Hot Rods Brake Problems; need advice, opinions please

I'm working on a friend's 41 Willys coupe hot rod. Car has front and rear disc brakes with a dual master cylinder mounted up on the firewall. No Power Brakes. Brake lines do have the proper residual valves going to the front and rear. He says that a couple times while driving he has hit the brakes and the pedal has gone to the floor. By the time I get to his house about an hour later the brakes are fine!! Full pedal just like normal. Master cylinder is full. I looked at the brake lines and noticed both front and rear lines at one point are about 3" from the left header. I'm wondering if the fluid in the lines is boiling being that close to heat. I'm just really at a loss here. Any thoughts on this?

 

Could be.

But also check that the calipers are all mounted properly, and there is no end play on the rotors/hubs/axles. It's possible that when he drives, if the caliper or rotor are moving relative to each other, the caliper piston is getting retracted, and it takes a lot of pedal action to push it back out.

 

If the lines are that close to a heat source, it will become porous and the pedal will drop, yes it will lose fluid, but not much. The other thing I come up with, the master may be bypassing internally, so you have no loss of fluid. I would route the line away from the exhaust and see if that does it. If there is a line that gets hot, you should be able to see it glow red and fluid will bubble out of it.

 

I put on a sheet metal heat shield 20 years ago and the problem was solved. Locate about 1/2 inch from the brake lines so the sheet metal will dissipate the heat first.

 

Traditional brakes ,,,
Pump it once then hit them again to stop.

 

Master bypassing internally part of the time. Could be a small tear or cut in the rubber cup that opens up, then closes back to normal on the return stroke.

 

Back up for continued consideration.

 

The one I had close to a header was REAL close, like a 1/2". That pedal sank like a stone. If it were 3" away, I'm not so sure that is the problem. More likely the master is bypassing.

 

There is no reason to have residuals in a disc system when the master is higher than the calipers. It's possible one or more caliper may be lazy on retraction and/or being slightly pressurized because of a residual and the high master location, resulting in one or more brakes producing excessive heat.

 

A quick install of a heat shield will prove it one way or the other. Even temporary with a couple of C clamps to hold it.


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