The brake power is declining and the problem seems almost certain to be either the master cylinder, or the booster vacuum. It's tempting to simply replace the master cylinder because it's so cheap and easy, but would prefer to know how to diagnose the vacuum to determine if that's the cause before doing anything. Any help or advice appreciated.

 

Can you be more specific? Does the pedal go to the floor? The brakes work fine but the pedal is harder to push? Plenty of fluid in the MC? Any puddles of brake fluid underneath? Etc.

 

A loss of vacuum would not cause the "brake power to decline". It would make the pedal more difficult push, but the car should still stop OK. If the pedal still feels normal in this regard but you have to push it further to get the car to stop, then you might have air in the line, a failing master cylinder, a loss of brake fluid, or something else.

Have you checked the brake fluid level in the reservoir? What is it? Does the brake light come on? Do you notice any brake fluid on the ground under the car? Does the brake pedal feel more spongy but otherwise stop the car OK?

We need more information, and it seems that a little troubleshooting on your part could narrow down the problem. Simply replacing the master cylinder because it's easy to do is the wrong approach. You don't want to just start replacing parts *****-nilly as it's time consuming, gets expensive, and may not fix your problem, anyway. Better to properly troubleshoot the problem and determine the cause.

 

Okay, a friend of mine posted that for me, this is really me. When I push down on the brake pedal, it sometimes goes down more easily than it should, feels a little mushy. It does not really go to the floor. When I let up on it and push down again, it has just the right amount of pressure.
The brake fluid level is full. It's not happening all the time (yet.)

 

OK, the fluid level being full is good. The other symptoms you describe suggest air in the lines. The brakes need to be bled. If you're not able to do that, take it to brake shop, describe the symptoms, and have them do it.

Based on what you say, I would not yet suspect a bad master cylinder or vacuum booster.


If it does turn out to be air in the line, one question is, how did the air get in the line in the first place? Was brake work done recently? If not, perhaps you do have a slow fluid leak somewhere. Again, a competent brake shop should be able to diagnose the problem if you're not up to the task.

 

X2, sounds like air in the line. But as jaunty questioned, where did it come from? Don't put this off. A slow leak can become a quick gush and pressing on that pedal and having nothing happen can be quite a shock. Ask me how I know :-)

 

I am with someone who is convinced it is the master cylinder and talked me into getting a new one today, which he will put in tomorrow... fortunately I live next door to an actual mechanic, which might be helpful if this doesn't work and it is an air leak instead. But my regular mechanic can't look at it that soon. And I need the car Monday, naturally. I'll update as I know more. I appreciate the responses.

 

Have you looked at the brake pads and shoes?.
I had similar symptoms in my '87 Olds 88, it turned out one of the rear brake cylinders had a slight leak, not enough to make the fluid level drop alarmingly, and the shoes were contaminated. The self adjusters on the rear brakes weren't working either, new cylinder, shoes, an overhaul of the adjusters and the brakes were like new again.


Roger.

 

I had the brakes checked a couple of weeks ago at Firestone (they have been really good to me) and they didn't see any problems with the brakes at that time.

 

This is an incomplete comment. Why did you have the brakes checked a couple of weeks ago? Were they exhibiting the symptoms you describe now? If so, what did Firestone say about the situation? If not, why don't you go back there now, describe your problem, and see what they say?

In other words, the question raised by your comment is, was your visit to Firestone a couple of weeks ago connected to your current brake problem, or not?

 

I'm sorry, I was distracted by the situation here and couldn't reply completely. I had the brakes checked as a precaution because my son was concerned that the rotors might be wearing unevenly as he felt something not quite even when coming to a stop. I didn't notice anything but I had it in for an oil change so I had them look at the brakes. This was unrelated to what I'm experiencing now.
Latest is that they are replacing the master cylinder. I will update when I know more.

 

Ok, thanks. Do let us know what you find out.

 

Okay, the final word is that they felt like replacing the master cylinder did need to be done, as it's braking better now, but they had to bleed the brakes, and there was a caliper they couldn't get off without busting it (they did bust some nut or something and couldn't get it out) so they put it all back together for me and it's waiting to be fixed at Firestone. So I saved some money by having them do that, and who knows what it will actually cost for the rest of it.
I appreciate the comments here. if there is anything else unusual, I'll post it. Night, all.

 

They "felt" like the master cylinder needed to be done? They FELT like it? What, they were getting vibes from it or something?

It sounds to me like they're covering their butts. "No, replacing the master cylinder didn't fix the problem, but it needed to be replaced, anyway."

Yeah.



AHA!

 

I'd translate "they felt like replacing the master cylinder needed to be done" as "They diagnosed a faulty master cylinder". Plenty of repair shops are competent and honest.


Of course the brakes needed bleeding after the master cylinder was replaced, and I guess they broke off a bleed nipple on a calliper when doing the job. If they fix the calliper on their own dime I'd say they are worthy of your business.


Roger.