Question About My '74's Brakes and Proportioning Valve

[Tidybuoy]

I rebuilt all 4 calipers on my '74 about six months ago. The rears had a fair amount of rust on the piston tops (not the sides). I kept and reused all the old pistons. After the project, braking improved 100% plus.

However, I still feel that braking is not 100%. I use my 944 for comparison which has great braking, however the 944 has boosted brakes and the 911 does not. The 944, base model, has much larger calipers and pads than the 911. The 911 stops fine but if I need to stop hard, I usually downshift and use engine braking for added braking assistance.

At auto-x last week, I felt that the front end locked up at times and I attributed this to a dirty parking lot surface with bits of loose gravel. But, a friend of mine mentioned that it looked like my front brakes were locking at hard braking points. He told me that I probably need to adjust the proportioning valve. I told him that the car does not a proprotioning valve and he recommended that I add one.

My questions: does this sound like I need to redo or replace my rear calipers? I flushed the brakes several times and had no problems with fluid flowing to all calipers. I did not do anything with my master cylinder.

Any advice would be great........Vern

[Steve@Rennsport]

Hi,

As you already know, the '74 cars didn't have, nor need, a proportioning valve. 944's have ABS, hence less propensity for lockup.

YOU are the "ABS" on your '74 and thus some driver technique changes (and perhaps pads) are the key toward prevention.

FWIW,...I would caution you from using the engine to slow the car: pads & rotors are FAR cheaper than engines as the rods and bearings take a real beating when using engine braking.

[kodioneill]

Did you replace the rubber brake lines? They choke themselves off after many years and become plugged I've seen it many times.

[Tidybuoy]

For what it's worth....My 944 does not have ABS. That was available as an option on '88 and standard thereafter. I don't like using the engine for braking but it can be necessary to avoid a colision - this is why I think something is wrong.

My '74 has braided stainless lines, and I'm not sure (without getting under the car) if there are any rubber lines up to the point they are braided SS. But, that said, the fluid was flowing well when I bled the brakes.

One more thing...I plan to replace the rotors but at the time, i had them resurfaced and the shop told me they are at their limit and need to be replaced. Funds were tight but I will be replacing within the next couple of months.

[manbridge 74]

Have an IR temp gun? I haven't shot mine, but I'm guessing the temps should be close to the same at all four corners if operating properly. 911s don't experience the same weight transfer in normal driving that most front engine cars do so I'd expect somewhat equal temps.

That said, I'm sort of in the same boat as you. My fronts would also lock up on my 74 so I'm doing brakes as well. I notice I have later A calipers which may or may not make a difference.

Do you have the stock M calipers? Have you attempted re-bleeding after some previous bleed? I like to come back after a day or so. Usually get a tiny amount of air to come out again.

[Tidybuoy]

Originally Posted by manbridge 74

Do you have the stock M calipers? Have you attempted re-bleeding after some previous bleed? I like to come back after a day or so. Usually get a tiny amount of air to come out again.

I currently have M calipers. I'm hoping to upgrade to A calipers at some point but as you know, this requires also purchasing 3.5" strut/spindles.

I did bleed my car several times and I used both a power bleeder as well as the old pedal method. I went thru 2 quarts of ATE Blue. My primary concern is that fact that the rears were in very poor shape prior to rebuilding. I want to find a good set and then send to PMB for restoration (which includes re-plating).

Also, I'm not sure if this is normal for a '74 non-boosted system but I can get the car to stop well with significant pedal effort. But the regular stopping without stomping on the pedal seems very weak.

[Steve@Rennsport]

Originally Posted by Tidybuoy

Also, I'm not sure if this is normal for a '74 non-boosted system but I can get the car to stop well with significant pedal effort. But the regular stopping without stomping on the pedal seems very weak.

M calipers are marginal on these cars and do require considerable "leg" to stop the car. Pad choices become critical for that reason, alone.

Installing A calipers with good pads will REALLY help, but its important to understand the fundamentals of brake bias. In this way, you'll know why the rears do not lock up and how dangerous it would be for all four tires to lock at the same time.

