Possible brake issue

[Jim Nolan]

I took the Z out for some exercise yesterday and had a problem. After 20 minutes on the road it started to bog down in 5th gear at 60 mph, I down shifted and still bogged down, power key on. I pulled off the road clutch in and when I applied the brake the pedal was all the way up and stiff with no cushion, rock hard. I backed up about 50' and everything seemed normal. The brake pedal felt normal also.

I drove about another 10 miles gassed up and was on the way home and the same thing happened again. I backed up with the same results. Either time it did not pull to either side. I spent the rest of the afternoon reading up in the service manual. The only thing I could figure out is that the master cyclinder is failing or the proportioning valve is going bad and appyling pressure to the rear. No codes were set in any of the systems.

Any thoughts.

Thanks!

Jim

[scottfab]

Others might care to guess on this but for me
there's just not enough info.
When this "bog down" happens does it idle ok?
(take out of gear and coast then take a look at idle)
Can you accelerate from a stop ok?
You'll need to isolate a bit as to whether it's fuel or spark
related.

[batchman]

The hard pedal says to me booster or vacuum problem. The power falling off seems to nudge it toward vac problem to my eye.

If you had a brake misbehaving it should definitely pull to one side and also be noticeably hot compared to the others. If you have or can find a cheapie handheld pyrometer it can help prove this if it happens again.

Good luck,
- Jeff

[VetteMed]

Internally collapsed brake hose(s)?

Sounds like brakes dragging that led to the bog, rather than an engine problem.

Bad brake booster or vacuum leak could cause hard pedal and poor running.

[Paul Workman]

If NOT for the hard peddle situation, I would be thinking the "ZF" was freezing up (a common problem with some of the later ZFs used in the C4, according to a discussion with Bill B some time ago - had one freeze up on me.)

I'll be :happy1: this one!

P.

[Jim Nolan]

When I backed it up the problem would go away. This is what led me to a brake issue. The brake was never applied when this happen. No pull to either side. I don't like changing parts 'til the problem is fixed. The fact that there was no pull to either side leads me to the proportioning valve. It's job is apply brake pressure to the rear if the front brakes fail. When this happened it was not a nose down like braking action. I looked at the rotors and there appeared to be no scoring or overheating. Backing up curing the problem is strange. If I don't get it figured out I'll have to take it out this weekend do a little more troubleshooting. I had a brake hose collaple on me a few years back and replaced them all so I don't that is the problem.

[scottfab]

When I backed it up the problem would go away. This is what led me to a brake issue.

backing up seems to be unrelated. Kinda like opening your door and each time the problem goes away. Could seem related.
Still not enough info.
Is "bog down" related to engine rpm. (engine partially dying due to low fuel)
That could be a fuel pump intermittently failing. In that case the engine would be revable but no power. Also no codes would indicate the loss of fuel is not long enough to set a lean code.

At this point though the "bogging down" could be the electricals failing too. Need to isolate one or the other.
This can be frustrating if the problem does not happen on command.

[tomtom72]

Did you try pulling up on the brake pedal? That happened to me once, not quite to the same degree as I never made it to a highway. The rear brakes were hanging up in the "on" position. I don't know what I did to cause the situation, but pulling up on the brake pedal was suggested in the FSM's brake section trouble shooting guide. They have a list of symptoms and possible cures/causes listed on the last page in the brake section.

The only thing I don't recall was the pedal behaving like you describe, rock hard like as in a booster failure. My symptoms were that I could feel a 'drag' on the car at 30 mph, and I smelt the rear brakes overheating after about a block.

:cheers:
Tom

[Jim Nolan]

Well now I'm really lost. Took the Z out for a long drive, ran it hard , cruised, all the gears, key on/off and nothing. The weather was the same, hot and humid. It never missed a beat. Now I don't know to trust it on a trip. Before going out I pulled plugs while still cold, they still looked new (500 miles). I guess have to drive it more local hoping it happens again. Any other suggestions?