Early 911S brake balance

I recently purchased a 1973 911S and am now trying to sort the car out. This is the first 911 I've owned so I'm not 100% familiar with them and their systems. Is there a proportioning valve in the brake system like there is in the 914s? I'm trying to figure out why the car locks the rear brakes before the fronts. I bled the system tonight, and other than some old fluid didn't find anything unusual. All rotors and pads are in good shape. The system is capable of locking all four wheels but it consistently starts with the rear tires.

Is this typical for the 911s? Seems backwards of what is desirable to get the car to perform. Especially with the tendenacy to bring the back end around in a corner. I don't need something else working against me.

There is no proportioning valve in the early 911 brake system, unless added in the aftermarket. Porsche accomplished proper proportioning by sizing the rear caliper pistons and pads much smaller than the front on your car. You definitely have something wrong with your braking system if you are locking the rears first. That is a very dangerous situation and could easily cause a spin under hard braking, especially trailbraking.

Have you taken a good look at your hoses? The old rubber hoses can deteriorate over time and it's possible the fronts are "ballooning" and not transferring as much force to the pads as the rear.

Are you sure there are no leaks in the front that may be contaminating the pads or rotors with hydraulic fluid, causing them to slip instead of grab? Are the front and rear pads the same compound? Is it possible the fronts are glazed and the rears are not?

Did you "exercise" the calipers with the pads out and make sure they travel freely? With the pads in, are they retracting the same (slight) amount front and rear when released?

Are the dust seals intact? I had a problem with pieces of rubber from the dust seal working loose and getting between the pad and rotor once. This happened on one side only, though, and resulted in a radical pull towards the opposite side. There would be only a very small chance that it would happen on both sides simultaneously.

I would definitely get to the bottom of this problem before driving the car hard, even if it means rebuilding all 4 calipers, turning the rotors, and installing new hoses and pads. You gotta have "whoa" before you can "go".

TT

Thanks for the info TTWEED. I couldn't beleive this system was operating as intended. As you noted, trailbraking is definitely out of the question until I get this straightened out (no pun intended).

I spent more time under the car last night and everything looks great! Piston were moving freely and retracting properly, hoses appear near new (my understanding is that old hoses will lead to the opposite problem I'm having: brakes apply fine, but they won't release). The front pads also look newer than the rears, ie have more meat left on them. Could be because this condition has existed for a while and no one noticed or bothered to fix, a dangerous situation. Or, I suspect the front pads were replaced because they worn our before the rears. And, in replacing them, a less agressive set was installed up front leaving the rears with more bite and leading to this condition. I think I'm going to start with new pads and see if that solves the problem.

Thanks for the info. At least I know I'm not expecting something the car isn't capable of delivering. I'm excited to see how well the aluminum S calipers work!

The other things I can think of (apart from TTWeed's comments) are that:

a. you have a problem with your master cylinder,
b. a kinked brake line somewhere,
c. calipers jamming by being out of alignment or
d. that your rotors are "glazed" and thus the front pads are not bitting as much as the rears.

Usually, rear brakes (especially on Porsches) account for a VERY limited part of the braking forces.

The S caliper stock pistons are known to rust out and not move freely. I'd definitely look into rebuilding them (the kit is only $20 unless you need to replace the pistons).

I have had a weird problem with on eof my cars (78 SC) in which the front calipers "separate" under brake pressure. You could even hear them creak and crak when you applied the brakes. This wouldn't happen on the S type. I have rebuilt the S on both of my 69 S's and it is not a hard job at all.

Hope this helps. Juan (119300740/119300398/9118201069)

Juan's idea about a bent or contstricted hard line is worth checking. I couldn't think of any way the master cylinder could cause the problem of differential pressure to the front and rear, since the same cylinder drives both circuits, but if there was a clogged port or fitting, I suppose it could. Most of the time their failure mode is the seal leaking, though, and you would not have a firm pedal and fluid in the pedal cluster, probably.

The "glazed" rotors is also a good guess- I have heard that high heat in track use can cause the metallurgy of the rotors to actually change, in spots, or they can warp. You should feel this in the brake pedal though, as a pulsing sensation.

I think trying the simple things, like cleaning the rotors well to make sure they aren't contaminated with grease or oil, and changing to new (same) pads F/R is a good start. Different pads can have very different grab, especially when cold, and if the front set was glazed??? Also, the obvious thing to check is the tires- same type front and rear? Inflations correct?

The less obvious thing is corner balance. Is just one wheel locking in the rear first? Depending on how tweaked your alignment and right heights are, you may be jacking weight pretty weirdly under hard braking. That could cause one rear wheel to get really light.

Hope it's easy to fix- you've got a great car there and should be able to enjoy driving it at the limits.

TT

Thanks for the input. I thought about the glazed rotors and/or pads and checked that when I was bleeding the brakes. Looked fine to me. When I bled the brakes, there was plenty of volume out of the bleeder screws so I don't think a kinked line is the culprit. Didn't have an assistant that night so I wasn't able to check for piston travel. But, I suspect they are working fine. It is possible to lock all four wheels on the car, it's just that the rears come in too much before the fronts to be correct.

TTWEED, to answer your question: both rears lock at essentially the same time as do the fronts when you lock them. I suspect alignment and corner balancing will help with the overall handling of the car, but I don't think it's at the root of this problem. I'm still leaning toward mismatched or contaminated brake pads.

Try the obvious first, the front pads are newer than the rear, (replaced at a different time - so are they the same compound?) Try both with the same compound pads. Let us know the fix.
Thanks, Bob

Playing with cars et al:

I admit to limited knowledge on these cars, but on my '67 S the factory flex lines were replaced with larger diameter stainless lines. The result is that a pump is required to bring the pressure up then apply pressure for the stopping effort.

Is it possible that the front lines were replaced and the rears were not? Perhaps the higher volume required by the larger inner diameter brake hoses is enough to emphasise the rear brakes?

Let us know what you find out please.

Regards;

Mark ****inson
Calgary, Alberta

Just to cover all the bases - you know that it's not the brakes that stop the car, but the tires. Different brand front to rear; new full tread on front versus older worn on rear; all tires old and hard?

In effect, it could be that your brakes are working normally, but the rear tires are loosing grip before the front.

Vern

ALL flexible brake lines should be the same diameter - which is -3 (AN3) - never -4. You are looking for pressure and not volume. Check the dual piston master cylinder in '68 and newer cars. Often, one will leak by and the other won't.