I plumbed the brake system in my one-off Mini using braided teflon "-3" brake line. Someone said it is why I'm suffering from a soft brake pedal. He contends the flexible line should be used only where needed (only flexing areas) and to use steel everywhere else. Right now I disagree with that, I believe the soft pedal is because the master cylinders weren't sized correctly (too small.)

I haven't swaped out the cylinders yet, but I'm wondering if anyone else has used flexible line throughout their car and suffered any problems. I replaced all the line in another car years ago and don't remember any problems.

[Edited on 3/28/05 by kb58]

HI

WE use teflon brake line wiyh rally cars and rallycross all the time brake pedal is ok
you can try the cylinder
put see if you have air in the system this cangive you a soft pedal.

Personally I wouln't fit teflon lines to a road car if for no other reason than you can't clamp them when working on a corner.

Long teflon lines were always the way most single seaters were plumbed. As for master cylinder size unless you have fitted grossly oversized calipers something else is wrong. The usual way of tracking down the cause of a spongey pedal is to clamp the rubber hoses -- but clearly you can't do this so here are a few ideas.
I assume you know how to bleed brakes properly and have checked the brakes over for leaks, the caliper pistons are completely free (a seized caliper piston or pad gives a spongey pedal which gets progressively worse as the pads wear).

(1)Late 70s 1275GT Minis with diagonal circuit brakes used to get a problem with spongey pedal when the pads were 50% worn, the quick cure was to fit new pads -- just pulling out the pad and working the caliper piston out a few mm then push them fully home should have the same effect. While doing this job check the pads aren't wearing tapered.
(2) If that dosen't work check the calipers are fitted the rightway round -- nipple at top above fluid inlet.
(3) Check the hand brake cable isn't too tight, the brake hold down springs are fitted correctly and the rear brakes correctly adjusted.
(4) Remove fluid resevoir cap and check fluid isn't rising a few mm when you press the brake - indicates master cylinder fault.
(5) If none of the above shows a fault try disconnecting the pipes on the mastercylinder and screwing bleed nipples in -- then bleed. if the pedal is rock solid the mastercylinder is OK so try reconnecting the brake lines one at a time.
(6) If none of the above shows any fault and you have non standard brake calipers check the caliper brakets are set up 100% true and square to the disc surface --- easy way to check this is hold your hand on the disc and caliper and get an assistant to pump the brakes on and off vigourously. If you feel any movement at all the caliper brakets aren't right or you have a seized piston or pad.

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[Edited on 28/3/05 by britishtrident]

quote:

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Originally posted by kb58
.........He contends the flexible line should be used only where needed (only flexing areas) and to use steel everywhere else....

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I should have said the "Mini" in question doesn't have any actual Mini parts in it!

Good point on calipers not being square to the rotor. I'll double check that. All calipers have been rebuilt so there "shouldn't" be any sticking pistons. I hadn't though of clamping the hoses (even the rubber ones) because I thought it would weaken them.

There is a spreadsheet for figuring out proper brake master cylinder sizing: http://www.7builder.com/Downloads/MikesBrakes.xls

It says I'm 30% too small on the cylinders which would explain much of the sponginess. Of course the prime suspect is that I still have air bubbles... I'll try harder to make sure there are none once the larger cylinders are installed.

I have managed to shift persistent air bubbles using a pressurised bleeding system in conjunction with pressing the brake pedal. Attach the pressurised system first of all then get an assistant to apply firm pressure to the brake pedal while you release each bleed nipple in the appropriate sequence. This can sometimes dislodge a trapped bubble of air due to the speed of the fluid in the lines. Once the pedal goes to the floor, make sure it stays there until the nipple is tightened up as usual.

HTH,
Craig.

It's not the teflon lines, you are right in that dispute!

If they're not fastened down thoroughly a lot of the pedal effort goes into trying to straighten out any curves in the pipe. It was tried in a rally car by a local crew and they took them back out and replaced them with solid pipe as they could see them moving under heavy braking. They also thought about the heat dissipation not being as good as metal pipe.

If they are moving then they are expanding, but I have used full aeroquip setups on rally cars with no such problems.

I have usually used copper-nickel though purely on cost grounds.......it's a lot cheaper!!!

My reasoning(?) is that while hard-line is cheaper per foot, it will require at least four additional couplings between it and the flex line, which drives the cost up. Flex line is also easier to install.

Next time? I'll look into hardline a bit more...

Theirs was aeroquip, brand new, not cheapo replacement stuff. Kunifer is best you can get in my opinion. Kb, the four unions needed will cost virtually nothing compared to doing the whole car in braided hose and the saving using solid brakepipe should more than offset them.

I used Earl's flexline, which should be as good as Aeroquip. I know not to use the cheap stuff in this application

quote:

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Originally posted by Peteff
Theirs was aeroquip, brand new, not cheapo replacement stuff. Kunifer is best you can get in my opinion. Kb, the four unions needed will cost virtually nothing compared to doing the whole car in braided hose and the saving using solid brakepipe should more than offset them.

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Update:
I replaced the smallish master cylinders with the larger ones. The results are predictable, the math said I'd have 30% less pedal travel and I do. The sponginess is less, but I think it's also about 30%, meaning there's still compliance/air in the system.

Having said that, I'll wait until it's driven to make any further decisions. What I don't know is how much pedal pressure is needed to quickly stop the car. The fact that it's still a bit soft will only matter if it takes a lot of pressure to stop the car.

Ah, car design...

I use just plain Tubes Nylon 12 1000 PSI lines in my off-road cars with braided lines out to the callipers. It works well for what's required of it, but on occasions when I've driven it hard on bitumen, the brakes do lack feel.
I've always used cupro-nickel lines for road-going cars.

Just a thought KB

It might be wise to drive the car for a while and then see how pedal feels.

I have found when I built completely new drum brake systems in the past that the travel and firmness of the pedal to be awful until the shoes bed in.

I'm guessing that disc brakes don't require so much 'bedding in' tho, but it still might be worth taking the car out and driving it (carefully! lol), then readjusting the pads.

chris

Not to belittle your intelligence, but have you checked to make sure you have the calipers on the correct sides?

Its a common mistake people make. I once had a friend who couldn't get the pedal hard in his racing Porsche after replacing the calipers. After bleeding and re-bleeding the brakes for 5 hours with no luck he called me. Figured out he had the calipers mounted on the wrong sides with the bleed screws at the bottom. DOH

He was mad mad mad One of those....."should have known better" things.

The car is a one-off, with front calipers from a Nissan 280ZX and the rears from the front of an RX-7. Bleeders are at the top on all corners. I should just bite the bullet and run an entire bottle of brake fluid through it to really make sure it's bled.