Peter,

Spongy brakes. Make sure the bleeders are on top of the calipers and not the bottom. Don't ask how I learned this...

Thanks Jim, a friend I know very well has tried that too, so I will not ask.

Bill,
Thanks for the reply

Jon

Peter,

If your brakes are still spongy, then a couple more things to try.

1. Replace brake lines if they are orginal
2. Try using a pressure bleeder (Pelican has them)
3. Try using brake bleed valves that have a check valve in them (Pelican)
4. If MC is old, replace.

5. tap the calipers dozens of times with a small metal hammer or wrench while bleeding

Thanks for the tips!

I guess I will get a new master cylinder and a pressure bleeder, since the rest of the brake system is virtually new. Had to change a few meters of hard lines when changing to steel braided brake hoses.

Peter,

Consider upgrading to the 930 MC. It has a larger piston which means less throw but more brake force from your leg. This is a great option for a track beast but may not be desireable for a street car. Depends on what you would like. The installion is the same as the stock unit.

Jim,

thanks, that was the answer to my next question! I have just bought a pair of 993 RS front calipers and a pair of 964 Turbo 3.6 rear calipers in the hunt of the spongy pedal, and I got the impression from reading threads here that the original MC might be a little on the small side. I guess I will feel a little embarrassed when it turnes out that it was "only" the MC.

Peter,

If you have the piston sizes of the calipers, I can go through the math and calculate the front to rear brake bias.

Hi Jim - could you explain how an old MC can lead to spongy brakes? Perhaps this is my problem too...

Thanks,

Jeff

edit - assuming it isn't leaking, which mine isn't.

Jim,

I did some math myself and found them more front biased than the stock, but I would appreciate it if you had a look at the numbers.

I have not measured the items myself, but according to Bill V's table:

Front disc is 322 mm in diameter (993 RS disc)
Front caliper piston diameters are 44 mm and 34 mm

Rear disc is 309 mm in diameter (930 rear disc)
Rear caliper piston diameters are 34 mm and 30 mm

By comparing braking moment at a unit brake system pressure and assuming the brake pad coefficient of friction to be the same for all the pads, I found the new brakes to have a 61/39 front/rear bias compared to the stock which have 54/46 front/rear bias.

To calculate the braking moment I used the following equation:

http://forums.pelicanparts.com/uploa...1145791182.gif

p_sys is brake system pressure, (I chose 1 MPa (1 N / mm2)), A_pistons is the area of the pistons in one caliper, mu_pad is the coefficient of friction (I chose 0.55). Ø_disc is brake disc diameter and h_pad is the height of the pad. The part within the brackets is the average lever arm for the brakes.

Does this look reasonable?

I run the Wilwood Superlite Series on my Carrera rotor for years; I've changed the model a few times, the Forged unit is the latest. With (4) 35 mm pistons and larger pads it works well on the track; I realize the caliper is overkill for the rotor, but, it's an inexpensive switch. The rear is OEM 3.2 Carrera, running with the 55 bar p/v.
j.p.

http://forums.pelicanparts.com/uploa...1145797448.jpg
http://forums.pelicanparts.com/uploa...1145797540.jpg
http://forums.pelicanparts.com/uploa...1145797603.jpg
http://forums.pelicanparts.com/uploa...1145797633.jpg

The pics of the inner side, show a BBS 15" modular race wheel and the custom hard line to the strut.

Peter,

Great equation. Is H_Pad measures just the pad? (i.e. if the pad height is 50mm). Or is it in relative to the position of the pad to the rotor?

Anyway, I went through the math and the brake bias is 60.1% to the front and 39.9 to the rear. The math is based of a 23mm MC and the caliper piston areas on one side of each caliper

Jeff,

Spongy brakes.... If all else is done, (new lines, new/rebuilt calipers, good pads, system bleed, proporting valve OK), then the only thing left is MC. As these cars are 20+years old, sometimes things are placed as preventative maintenance.

Jim,

good to see that my calculations are at least reasonably close to yours. Do you think the bias is to front heavy? I was playing a little with the idea of getting an adjustable brake bias.

H_pad is just the height of the friction material of the pad (not the steel backing plate), and I assume that the outer edge of the friction material is flush with the rotor. That is not exactly true, but quite close to the real thing.

I measured my brake pads with a slide caliper, but the sizes differed a little from the ones in Bill V's table. Since I am going to use the pads that I have, I used my measurements.

My stock pads measured 54 mm and 40 mm in heigth, and the new ones were 60 mm and 49 mm.

Thanks for helping out!

jpahemi,

nice setup you got there. Do the calipers bolt right on?

Peter,

A 60.1/39.9 is split good. Some say about a 38.5 / 61.5 is about ideal for the street. Measure rotor temps after a few hard braking stops. If the rears are not working enough (cold rotors), then rethink the bias thing. For a track car, the rear bias could be more, but then one has to watch the rear brakes locking up.

FWIW, Porsche increased the size of the rear pistons in the Carrera as compared to the SC's. This was to even out the brake pad wear. Of course the Carreras had a proporting valve.

Peter:
The caliper mounting ears need to be enlarged from @ 10.9-11.01 mm to 12mm; the spacing is 3.5" which allows direct bolt on. The hard line from caliper to strut needs to be made; I found an outfit that makes custom stainless hard lines with metric fittings; I ended up sending them a coat hanger wire model of what the line needed look like.
j.p.
Ps. I realize the rear needs to be upgraded as well, for now all I did was replace the 22 year old calipers with new ones. It's nice seeing the new yellow zinc finish show through the wheel.

Freudian slip?