1973 brake pedal effort higher/harder?

[Jay Laifman]

I've driven my '73 911 for 25+ years. Over the last 3 years, I've been driving my '67 912 mostly. The '67 has the single circuit, smaller bore MC. So, it has more travel and less effort. Now when I switch back to the '73 911, with the dual circuit, larger bore MC, it feels especially high and firm - higher and firmer than I seem to remember. Maybe I'm just used to the 912, or there is something wrong with the 911.

Years ago, I remember a similar change to the 911. As I recall, my mechanic at the time (I think it was Pete Zimmerman or Bob and Connie Fern) said there was something in the pedal assembly that had to be changed. If I recall correctly, it was going from a plastic bushing to a brass bushing. I'm wondering if something like that is the problem again.

I will say that the brakes are NOT sticking - but when I had this problem before, they definitely were sticking - such that as I slowly coasted to a stop, at some point they would grab enough to stop the car. Now the car just slowly rolls until it runs out of flow, and will roll back again if there is any slope backwards.

Any thoughts on what this could be? My imagination? Or something else?

Thanks.

[chris_seven]

I thought that the nominal diameter of both the single and tandem master cylinders were 19.05mm and hence have the same areas.

The caliper piston areas are also similar although the later calipers have a slightly greater pad area.

Providing there was no change in the pedal ratio then the pedal force should be similar.

The volumes displaced would also be the same for both single and tandem cylinders.

There may be an increase in seal friction between the single and tandem cylinders but this would only make a very small change.

If the pedal box bushes are badly worn and binding then the pedal effort would increase but if it was jamming up you should be able to tell.

[edmayo]

Maybe it has something to do with the retractors in the early calipers pulling the piston back slightly more. Chris,,,,does that have merit?

[chris_seven]

Ed,

I had always believed that the pins and associated bushes fitted into early calipers were meant to prevent 'pad knock off'.

Pistons pulling away is generally caused by stored energy in the seal and is undesirable and on race cars it is common practice to fit either light springs behind the pistons or low pressure check valves to prevent this from happening.

The bushes fitted to the rear of the piston in the early calipers have a bronze sleeve and I think that the small clearance between this sleeve and the pin creates a swash which prevents the piston moving back.

In any case piston retraction will only increase pedal travel but will have little effect on pedal force.

[Jay Laifman]

I'm not sure what the difference here is. But I've always known it was common knowledge that the earlier cars had more travel. I know there is much discussion about hysteresis with the 912s. This is what Chris mentions with the pistons pulling away with the stored energy in the seal. Thus, every time you need to press on the pedal again, you need to move more fluid against the piston to push it forward again. They discuss all sorts of ways to try to overcome it - most of which probably only last until the pads wear down a bit.

Perhaps I misunderstood that there was also a difference in MC bore.

I know I asked the guys at Schneider Autohaus about this once - and they said that the only way to make the early more like the late brakes as far as travel goes is to go with the dual system MC. And I could have sworn I've seen that explanation on one of these 911/912 sites too. But, that wouldn't change the calipers and the hysteresis issue.

To be sure, my real concern is whether or not my 911 brakes are doing something funny or if I'm just imagining it because of how much I drive the 912 these days (with the hysteresis issue).

[Jay Laifman]

Here is a vendor selling a caliper claiming that it has addressed hysteresis problems by less friction on the rods that hold the pads in: http://roadlok.com/pdf/XRB_Series_Install.pdf

Are these the pins that Chris you mention?

[chris_seven]

photoI think that there are two slightly different issues.

With regard to pedal force and travel there are only three significant variables.

1. Master Cylinder Area
2. Caliper Piston Area
3. Pedal Ratio.

Unless on of these variables change there shouldn't be any difference between the pedal effort and pedal force.

If we look at the early and later cars we find the following:

65-67

Master Cylinder 19.05mm dia (Single Piston)
Front Caliper Pistons 38mm dia
Rear Caliper Pistons 35mm dia (changed to 38mm on some cars)
Pedal Ratio 5.6:1

68-72

Master Cylinder 19.05mm dia (Tandem Piston)
Front Caliper Pistons 38mm dia
Rear Caliper Pistons 38mm dia
Pedal Ratio 5.6:1

Given these numbers there is no reason why there should be any significant difference between pedal travel and pedal force between the two cars. If anything the 35mm pistons on the very early cars should reduce travel and increase force but this would be a small effect give the overall brake balance.

It is fair to say that the caliper mounting arrangement on the front calipers may have improved slightly on the later cars and this may reduce pad knock off slightly but the differences in the two arrangements would be very small.

I can see no reason as to why a tandem master cylinder would have any influence as the two section operate in series and not in parallel.

The two sections of the master cylinder can move relative to each other and effectively you are compressing two springs each with a stiffness of K compared to the single cylinder which would act as a single spring with a stiffness of K/2. The ravel would be identical for both cases.

As the area ratios are also the same there is no reason why the force would change.

With regard to the discussion about pins I was trying to understand Ed's comments about piston retractors in early calipers.

There are devices fitted into the early calipers which I have heard referred to in the past as 'guide pins' but I believe as stated that they are designed to prevent pad knock off.

It would be bad practice to try to guide the caliper piston on two diameters as there would be some 'fight' between the pin and the OD of the piston and this would have a negative effect on the seal.


There is a pin inside the early calipers (both Cast iron and the Aluminium S Caliper) as shown below.



There is a corresponding bush fitted to the back of the caliper piston - difficult to remove on 65 cars but uses a snap ring on later calipers.



This 'bush' floats radially but not axially and these two parts combine to form what I believe is an 'anti- knock off' device.

As regards the Eringer Caliper, this is one of the finest set of claims I have seen in a while.

If you examine the hysteresis graphs closely you will see the issues.

[Jay Laifman]

Great stuff!

One question. You list 68-72. Was there something different in '73, or was that just a typo? My 911 is a 73. Actually a 73.5.

[chris_seven]

My mistake,

the PET shows the same part number for the brake pedal from 1965 to 1973.

The part number changes in 1974 when the non-servo pedal has a different part number.

I am not sure what the difference between 73 and 74 is because the brake pedal return spring and bushes remain the same as for the 73 pedal and work right back to 1965. The first servo-pedal also appeared in 1974.

I would be keen to learn the difference between the early non -servo pedal and the non-servo 1974 pedal.

[Jay Laifman]

I'm the second owner of my car. I guess there is a chance the PO swapped to the 23 mm master. But, the master was once replaced while I've owned the car. No one asked me if I wanted to stay with the 23 or go back to the 19. Is there a way to tell from the outside of the master while installed on the car?