Intermittent brake feel - sometimes firm, sometimes feels very soft

[c4koh]

Hi all,

Wonder if anyone can help (on my '92)

I am getting different feel in my brake pedal, and it the behavior is intermittent - happens when the car is cold, hot, just started etc.

I did get an ASR/ABS warning lights, and checked the codes - 83 low fluid, which I topped off to max. Fine. No other codes.

The brake pedal is sometimes firm (needs very little travel to engage the brakes - and feels confident), and sometimes it's very soft (needs a lot of travel to engage brakes, and does not feel confident). Can be soft from cold, soft when hot, hard when cold, hard when hot - no pattern to it that I can see.

If there were air in the system, I'd imagine the pedal would be spongy and soft all of the time - is that a correct assumption, or not? Or can the air move around so it can give different results.

There are no leaks that I can see anywhere.

Any help appreciated - I've read through the various posts as best as I could find, and have some ideas - but would appreciate any input.

Many thanks, folks.

Steve

[WB9MCW]

Time for a new/rebuilt MC.
I had the spongy deal and did a bleed on all 4 lines only to have the spongy come back in no time at all (less than 15 days - only two times out for countryside drives). It was the MC pumping air back in the system from bad seals. One wonders how this happens in a closed loop system but it just does. Every time you stab the pedal more air gets into the lines and soon the spongy pedal returns.

So I bought a used MC from a Vette Salvage Yard and sent it in on/to Rock Auto rebuild deal. Got it back and had my Mech swap it out.

Rebuild works great and I now still have my old one for a spare core in case I ever have to do it again.

I did it this way so I did not have any down time.
It takes about a week for the rebuild center to turn the MC around plus to and back from shipping time. So about 2 weeks total.

If you are doing the work yourself and have the garage space then you don't have to do the way I did. I just could not clog up my mech/buddy's shop for all that time. It is nice that my best friend is my mech as well. LOL

The company Rock Auto uses for the rebuild does excellent work. Rock has the best price and you ship direct to the rebuild company. Rock sends you all the details after you do the online purchase of the rebuild service from them.

It sure is nice to have a full firm pedal the way it should be and no fear of the dreaded spongy pedal anymore.

Here is the thread I posted on the forum. There is even an upgrade solution available now as posted in the thread.
http://zr1.net/forum/showthread.php?t=17777&highlight=master+brake+cylinder

[c4koh]

I understand, however why would my pedal sometimes be very firm? I don't want to simply replace the master cylinder only for it to be something else, or indeed simply air in the system (in general).

If the pedal were consistently spongy / lots of travel, and I'd done a full bleed, and still got back to a consistently spongy feel - yes, I'd go with the MC. But as the pedal is sometimes hard, I wonder if it is something else.

?

[WB9MCW]

I understand, however why would my pedal sometimes be very firm? I don't want to simply replace the master cylinder only for it to be something else, or indeed simply air in the system (in general).

If the pedal were consistently spongy / lots of travel, and I'd done a full bleed, and still got back to a consistently spongy feel - yes, I'd go with the MC. But as the pedal is sometimes hard, I wonder if it is something else.

?

Did you look at the thread link?
You should do the MC test and see what results you get.
This will determine if you need a new MC or not.
You may want to do all 4 lines bleed down for air bubble purge first and see.
If the spongy comes back then really all that is left is the MC to check into.

[gbrtng]

Intermittent vacuum to the booster or the booster itself.

[batchman]

Intermittent vacuum to the booster or the booster itself.

My thoughts exactly... I guess to test this theory you could pull the vac hose off the booster, plug it, and see if the pedal feel seems familiar.

Just a thought,
- Jeff