e30 soft brake pedal solved?

#1

I’ve long noticed that our MC moves under hard braking and I figured contributed to our relatively soft brake pedal. If you’ve ever run some laps in a Porsche, then you know what a brake pedal “should” feel like.

Tonight I put the wife in the car and with the engine off, had her repeatedly press hard on the brake. I could see the MC move 3mm or maybe a little more. Looking at the geometry of the brake pedal arm, I’d say there’s a multiplier of 4x so that’s 12mm of soft pedal movement due to MC movement. I’d call that a holy shit.

I couldn’t detect the firewall sheetmetal moving, but I wasn’t able to get eyes on to the underside of the vac assist where firewall movement, if any, would have been most apparent. I stuck my hand down between the bottom of the vac assist and the firewall and I could certainly feel the back of the vac assist moving, but I’m not sure about the firewall. So either the vac assist is moving away from the firewall or the firewall is bending. How the heck can I keep the MC from moving?

Go try that experiment. Lets see if this behavior is universal, or maybe limited to early models, a brand of (or simply old) vac assist, or whatever.

If it’s universal, it would be interesting if we could come up with a cheap and easy fix. Cheap and easy can be turned into a rule change well enough. But in order to fix it we need to better understand exactly what’s happening. Surely M3 MC’s don’t move 3-4mm.

#2

Boy can you show a girl a good time on a Friday night!

#3

To my surprise I found that RockAuto has 3 different size MC’s for my '87. I have the smallest.
15/16 (23.8mm)
7/8 (22.2mm)
13/16 (20.6)

Some say that we have a dual size piston in the MC with a larger piston for the front brakes an a smaller piston for the rear. Like 22.2/17.5mm. I’ve not been able to confirm that tho.

Smallest MC means more pedal travel but less pedal foot pressure. I was kind of thinking that maybe I could get a smaller MC so less foot pressure would make the MC move less. But since I seem to have the smallest MC already, that idea took a hit.

I would never have guessed that there’d be 3 different MC sizes for our car. I wonder what other parts have mystery variance and how one “proves” that one type of part is right and another one wrong.

MC movement isn’t the whole ballgame tho. There is some consensus that while ATE calipers are stiffer than Girling, there’s still a lot of caliper flex. Since the MC doesn’t move much fluid, even a tiny amount of caliper flex would be apparent at the foot pedal.

#4

I grabbed a neighbor today and did some more experimenting with the brakes. Conclusions:

  1. The ATE caliper is pretty darn stiff. It may not be as fabulous as a big brake kit, but it doesn’t flex much. It “positions” itself under hard braking, but that’s not flex.

  2. The problem child is my ATE vac boost. It flexes like a Mofo under hard braking and that allows the MC to move. Is a flexy vac boost unit common? Are Girling vac booster’s made of sturdier stuff than mine? Maybe mine’s just old and tired?

I tried to imagineer a way to stiffen up the vac booster, but I didn’t really get anywhere. It’s such an odd shape.

#5

If we are doomed to suck up wimpy boosters, we should consider a booster delete kit. Basically it just moves the brake MC connection on the pedal arm such that you get more mechanical advantage, 6:1 instead of 4:1. Then you remove the booster. It’s pretty cheap too.

If I can’t find a way to get a non-wimpy booster, I might try out this booster delete kit. If it works well, I’ll try for a rule change to allow it.

If I end up removing my booster and I win a race, DQ me for “inadequate vacuum assist”.

#6

I have the girlings and my brake pedals is as hard as a diamond in an ice storm. You need to check P/Ns before ordering from rock auto.

#7

I have the early car Girling caliper/ATE MC and ATE vac booster setup. MC visibly moves while someone is pumping the brakes, so no, it isn’t just your car. If you come up with a cheap and easy improvement, I’d go for it. My brakes tend to have a good bit of mush. Pump once and it feels much better, but that doesn’t exactly help lap times.

#8

What about a simple master cylinder brace. Something coming off a strut tower bar and then contacting the master cylinder not allowing it to move. They are pretty popular on some other makes.

#9

Two non-is, four door chassis, '86 and’87 cars= mushy brakes.
Laura’s '86 es chassis has firm brakes.

Correlation?

RP

The only place where the mush is a problem is at CMP where you have to use lots-o-brakes. I HATE that track.

#10

[quote=“Patton” post=70977]Two non-is, four door chassis, '86 and’87 cars= mushy brakes.
Laura’s '86 es chassis has firm brakes.

Correlation?

RP

The only place where the mush is a problem is at CMP where you have to use lots-o-brakes. I HATE that track.[/quote]

Can you do my test on your cars? Hard push on the brake and look for any MC movement. If it moves, stick your hand around the vac booster as if you were reaching for the starter and then get your hand between the bottom of the vac booster and the firewall. Then with move your hand around to different positions as someone repeatedly brakes hard and feel for what is deforming. With different hand positions you can sense if the firewall is bending, if the booster is trying to move away from the firewall, if the two booster halfs are trying to separate, or if the booster shell is simply deforming. Mine is the latter.

If the person sitting in the driver’s seat is a practical joker, remind them that these tests are to be conducted while the car is STATIONARY.

#11

You mean Girling MC and booster, or Girling front calipers?

#12

Save yourself a lot of grief and replace the booster.

#13

89 4 door chassis, All Girling, and the pedal is rock hard. In fact some of the best brakes I’ve experienced on track and I’ve driven and owned Porsches. My buddies 87is ATE system is another story altogether.

#14

More data - mine is an '87 with a soft brake pedal as well.

#15

The question isn’t whether or not you perceive your pedal is hard. The question is whether or not you can detect your MC moving when your lovely bride pushes like hell on the brake pedal.

#16

Last night I put wifey in the car and had her press on the brakes hard repeatedly. Car was off. Objective was to attempt to judge if my MC restraint strap was doing any restraining. I’d say yes. Whereas it was moving out and up 3-4mm, it’s now moving maybe 2mm. I won’t know for sure about any change in pedal feel until the car goes back on the track which is some weeks out.

I also bought a used Girling vac booster on ebay.

#17

Where is this “restraint strap” allowed in the rules?

#18

My bad… I thought you were looking for data points…How many years have these cars been running… Now your latest car has a problem and I detect a rule change in the air???

#19

[quote=“cosm3os” post=71008]Where is this “restraint strap” allowed in the rules?[/quote] Under “fasteners”.

Here we may be on the cusp of figuring out the most interesting development ever in SpecE30 braking and your response is to throw sand in the works?

Where does it say you can clearance a subframe bushing so that there’s room to get a tool on an adjustable RTAB? Where does it say you can imagineer a solution to help keep an rtab from moving? What we’re trying to do here is make something work properly. In order to figure out solutions, sometimes a person has to experiment a bit at the outer edge of the rules. If it works, fine, one can submit a rule change, and the community is done a big service because a problem is solved. If the idea doesn’t work then the community learns from that too and we try different solutions.

One can be trying to fix problems, standing on the sidelines and waiting for someone else to figure it out for them, or they can throw sand into the works. I’m not suggesting that everyone go out and slap some strap on their MC, I’m just trying to figure out a problem. Maybe the result will be an awareness that Girling boosters are superior. Maybe the result will be acceptance of a strap on the MC as a “fastener”. Maybe the result will be a rule change. But lets give me some space so I can figure out what’s going on and test solutions, eh?

#20

…or just thinking new ones up.

Palacio never complains about his brakes. In fact, I’m not sure he ever uses them. :evil: