Clutch Hydraulic Problems

[Fletch189]

I have had my Backdraft for almost 7 years now and I have replaced the Master cylinder about every 8000 miles and the slave every 15,000. I have added a heat shield but haven't driven it enough to tell if it is going to make a differance. I have chalked it up to a bad design and just keep extra cylinders around just in case. I would have thought Backdraft would have fixed the problem by now.

[jhv48]

So, if I get this right.

Clutch operates fine when cold but fails when good and hot?

No visible leaks probably means one of two things.

1. Master cylinder or slave has an internal leak when the seals get good and hot allowing the hot (perhaps boiling) fluid to slip past the seals.
2. The fluid is boiling in the lines causing a loss in pressure. Try running with the electric fans on continuously. Does the problem persist?

Drain the hydraulic fluid first and see what color it is. If dark brown/black, it probably has been burned. Change to a high temp DOT 4 fluid such as Wilwood 570. If it has boiled, time for some custom heat shields.

Now drive it and see if the problem recurs. If so, then climb under the car (while the thing is still good and hot) and have a friend depress the clutch pedal. Observe the travel that the clutch fork makes (or doesn't). Is it the same as when dead cold? If not, then the M/C or slave are losing pressure internally. That will necessitate replacing first one then the other. Start with the one closest to the heat sources (headers). If that makes no change, then go to the other one. One or the other will prove faulty.

And this is not ONLY a BDR problem. I've been chasing my heat devils in an SPF for the last few weeks.

[bobinyelm]

Originally Posted by jhv48

So, if I get this right.

Clutch operates fine when cold but fails when good and hot?
Yep.

No visible leaks probably means one of two things.

1. Master cylinder or slave has an internal leak when the seals get good and hot allowing the hot (perhaps boiling) fluid to slip past the seals.
Yes, bypassing master seal was my first thought.
2. The fluid is boiling in the lines causing a loss in pressure. Try running with the electric fans on continuously. Does the problem persist?
Could be, but hood was open when this occurred, and car hadn't been driven that day-only idled for under 10 minutes. Another thread showed someone made new lines and routed them differently. I need to PM him to see if it cured his problem.

Drain the hydraulic fluid first and see what color it is. If dark brown/black, it probably has been burned. Change to a high temp DOT 4 fluid such as Wilwood 570. If it has boiled, time for some custom heat shields.
Color is normal (sample from master reservoir).
Fluid is DOT 4, and installed less than 10 miles and maybe 30 minutes engine-run-time ago (that's the total mileage on the Cobra since assembled). Is Wildwood DOT 4 fluid a much higher temp fluid than normal DOT 4?

Now drive it and see if the problem recurs. If so, then climb under the car (while the thing is still good and hot) and have a friend depress the clutch pedal. Observe the travel that the clutch fork makes (or doesn't). Is it the same as when dead cold? If not, then the M/C or slave are losing pressure internally. That will necessitate replacing first one then the other. Start with the one closest to the heat sources (headers). If that makes no change, then go to the other one. One or the other will prove faulty.
[color="red"]Yes, when I get it down from jack stands, I will drive it again. I no longer have a lift, so getting it up to see the fork move is time-consuming, but worthwhile.
I doubt the slave is the problem (if it's heat related) only because it likely did not get hot w/ the engine just idling under 10 minutes, especially w/ the hood open and cool air rising up past it.

And this is not ONLY a BDR problem. I've been chasing my heat devils in an SPF for the last few weeks.
A LOT of heat up under these cars! A small fan mounted behind the side grill blowing in continuously wouldn't hurt both master cylinders.
My car has the BDR heat shield, but hot air can rise past a heat shield.

Thanks-Bob

[CobraEd]

Replace all that mess with a cable and it will work perfectly forever !!!


.,

[jhv48]

Bob, don't sample the fluid from the M/C end. Open the bleeder valve on the slave and let it slowly drip into a glass bottle. Then check the color. That is where the heat will get to the fluid the fastest. You won't need to bleed the system if the bleeder is on the top of the slave and the master is higher than the slave. Gravity will cause it to flow without any air entering the system.

And, yes, Wilwood fluid will boil at 570 degrees dry where normal DOT 4 fluid can boil at 446 degrees dry. Big difference.

[bobinyelm]

Originally Posted by jhv48

Bob, don't sample the fluid from the M/C end. Open the bleeder valve on the slave and let it slowly drip into a glass bottle. Then check the color. That is where the heat will get to the fluid the fastest. You won't need to bleed the system if the bleeder is on the top of the slave and the master is higher than the slave. Gravity will cause it to flow without any air entering the system.

And, yes, Wilwood fluid will boil at 570 degrees dry where normal DOT 4 fluid can boil at 446 degrees dry. Big difference.

Man, if the fluid boils and is destroyed after 10 miles and a total of 30 minutes engine run-time, it would see, that there are more design problems present than
switching to a new fluid type will cure.
Reaching over 446 degrees where fluids are present is pretty shocking, not to mention the brake master cylinder parts and fluid!
Do people suffer brake failure frequently as well, or is that why DOT 5 silicone fluid is used for the brakes?