ether in area of hoses. I could have missed a crack. Other theory: Could there be some kind of vacuum "switch" that is supposed to react to changes btw accel. Vacuum & regular & isn't? Any futher input? The brakes work fine, except after a steep climb.
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Mack
May 3, 2012.



Do you mean the pedal gets hard to press? That isn't brake fade; that's loss of power assist. When accelerating or under load, engine vacuum goes way down so there is a check valve in the rubber hose going to the power booster. The booster will hold enough vacuum for two or three pedal applications. To test it for leaking, let the engine idle and check that the power assist is there. Next, stop the engine, wait five to ten seconds, then press the pedal a few times. It should be easy to press at least twice. If it's already hard, the check valve or booster are leaking. Use a hose pinch-off pliers while the engine is idling, then try it again. If you get two power assisted pedal applications now, the check valve is leaking.

Caradiodoc
May 3, 2012.
Nope, once I realease the accel. The pedal will floor, for 30sec to a min, unless I press pedal b-4 releasing accel. As the cat in the cartoon said, "That sounds logical." I'll try it. Thanx, Mack

Tiny
Mack
May 3, 2012.
Sorry, I'm not following your description. The brake pedal goes to the floor? The brake pedal is hard while accelerating, then there's no resistance and it goes to the floor after you release the gas pedal? How are the brakes during normal, slow driving? Is the brake pedal firm and high or does it go too far to the floor?

Caradiodoc
May 3, 2012.
Brakes work fine, all other modes of driving. If I go up a steep hill, when I release the gas, I have no brakes, for about 30 sec+.
If I depress the brake BEFORE I get to the top, when I release the gas I have brake. Been driving 49yrs & wrenching longer than that & NEVER saw this b-4.

Tiny
Mack
May 4, 2012.
"No brakes" can mean different things but if the pedal is hard to push, that's a loss of vacuum. If the pedal goes to the floor, I'd suspect the brake fluid level is low. You might try looking for a kink in the vacuum hose going to the power booster. Perhaps it gets restricted when the engine rocks from pulling up a hill.

There should be a low fluid warning light but even if that isn't working, if you look at the design of how the reservoir feeds the two sections of the master cylinder, there is enough above the pistons that going up a steep hill shouldn't matter. If the fluid is that low, you should have the same symptoms all the time, not just on a hill.

I'm trying to think of other common causes. What exactly is the symptom when you say "no brakes"?

Caradiodoc
May 4, 2012.
As in pedal goes WAAYY down w/ no braking effect. Reservoir above the master cyl is full, unless I'm getting a false level & I don't know how, as it is a straight drop in m. Cyl. Checked hoses & saw nothing. 1 puzzler is that depressing pedal, b-4 crest of hill, causes brakes to work.
This came on, suddenly, & I haven't been able to correlate it with an event. Just 1 evening, coming home, I crested the hill & the pedal went DOWN. Thought about buying a m. Cyl, but w/ no other problem, I DON'T think it's the m. Cyl. When I get to H.Q. I'll give it another look. Sometimes 3rd X, etc.
Thanx, Mack

Tiny
Mack
May 4, 2012.
Further on "Brake fade after steep climb". On way home from reserve drill, I tried a slower rate of climb w/ lower revs.
No loss of pedal. Strange thing is brakes work FINE, except under those conditions: steep climb (at moderately high revs) & back off gas, at top of climb. Slower revs, got brakes.
Go figure.

Tiny
Mack
May 7, 2012.
I'm still thinking about it and can't offer any suggestions if the reservoir is full. There are two independent hydraulic circuits and to lose braking in both of them is very unusual under any circumstances, let alone such a repeatable one. There can't be air in the system because you'd have a low and mushy pedal all the time.

What you might try is holding pressure on the brake pedal for half a minute to see if it slowly sinks to the floor. With no external leaks and no loss of fluid, a sinking pedal is due to internal leakage inside the master cylinder. A trick to get an intermittent internal leak to show up is to vary the pedal pressure with your foot but never totally release it. Often the higher foot pressure will put more outward pressure on the lip seals causing them to seal better. Since you want a leak to show up, reducing the pedal pressure intermittently can allow the lip seals to relax to where they let fluid sneak past them.

If the pedal starts to sink, replace the master cylinder. It would be very odd that you haven't noticed a loss of pedal at any other time if it's due to the master cylinder, but obviously we're looking for something unusual.

Caradiodoc
May 7, 2012.
You said a MOUTHFUL. Haven't tried the varied pressure, so that's next. Thanx

Tiny
Mack
May 7, 2012.