Brakes, Brakes, Brakes.....

[333pg333]

I've put this up on a few forums and thought you guys deserved your chance to voice opinions too.

This is in response to my continuing quest for a more solid pedal and I'm not the only one out there. I have found that many people in fact complain of a 'mushy pedal' on our cars.
I've done a bit of research on this issue and some of you may have seen me on other forums asking similar questions. I was under the impression that it may have been due to the stock m/cylinder not supplying enough hydraulic pressure to the upgraded or upsized calipers with larger pistons. I have heard what is so far turning into a story of mythical proportions that there was a 7 series BMW from the 90s that had a m/c with the same fittings as ours but bigger pistons. This was the advice of Sam Lin on 951Forums http://951forums.com/forums/index.php?topic=204.0
I have pressed him for more info but he has remained dormant on that thread.
I had also thought that perhaps it was something more common to cars with RHD, but I now don't believe this to be true as others can testify. I contacted ATE who make the m/c for our cars but they didn't want to know me as modifying the stock setup could have legal ramifications if I crash due to their advice. Here's another idea from Tony G on Rennlist.

"If the issue is simply pedal travel, then you could simply pull the brake pedal and drill in 2nd hole in the pedal so the the clevis ratio is faster (which would be away from the pedal pivot point). Go small. 1/4" should make it quite a bit faster and should make the pedal harder since the leverage will be reduced."

This could be an easy fix and would be interested if anyone's tried the above?

I put the larger m/c concept to Markus Blaszak in Canada (long time Porsche mechanic) and this was his reply to my question below:

Markus,
If the piston size in the caliper is increased like going from standard
> 951
> brakes to Big Reds or 6 piston calipers then wouldn't the larger m/c move
> correspondingly more fluid commensurate with the larger calipers?
> I'll check the firewall, but in the past I've had my pedal go to the floor
> on the track. That was a long time ago before I made some changes though.

"Yes and no.

You are dealing with pressure and MINUTE piston movement... what you are
saying applies ONLY if your brake pads are 1/8" AWAY from your rotor!
Think about it, your pads are always touching the rotor. The piston is
always touching the pad backing plate... so how much movement is there to
lock up the brakes??? About 0.015" that is it!! How much fluid is that in
each system?? about 1cc of brake fluid! So, what you want is pressure,
not fluid volume.

Look up hydraulic principles on the internet or go to a good hydraulic
shop near you... To lift a huge load they use a large piston and the pump
feeding it has a small piston. The larger the feed piston the faster the
main ram moves but the less pressure it can exert! Since you need minimal
fluid (you are not moving a huge distance) your focus needs to be on
pressure.

If you have excessive pedal movement and a soft pedal, you have:

1) air in the system still
2) swelling lines, yes even if they are SS lines!
3) a leak! check calipers, line fittings, hose joints and master cylinder
4) internal leak in master cylinder... pressure is blowing past seals
internally so you do not get press. to calipers AND you get a long pedal
5) cracked firewall and master moves when brake pressed."

So I'm not sure of the answer. I have changed my pads to Pagid Blues at the behest of my mechanic who is very happy with them in his '86. I have upgraded my setup over time and unfortunately due to a lot of bad luck, haven't had much tracktime recently so hopefully I can do some more testing of the current setup pretty soon.

My setup is direct to rotor venting/ducting, 928GTS discs, Big Reds, s/s braideds, SFR fluid, new M/C, rear bias 5/33 valve, PFC pads (now Pagid Blues), pretty decent sized sticky rubber, and yes, plenty of power bleeding.

PS as discussed with Scott on Rennlist. He mentions ABS as a possible cause.
"Well that's another angle that I neglected to mention Scott. I have had some experienced racers suggest that ABS can indeed be the cause of brake issues. My understanding was that they were implying that with a lot of repeated heavy brake usage, the ABS just can't keep up and this can lead to brake failure. Well that would almost certainly only be on the track and brake failure (pedal to the floor) is not necessarily what we're discussing here. Having said that, I did have maybe 3 occasions of this back when I first started to track the car. Guess what pads I was using. EBC Greens & Yellows. I changed to PFC pads and the difference was immense. Yet even though the bite and retardation was increased 100 fold, the pedal still isn't as solid as I'd like. Also I'd like to say that 99% of the time my brakes pull me up very well, it's more the modulation that I want to change.
I am quite sure that the ABS could be problematic but I don't want to remove mine from the street car. I like it for backup."

 

[Tam Lin]

ORIGINAL: 333pg333


If you have excessive pedal movement and a soft pedal, you have:

1) air in the system still
2) swelling lines, yes even if they are SS lines!
3) a leak! check calipers, line fittings, hose joints and master cylinder
4) internal leak in master cylinder... pressure is blowing past seals
internally so you do not get press. to calipers AND you get a long pedal
5) cracked firewall and master moves when brake pressed."

Well I have a soft brake pedal
and
1) The system has been completely bled twice, and all fluid changed from ATE Blue to high temp (dry) boiling point race brake fluid
2) All (yes all) brake pipes are brand spanking new, OEM except for wheel wells, which have braided aeroquip, pedal feel is the same
3) Pedal is same with old floating calipers & new 4 pot fixed calipers
This does rather indicate that the culprit is a master cylinder internals or firewall mount.
I must say, this is the only instance where my old Peugeot 309 GTI rally car was vastly superior to the current Porsche track toy.

 

[Diver944]

Patrick I had a new standard master cylinder after my brake failure (as well as new Big Black callipers, disk, pads, lines, hoses etc) and my pedal is now as firm as I have ever felt it in any 944 and I am more than happy with it.

Historically I always felt that my S2 brakes had a much firmer pedal and were crisper in operation than my Turbo and I always put this down to the smaller S2 callipers compared with the Turbos slightly larger versions. Now that everything is new on my Turbo it makes the S2 feel postively mushy and archaic in comparison.

Are you still using your original master cylinder? It may be worth trying a new standard one before trying to go too far off from the norm?

 

[zcacogp]

Paul,

While I don't disbelieve your experience, how would a master cylinder cause a soft brake pedal? I would assume it can only be if it ha less-than-perfect seals in it, and fluid is leaking back past the piston.

If this is the case, where does this 'leaked' fluid go when the brake pressure is released? Does it return to being in front of the piston (unlikely, unless the piston seal is dire), or does it leak out somewhere?

If it does leak out somewhere, you will also see the brake fluid level (in the reservoir) drop over time.

Does the level drop in your brake fluid reservoir? Anyone? Mine doesn't ...


Oli.

 

[Diver944]

ORIGINAL: zcacogp

how would a master cylinder cause a soft brake pedal?

I certainly can't argue with you or even explain why, I just know that beforehand my S2 was the crisper braker and now it's not. The Turbo was by no means soft, but it was definatley softer and you had to press harder

We all know that perfectly sealed 15 year old shocks that feel fine, turn out to be useless when you fit new shocks. I'm just hypothesizing that maybe our master cylinders are display a similar aging [&:]

Reading back over some of Patricks input from other people it seems we are talking about tiny amounts of fluid that make a big difference, so maybe just softer seals are 'moving' around a tiny amount?

 

[zcacogp]

OK, fair enough. 'Softer' seals moving a bit sounds plausible, for sure.

(An aside - can we define what we mean by "soft" here? Do we mean that, if you sat with your foot on the brakes, the pedal would slowly sink to the floor, or just that the pedal moves quite a long way to effect a given amount of retardation? My discussion 2 posts up would apply in the former circumstance, but not the latter.)


Oli.

 

[Diver944]

Good point.

