Brake Bias Questions

[Artfrombama]

Ever since I got this car I've noticed it took quite a bit of brake pedal pressure to stop. Rebuilt calipers and new Porterfield street compound pads.
Seems like most of the braking is done by the rear. Much more brake dust deposited on the rear rims and even with the "S" calipers I don't have the stopping power I think I should have.
Master cylinder? Bias valve?
Comments, suggestions, bring on the tech!

[Bill Verburg]

Originally Posted by Artfrombama

Ever since I got this car I've noticed it took quite a bit of brake pedal pressure to stop. Rebuilt calipers and new Porterfield street compound pads.
Seems like most of the braking is done by the rear. Much more brake dust deposited on the rear rims and even with the "S" calipers I don't have the stopping power I think I should have.
Master cylinder? Bias valve?
Comments, suggestions, bring on the tech!

Assuming that you have a stock '73.5 T , You have M calipers front and rear, on 282& 290 x20mm rotors

The same pads are used , f/r, but the bias is 1.491 mostly because the front calipers have bigger 48mm pistons than the rears w/ 38s


hydraulic pedal ratio is 35.328 and mechanical 5.4, This makes for a healthy amount of led effort for those that are used to more modern setups

On these cars one would expect ~50% faster wear on the front, there is no p/v.


A very common issue which arises from a lack of maintenance is frozen caliper pistons and internally swollen rubber brake lines. Either could contribute to what you describe, To address the issue exercise the calipers manually. Any piston that can't be pushed back is a likely suspect, and change the rubber hoses especially so if they are date uncertain.

You mention S calipers, it's possible that someone swapped in aluminum S calipers sometime over the cars history, These wouldn't change the bias or pedal feel, but it's god to knw what you have A or S calipers have a 3.5" OC bolt mounting pattern, M front or rear have a 3" OC bolt mounting pattern.

[Walt Fricke]

No bias valves on a '73.

I'd take my handy infra red temperature gun with me, go drive doing a lot of hard braking, and see what caliper and rotor temperatures were. Front ought to be hotter than rear. If front is lots cooler, maybe you have an internal MC leak. You should be able to get a kit to rebuild what you have, or get a good used one, or a new one.

A better way would be to attach pressure gauges to the front and rear lines, and see what they read. But if you are like me you won't have the fittings or gauges for that.

You can jack the car up on jack stands, have someone press the brake pedal, and see how hard it is to rotate front vs rear tires. With rims off you could use a long 2x4 or metal bar as a lever so you could get a decent torque on things.

I think typically an internal MC leak involves a pedal which will slowly depress when you push down hard while at a stop - that would be fluid leaking around a seal. The MC is designed so that if either front or rear circuit fails, the other works. The rearmost circuit in the MC gets its pressure directly from the rod the pedal pushes into the MC. The front circuit is hydraulically pressed forward. There is a metal rod in between the two, but it is shorter than the normal spacing from piston to piston, so it only comes into play if a seal or line has failed. Pedal travel is longer, but there is something there eventually to slow you down.

Anecdotes from guys who have had this problem on the track suggests it feels like a lot less than 50% braking, though.

If you have the original rubber lines in the system, perhaps one of the fronts bulges under pressure?

Here is another stopping power test: On a suitably safe place, try a maximum brake from 60 or 70 mph. Hit the brakes very hard (like a deer running across ahead of you), and keep near max pressure on all the way to a stop - no modulation. If you don't lock up a wheel or wheels as you get down to 5 or 10 MPH, you have a problem. The force which slows but won't lock wheels at 50 should lock them at low speeds.

Just some thoughts.

[Artfrombama]

Originally Posted by Bill Verburg

Assuming that you have a stock '73.5 T , You have M calipers front and rear, on 282& 290 x20mm rotors

The same pads are used , f/r, but the bias is 1.491 mostly because the front calipers have bigger 48mm pistons than the rears w/ 38s


hydraulic pedal ratio is 35.328 and mechanical 5.4, This makes for a healthy amount of led effort for those that are used to more modern setups

On these cars one would expect ~50% faster wear on the front, there is no p/v.


A very common issue which arises from a lack of maintenance is frozen caliper pistons and internally swollen rubber brake lines. Either could contribute to what you describe, To address the issue exercise the calipers manually. Any piston that can't be pushed back is a likely suspect, and change the rubber hoses especially so if they are date uncertain.

You mention S calipers, it's possible that someone swapped in aluminum S calipers sometime over the cars history, These wouldn't change the bias or pedal feel, but it's god to knw what you have A or S calipers have a 3.5" OC bolt mounting pattern, M front or rear have a 3" OC bolt mounting pattern.

Both front calipers were frozen when I bought the car, I was able to remove the pistons, remove the corrosion and install kits, rebuilt the rear calipers at the same time. Wheels spin freely now.
Flexible lines were upgraded at some time to steel braided racing type flex lines F&R.
I was told since my car came with Koni's the S calipers were part of that option.

I'm thinking bad master cylinder.

[Bill Verburg]

Originally Posted by Artfrombama

Both front calipers were frozen when I bought the car, I was able to remove the pistons, remove the corrosion and install kits, rebuilt the rear calipers at the same time. Wheels spin freely now.
Flexible lines were upgraded at some time to steel braided racing type flex lines F&R.
I was told since my car came with Koni's the S calipers were part of that option.

I'm thinking bad master cylinder.

It's possible

[Artfrombama]

Originally Posted by Walt Fricke

I think typically an internal MC leak involves a pedal which will slowly depress when you push down hard while at a stop - that would be fluid leaking around a seal. The MC is designed so that if either front or rear circuit fails, the other works. The rearmost circuit in the MC gets its pressure directly from the rod the pedal pushes into the MC. The front circuit is hydraulically pressed forward. There is a metal rod in between the two, but it is shorter than the normal spacing from piston to piston, so it only comes into play if a seal or line has failed. Pedal travel is longer, but there is something there eventually to slow you down.


Just some thoughts.

Thanks, your input is very much appreciated!
A problem with this car that I was hoping would work itself out but persists. I feel like I'm getting maybe 30% of the front braking.
Had I not rebuilt the calipers and noticed all upgraded hoses I would suspect something else but believe calipers and hoses are in top shape.
New URO 911 355 012 02 master cylinder on order.

[Bill Verburg]

Originally Posted by Artfrombama

Thanks, your input is very much appreciated!
A problem with this car that I was hoping would work itself out but persists. I feel like I'm getting maybe 30% of the front braking.
Had I not rebuilt the calipers and noticed all upgraded hoses I would suspect something else but believe calipers and hoses are in top shape.
New URO 23mm master cylinder on order.

The correct m/c for your car is
Porsche # 901.355.012.02
this a 19.05mm

you can get a better pedal feel at the cost of a bit of leg effort if you can find
Porsche # 901.355.012.04
ATE # 3.2120 - 3901.3
This a 20.5mm version only used for part of '69

If using a 23.8
ATE (ports face left) # 03-2123-3402.3

I know a few racers, mostly w/ 2.7RS cars that use this w/ their early A or S & M brakes but it takes a lot of leg to operate, it's not going to be for everyone

You cannot use the later 23.8 from a 930 on a non boosted 911

Just for illustration here are the hyd pedal ratios each imparts, all have the mechanical pedal ratio of 5.4

19.05 41.311
20.5 35.674 this is a good place to be for an early non booted car
23.8 26.467

[Artfrombama]

Originally Posted by Bill Verburg

The correct m/c for your car is
Porsche # 901.355.012.02
this a 19.05mm

you can get a better pedal feel at the cost of a bit of leg effort if you can find
Porsche # 901.355.012.04
ATE # 3.2120 - 3901.3
This a 20.5mm version only used for part of '69

If using a 23.8
ATE (ports face left) # 03-2123-3402.3

I know a few racers, mostly w/ 2.7RS cars that use this w/ their early A or S & M brakes but it takes a lot of leg to operate, it's not going to be for everyone

You cannot use the later 23.8 from a 930 on a non boosted 911

Just for illustration here are the hyd pedal ratios each imparts, all have the mechanical pedal ratio of 5.4

19.05 41.311
20.5 35.674 this is a good place to be for an early non booted car
23.8 26.467

Thanks Bill, I have the 911.355.012.02 ordered. Since I have an eternally burning brake (dash) light I thought I should go back with the OEM style.

[Bill Verburg]

Originally Posted by Artfrombama

Thanks Bill, I have the 911.355.012.02 ordered. Since I have an eternally burning brake (dash) light I should go back with the OEM style.

That looks like the latest version

[Jonny042]

To the OP.... I have new brakes in my backdate project. When I say new, I mean new. Every single part, including the "S" calipers. I was very surprised at the pedal effort required to stop the car. Enough so that I went searching for pads with a higher coefficient of friction at cold temps. Not yet sure if I have found the ultimate pad yet but am running the latest compound revision of Hawk HP+.

Another option to consider might be a 17.0mm master. My pedal is firm enough that I'd add a bit of travel for the increase in stopping power. Not sure if there is a bolt on option, however.

[Bill Verburg]

Originally Posted by Jonny042

To the OP.... I have new brakes in my backdate project. When I say new, I mean new. Every single part, including the "S" calipers. I was very surprised at the pedal effort required to stop the car. Enough so that I went searching for pads with a higher coefficient of friction at cold temps. Not yet sure if I have found the ultimate pad yet but am running the latest compound revision of Hawk HP+.

Another option to consider might be a 17.0mm master. My pedal is firm enough that I'd add a bit of travel for the increase in stopping power. Not sure if there is a bolt on option, however.

The m/c he ordered is the 19.05mm stock size

a 17mm 914 version would be dangerous. Yes, it reduces the leg effort but it is too small to properly feed Any combo of M/M, S/M or A/M