Odd hydraulic problem.

[joyridin']

On my Arntz, I have twin master for the brakes, and a single for the clutch. All 3 have remote reservoirs with -4an lines run to them.

After you drive the car for a little while, the brake and clutch pedal height will change. The brakes go from roughly and engagement height of 1/2 way down to 1/4 the way down, and the clutch release goes from roughly 1/2 the way up to 3/4 of the way up. It acts as though the fluid is expanding from hear or something, but there really is no heat source near the lines. The reservoirs can get warm, but not hot.

Any ideas on the cause of this? The brakes do not drag in any way from what I can tell. The car is easy to push cold or hot and the clutch does not seem to slip.

This has me baffled as it happens every time I drive for more than maybe 15-20 minutes.

[spdbrake]

From what you described, I think your saying the pedals are closer (less free-play) to you once the car is warmed up. If so I'd suspect water. You may also take a look at the line routing around the headers and if you can add a heat shield below the master cylinders. If - like most Cobra the headers are less than a foot away.

The next to last paragraph in this article describes the pedal issue.
https://www.good-guys.com/hotnews/te...d-brake-fluid/

[cycleguy55]

Originally Posted by spdbrake

From what you described, I think your saying the pedals are closer (less free-play) to you once the car is warmed up. If so I'd suspect water. You may also take a look at the line routing around the headers and if you can add a heat shield below the master cylinders. If - like most Cobra the headers are less than a foot away.

The next to last paragraph in this article describes the pedal issue.
https://www.good-guys.com/hotnews/te...d-brake-fluid/

So, DOT 4 can replace DOT 3, and DOT 5.1 can replace both DOT 3 and DOT 4. It seems like the prudent thing to do for our cars is use DOT 5.1 not just because it's 'the latest and greatest', but primarily because it has the highest wet boiling point and is also the least likely to absorb water - at least among the 'cross-compatible' types of fluid.


As noted elsewhere, most people would be well advised to avoid DOT 5.


Edit: I note Summit Racing has numerous types of DOT 3 and DOT 4 from which to choose, but only 3 listings for DOT 5.1 fluid. Limited selection, but at least it's reasonably priced so we don't have to give up our first born - though there are times we might want to do just that... LOL

[joyridin']

Hmmm...it has new synthetic DOT4 fluid, but that sure matches the issue I am having. I am going to buy some DOT 5.1 and do a flush to see it that solves the problem.

Thanks for the replies!

On a side note, years ago I ran DOT 5 in a car I had. Never had an issue. We used to repair U.S. Mail Jeeps and they all had DOT 5 due to the stop and go and the brakes heating up. Never had an issue with any of them either.

[Jaydee]

Also make sure the the push rod is loose on the master cylinder when you release. Any pressure at all. and the compensating port will be covered in the master cylinder and it won't allow the fluid to go back into the reservoir. therefore building up pressure before you push the pedal. JD

[Gaz64]

Originally Posted by Jaydee

Also make sure the the push rod is loose on the master cylinder when you release. Any pressure at all. and the compensating port will be covered in the master cylinder and it won't allow the fluid to go back into the reservoir. therefore building up pressure before you push the pedal. JD

I'm so glad another understands this theory.

Gary