From my point of view I am meaning a bit of slack in the pedal before it goes rock hard and it doesn't move any further once it's at that point, no matter how hard or long you press the pedal

 

[zcacogp]

By 'slack in the pedal', you'll not be meaning slack in the linkage to the master cylinder? As in, movement of the pedal will follow these three stages:

1. Free movement, taking up the slack in the mechanism between the pedal face (your foot) and the master cylinder piston. This should be minimal if the mechanism is well-adjusted. (Here I have a question which I keep on meaning to post on another thread, but there we are.)

2. 'Stiffer' movement, as the master cylinder piston moves, fluid moves around in the system, pads move towards the disks, pipes bulge, fluid compresses slightly, any air bubbles compress a lot. This can be minimised with good fluid, good bleeding and stainless flexi-hoses.

3. Pedal should reach a solid stop, as all 'slack' in the entirety of the system (between your foot and the brake pads) is taken up. Any extra force applied here will cause a greater pressure to be built up in the brake system, and the pads to be forced onto the disks with more force - i.e. the braking 'force' will increase. (There may be fractional extra movement at this point as the linkage, bulkhead and pedal flex a little, but this should be minimal.)

I guess the question is, which part of the process is it that causes things to feel 'soft'? If the master cylinder (or anything else in the system, come to that) is leaking, stage 3 above will not be reached as the pressure in the brake fluid is lost through the leak. Although a leak will also affect stage 2, as this involves pressure in the system as well.


Oli.


Oli.

 

[Peter Empson]

I'll read this thread properly when I've got a bit more time, but the best pedal feel I've had in a 944 was on my Lux with the original calipers, when they were rebuilt, running with Porterfields, standard pipes but good fluid they were brilliant and not remotely soft (I've compared them to the switch like pedal in my old Sh*troen BX before). If I could get something like that setup again I'd be delighted as I too have that slightly spongey feeling pedal now.

 

[sawood12]

I'm not sure how much of this is due to us getting used to modern cars with over-servo'd brakes. In modern cars it seems the pedal doesn't move at all and they are more like an on/off switch. I get what Patrick is getting at but i've not decided if it is how it is. My pedal certainly has alot more travel than a modern car - which I guess is by design so you have better feel and modulation - and I believe it is similar in brand new Porsches. But once the brakes bite the pedal doesn't feel rock hard. Yes you can press as hard as you like for as long as you like and the pedal wont move, but it just doesn't feel rock hard. Again i'm not sure it is supposed to - I certainly have no probs overcoming tyre traction so there is plenty of power there.

All I know is that 21yrs without servicing is alot to ask of most engineered components and it is probably about time for the vast majority of us to replace our ageing master cylinders. Would you take your family on an airliner knowing that the brakes had not been serviced for 21 years apart from the usual consumables? Another thing i'm suspicious about is the ABS unit. I don't know how they work but they must have many internal moving parts and pistons with seals and how many of us have even thought about replacing/refurbing the ABS unit. If the master cylinder is a possible cause of give then the ABS unit must also.

I'm switching back to standard pads and when I next replace brake fluid I will get a new master cylinder (they are cheap afterall) and after than I'll be satisfied with whatever i've got.

 

[wibble]

I have a late turbo with the larger calipers. Earlier in the year, after routine maintenance I found that a rear caliper had siezed and the fronts were suffering from plate-lift. I decided to have the whole lot refurb'd, and fitted braided hoses, ATE blue fluid with standard OEM pads. The brakes now feel great and progressive - they do the job. Admittedly, I don't use my car on the track, so I cannot comment on continued hard use that a track demands.

If you read some of the 911 GT3 threads, their brakes discs suffer from hard driving.

 

[333pg333]

I have a new m/c in my car for the record. Due to circumstances I have not had the car on the road / track much of late, however I'm due to get it back this weekend and will hopefully have it for long enough to try the latest incarnation which is about as good as it gets. It may be something as simple as changing the ratio of leverage as described by Tony G in my opening? That would be sort of funny. Markus' assessment of going up in size with the m/c is interesting. I'd still give it a go if I can find something compatible.