I have a 1971 Chevelle with power front discs & rear drum brakes. The brakes are intermittently spongy when stopped in traffic. While waiting for a light to turn with my foot on the brake pedal, sometimes but not always, the pedal will depress further , stop and become firm at a lower level. Pump the brakes once and it returns to normal position. Sometimes it then repeats and sometimes it normalizes until it does it again. Sometimes at the next light, sometimes not for five or more lights. Other than this maddening symptom the brakes are good.

I have replaced the master cylinder twice. All 3 master cylinders have the same problem. I am reasonably sure it can’t be the master cylinder. I have replaced the wheel cylinders. The discs, calipers, pads and drums are in good shape. I have not replaced the proportioning valve. The power booster appears to be behaving well. There are no leaks. The master is retaining its fluid level over time.

I am convinced that the system must be getting air from somewhere or has not been properly bled . I have had two different mechanics bleed the brakes when replacing the master cylinders. I have owned the car since ’74 and never had problems bleeding the brakes in the past. I should say this did not start as a result of working on the system. It simply started on its own accord. Every mechanic I have spoken to says master cylinder. I’m reluctant to buy a 4th but I’m at wits end. Please advise.

Sounds like the booster gone bad.

Tomas:

I would be interested in knowing why you think it's a booster gone bad. I replaced the booster in this car a number of years ago and the symptom was a very hard pedal requiring a lot of physical effort to apply the brakes. My brakes are very responsive and not hard to apply - I just lose pedal after a period with my foot on the brake.

Somehow I missed this thread, but I am having a very similar problem in my 69. I started another thread with my symptoms.

I feel the OP's frustration, at least I havent started throwing parts at it yet.

I would defiantly look for any visible leaks and if you have none I would suspect your proportioning valve. Your right about the booster, a bad one will only cause poor power assist. Good luck on your fix.

You wouldn't happen to have a brake line against or very close to a header tube? You end up sitting it traffic, temps go up because no air flow under the car and you boil the brake fluid in the tube.

Zeke:

Interesting thought but I don't think so. I do have headers but they have been on the car for a long time and I haven't replaced a brake line for years. In other words, if that were my problem I don't know why it would suddenly show up now.

Murphdog:

I have looked & looked for leaks. During the winter the car sits for long periods. I have scoured and replaced the MC twice & wheel cylinders. Fluid in the MC is stable. The proportioning valve is original & I have no experience with one going bad. Can you provide symptoms for a bad proportioning valve? Can anyone?

I had a very spongy pedal on a C/K truck a couple weeks ago that improved greatly with a new booster. I know the system still has a bit of air in it, as work was done on it, but for the most part, the sponginess is gone. It was a new one to me, usually when a booster fails, it gets harder to depress the brakes, this one just got spongy.

Master cylinder is leaking past some of the seals INSIDE its self ..... main causes are rust & pitted bores .....

P.S. one of these times its going to the FLOOR !!! & your going to be replacing sheet metal :eek:

68 kmeno

MC is new. I have replaced it twice thinking the first replacement was bad. All 3 MC's displayed the same sponginess of pedal after the car has stopped and waiting for a light. I don't think all 3 MC's could have bad seals, rust and pitted interiors.

If you haven't done so already, I'd replace the rubber lines at the rear axle and at each wheel. They wear out from the inside sometimes, and when they go, you won't like it.

Although I can't see why they would cause your problem, but you've already tried everything else...

Xtreme:

I replaced the rubber lines at the front wheels a few years ago. You are right, losing a rubber line is not fun. I was not aware that there was another at the rear axle. However, there are currently no visible leaks anywhere. It's a good idea, not a big deal or expense to replace them and I have nothing to lose.

Generaly when a proportioning valve goes bad it will cause the rear brakes to lock up when you apply the brakes. But it could be pulling air into the system. Also if you have a residual valve on your system that could cause it. When it goes spongy can you pump it back up and get your pedal feel back?

Murphdog:

The brakes only go spongy after the car has stopped and your foot is holding the brake pedal in drive.. Other than that, application of the brakes is smooth, responsive & firm and the car stops normally. Again, only after the car stops and I have had my foot on the brake for a while then the pedal gently decends and stops. I can pump it once and the pedal returns to its normal position. Sometimes the pedal will again descend but not always. Sometimes it recurs with the next light, sometimes not for the next five lights. I see no cause and effect. Maddening.

my wagon did the same thing the next step is no brakes and I crashed into my shop building so I just replaced everything. \
be careful

You didn't mention the flex lines. Did change or check the flex lines? I was having really spongy brakes on my 67 big block el camino. I had never checked the brakes when i bought the car. i wasnt loosing fuid from the system. When I took the driver side wheel off, there was an original four piston caliper, that belonged on the passenger side. Being on the wrong side caused the flex line to slowly wear against the control arm and the rubber sheath had deteriorated, so that when u stepped on the brakes the pressure would cause the line to bulge and cause the brakes to feel spongy. at least thats what i thought. I have since bought wilwood front brakes among other components and am having all kinds of problems to say the least. ANYWAY, check the flex lines, and replace them with ss braided ones, if u already haven't.

-67 el camino custom 402
-72 chevelle ss454

Even though you've replaced the master 3 times I still think it's the master that is bad. A good test for it would be. With the car not running pump up the brake pedal and push with hard steady pressure for a minute or so. Then see if the pedal moves. If it slowly sinks to the floor you are losing fliud/pressure somewhere. You will either find it leaking on the ground,into the booster,or it is bypassing the cups in the master returning to the reservoir . Also have someone inspect all three flex lines while you are doing this to look for bulges in the lines. That "new booster " might have sat on the shelf for 10 years.