[chrismorse]

If i recall correctly, the A caliper has the same piston diameter as the M but it has a larger, (S) sized pad, (as well as requiring 3.5 spacing on the strut), so, am curious how going to a larger pad is really going to help, other than being more durable than the M pad at higher temps.

I am rebuilding my 74 brakes and suspension, replacing all rotors with plain zinc plated Zimmerman pieces and overhauling the calipers. I am going to give Caswell's home Zinc plating kit a try, along with EBC yellow stuff pads and stainless flex lines.

While i am in there, i am going to build my own backing plates and cool air intake for the front rotors.

With the stock 48/38 piston diameters, I don't know what else you can do to improve the brakes. I am running slightly shorter 195/55 tires, which will give the brakes a bit of an advantage. You could go to a slightly smaller master cylinder to reduce the leg effort but this might lead to a sbit softer or long pedal.

Other than loosing a lot of weight, increasing the disc diameter, or going to larger pistons in the brakes, you are going to have to just push harder.

I have very little experience with 911 brake improvement, so it might be good to talk to others about a pad with a higher coefficient of friction to get better braking with less effort. Fromwhat i have heard, the EBC yellows might be the way to go, but i definitely defer to those with the real experience.

hth,
chris

[FPB111]

Go to higher/lower friction pads.
Mix different pads front to rear. NOT ON THE SAME END. The 911 is much lighter in the front, so the brakes will lock sooner then a 944.
Porsche carbon kevlar brake pads - PowrPad
or
http://www.federalmogul.com/en/AftermarketSolutions/NorthAmerica/BrakingSolutions/Products/BrakePads-Shoes/FerodoRacingBrakePads/

Ferodo DS2500 Brake Pads
Ferodo 4003 Brake Pads
Ferodo DS3000 Brake Pads
Ferodo DS3000 Plus 03 Brake Pads
Ferodo DS3000 Endurance Brake Pads
Ferodo DS1.11 Brake Pads
Ferodo DS2.11 Brake Pads
Ferodo Racing Compound Selection
Ferodo Racing Part Number Suffixes
Ferodo Racing Pad Bedding

or
http://www.braketechnology.com/techinfo.html

choices are endless have fun experiment

[manbridge 74]

For what's it worth some old road test 70-0 mph figures.

74 coupe.................180ft
74 targa .................186ft
74 carrera ..............187ft
74 corvette(L82).....208ft

78 SC ....................177ft
Ferrari 308 ............220ft
Esprit ....................208ft
Corvette (L82)........172ft

So clearly the stock brakes must have been fairly capable for the times.

I always remember reading the old tests and how little brake fade the testers noted.

[Tidybuoy]

Originally Posted by Steve@Rennsport

M calipers are marginal on these cars and do require considerable "leg" to stop the car. Pad choices become critical for that reason, alone.

Installing A calipers with good pads will REALLY help, but its important to understand the fundamentals of brake bias. In this way, you'll know why the rears do not lock up and how dangerous it would be for all four tires to lock at the same time.

Thanks Steve. My plan is just to upgrade to the best of stock so SC A Calipers and Rear M's. I believe I used Mintex pads but I will have to check. Also, since the shop told me my rotors are at there limit, this may also have some effect as the shop showed me how one side of the vent was thinner than the other on each rotor.

Originally Posted by chrismorse

I am running slightly shorter 195/55 tires, which will give the brakes a bit of an advantage.

You bring up and interesting point as I have 195/55/16's on the front and normal is 205/55/16. The prior owner used this size I believe for balance to the rear 205/50's but maybe this affects the front braking with a narrower front tire, also my tires are starting to show some wear after a few auto-x's this year.

Vern

[Driver_X]

The proper size front tire would be a good place to start.

[manbridge 74]

Proper size front tire was a 185/70 15 all around and they stopped pretty well with the rubber compounds used 38 years ago.

[Driver_X]

My point exactly, same size tire front & rear regardless of wheel diameter.....

[manbridge 74]

No experience here with mis-matched tires but I agree in principle.