brake line - hard vs. soft

[Kurt Bilinski]

For my Mini, I plumbed the car with stainless flex line (the proper stuff). I'm now suffering from a spongy pedel, and after bleeding it a million times, I'm wondering if it's the flex line expanding. I'm wondering if other's have plumbed cars with flex line only and experienced the same thing. It would be a big PITA to redo it, but a spongy pedel gives me the creeps. What I don't know is if the real problem is the flex line, or something else. I'd hate to redo it all only to find it's still there.

[Kurt Bilinski]

For my Mini, I plumbed the car with stainless flex line (the proper stuff). I'm now suffering from a spongy pedel, and after bleeding it a million times, I'm wondering if it's the flex line expanding. I'm wondering if other's have plumbed cars with flex line only and experienced the same thing. It would be a big PITA to redo it, but a spongy pedel gives me the creeps. What I don't know is if the real problem is the flex line, or something else. I'd hate to redo it all only to find it's still there.

[Dave M]

Thats probobly it. The stainless flex line is much much better than rubber but can still flex a little in large lengths. You should use stainess hard line as far as you can, then just use enough flex line to clear for full bump, droop, left, right, etc

[Brian Evans]

While the theory that hard line flexes less than teflon/stainless line is absolutely correct, a vast number of racing cars are plumbed completely with flex line and they don't suffer from spongy pedal at all. Before I threw away all of that line, I'd try to find the real reason. System ratio can cause this: if you use, for example, four pot calipers with 1.75" pistons and a 3/4" master cylinder with any normal pedal ratio, you can have a very long pedal travel and spongy brakes - and way too low a pedal effort as well. Been there, done that, changed master cylinder size to fix it.

Brian

[Courtney Waters]

Got any specs on piston sizes, # of pistons, pedal ratio & length, etc? Your car is short enough that I can't imagine quality flex line would be the culprit. It's possible though. Even running stainless hardline just down the center of the car would help. The mounts for your pedal assembly aren't flexing are they?

It's hard to tell from the pics on your site - are the master cylinders mounted at an angle or anything such that air might be trapped in them? What procedure did you use to bleed them? Also, I recall when bleeding our FSAE cars (with dual MC's & balance bar) that the bleeding process could get tricky at times if one cylinder fully bled and pressurized so it would prevent the other from being bled. I don't recall whether we disconnected them from the pedal and hand pumped each one or adjusted the balance bar to get more travel to the second one or what.

[Schulberg J]

If you are running a dual master cylinder and balance bar you need bleed both the front and rear circuits simultaneously. If you only bleed one circuit at a time, the circuit not being bled will prevent the master cylinder in the circuit being bled from going through its entire travel and you will get problems like Courtney described.

[Chris Davin]

I agree with the previous remarks: quality armored flex brake lines (Earl's or Aeroquip)should not give a spongy pedal, especially on such a small car. I would think that caliper flex (clam-shell action) or improper bleeding would be the first places to look. In my experience, good flex line does not expand like you're describing.

Also, might it have something to do with the booster? (I am assuming you have power brakes.) I have never worked with a power brakes system before, and do not know much about boosters besides the basic principle. But I thought power brakes systems tend to have more travel than their non-boosted counterparts. (??)

[DaveC]

Yeah, I'd agree its not likely your issue is from the lines. I dont think there would be enough flex in Teflon/SS lines to be a noticable problem unless there is a defective line somewhere. I'd guess: 1.Master Cylinder bore size is too small 2.Booster or Master is defective 3.Caliper/Disk Alignment is off. 4.The system isnt bled.

[Frank]

the caliper effective piston area, and effective disk radius are the two variable that will dictate your brakeline pressure

we use the wilwood dynalight single long pistons

they have heaps of area, and hence the brakeline pressure is very low, causing little line bulging

[Kurt Bilinski]

Thanks for the input. Here's the spreadsheet I used: http://www.7builder.com/Downloads/MikesBrakes.xls

My data:
Calipers are single piston.
front caliper piston dia 2.265"
rear caliper piston dia 1.77"
wheelbase 80"
CG height 16"
Pedal ratio 5.5:1
Front and rear tire radius: 10.75"
Balance bar presently set at 50/50

Front master cylinder dia: 0.825"
Rear master cylinder dia: 0.812"

So how's that stack up? Oh and I used Earls stainless/teflon brake line.

What bothers me is it's spongy AND requires a lot of pedal pressure. Makes me nervous.

I guess I need to figure out how to determine if there's any remaining air. A race car mechanic said if I can pump the pedel and the pedal gets hard, there's air in the lines - it makes no difference in my case...

[BryanH]

Replace as much of the fixed line as possible with steel bundy tube. .750 front master.
Hawk blue carbon brake pads all round