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I have changed the front brakes of my '78 from the standard tiny 924 style sliding caliper to the S version floating caliper (wsm part 4 repair section 46). To do this, I installed "S" version wheel carriers, bought new discs and dust shields, and of course the calipers(used) and a lot of new parts for the calipers.

I was wondering whether the brake balance needed to be corrected front/rear. I asked our club tech expert during a wrenching meeting. As I already expected, the brake pressure regulators have to be changed also. As described in repair section 47-p18a, the brake booster changed from 9" to 10" from 08.011979 (is this August or January?). For the 9" brake booster, two different brake pressure regulators were used, with switchover pressure 55 bar or 33 bar. The 10" brake booster always had the 33 bar pressure regulators.

If I read this correctly, all brake pressure regulators were changed from 55 bar to 33 bar pressure changeover, regardless of brake calipers installed. Both for the standard (small) brakes as the "S" version brakes.
My car being a 1978 car must have the 55 bar pressure regulators according to the WSM.

Now my very important question:
Do I need the 33 bar pressure regulators yes or no? Are these parts any good when bougth as a used part?
And: Do I also need a 10" brake booster to make this work? I hope not.

Your expert advise is highly appreciated.

hacker-pschorr

I don't have an answer and I'll be watching this thread closely, I can only share how my cars are setup.

My 81 was upgraded to S spec 14+ years ago after I experienced horrible brake fade at Road America (with good fluid and Pagid Orange pads). I used my car for a few DE's without any further brake issues after the upgrade. I did nothing with the booster or valve and the car never felt out of balance under braking.

I "retired" my 81 from track use when I picked up my 79, which has S4 brakes all around, no change in booster or bias valve. The two previous owners used this car pretty much as a dedicated track car, I use it on the street quite a bit too (haven't done a track event in years).
Every instructor that has ridden with me or driven the car at the track have all commented how awesome the brakes feel in my car. They also pointed out my #1 problem was not using them hard enough.....

FWIW - I have zero complaints with my cars and changing out the brake booster seams like a lot of work for what gain? Does the larger booster even fit in an OB without modifications?

Your 78 is also lighter than the "S" car those brakes came out of, something to ponder into the equation.
My 79 is just a hair over 3,000lbs so significantly lighter than any stock S4 those brakes came from.

928nut

I've done this a few years ago on my '79, never bothered with pressure regulators or brake booster. Car/brakes feel very good, no sign of any imbalance under braking. The original brakes were downright scary under heavy braking.

The Forgotten On

I second the terror experience under heavy braking in my 81. You won't need to change a thing under the hood if you upgrade to S or S4 brakes. Just as long as your master cylinder is in good shape.

If the OB braking system apart from the booster stayed basically the same in the euro S cars under the hood, I think it would be fine for what you are doing.

The only difference is the pedal will be a little firmer on your car than a car with a 10 inch booster.

Petza914

Not specifically for this thread, but related I case others are viewing it. One thing that many people don't consider is the piston size within a caliper and they think as long as the bolt spacing, caliper position, and wheel clearance match-up that a given caliper can be installed on a given car. This is more frequent when taking a caliper from one model of car and fitting it to another (like big reds from a 911 or Cayenne onto a 928). If the are a different size you can end up where you have to either put a lot of force into the pedal for good braking, or the opposite which is even worse, very little force for a lot of braking and hardly any range of modulation. The safe bet is to fit front / rear pairs from the same model of car (not necessarily from the same car). For example, on my '79 Spyder I have Big Blacks from the GTS on the fronts with the correct S4/GTS rears and slotted rotors at all 4 corners, without any other changes and the brakes feel great.

mark kibort

don't make such a big deal about the bias or the larger diameter rotors, as they both are relatively small compared to todays standards and grip capability of better tires.
we fitted the S brake 84s with 13" rotors, bigger than GTS, and it works perfectly with the stock bias system.. mainly, as its a naturally correcting situation. first of all, most any brakes will get to lock up the front tires. what the larger diameter rotors do , is disappate more heat, and give more leverage when the brake pad coefficient of friction goes down. you press harder, so it naturally gives more bias to the rear to make up for the fading front brakes. without the fade, you use less pressure in a threashold braking event, so naturally there can be less bias in the rear, but compared for how the bias was set up.(safety under emergency street driving stops) its a non factor the greater the stopping G forces, the more weight transfers to the front, and less weight is in the rear. engine braking alone on the track, almost hits the limit of a 1.4G stop of the rear tires. I remember at the track, just raising the bias setting from 18 to 33 bar, made the car undriveable for trail braking into 3rd gear hard 90 degree turns. putting the 18bar back in, and that solved the problem.

the point is. putting larger diameter rotors up front, wont change anything very much. Especially going from a S brake size to a 79 to a S size rotor.

Ad0911

I did a quick research on the workings of the brake preswsure regulator, or bias valve, or brake proportioning valve.

How it works:
http://www.classicperform.com/How/Ho...-Prop-Work.htm

and (type C):
http://stoptech.com/technical-suppor...tioning-valves

The inner parts:
https://rennlist.com/forums/964-foru...look-like.html

The valve obviously limits the pressure of the brake fluid for the rear wheels. In our case: 1978 cars are limited to 55 bar. Later cars are limited to 33 bar.

This does not make sence does it? Because you would expect that more brake force at the front by changing to a bigger piston but with the same pressure, should allow for more brake pressure at the rear using the same piston diameter.

mark kibort

There is a lot of confusion and parts with purpose to do the job under all conditions. the net net, is that you depending on the match of mastercylinder and bias valve, that will determine if you have enough or too much rear braking . in street mode, its better for more rear braking as you get more even wear of the pads . if you have ABS, there is no issue with rear lock up so that negates the downside. however, without it, you absolutely dont want that much rear bias unless you have HUGE front rotors and pads with race compound. (like anderson did with his racer equiped with 14" rear slicks, using NO bias and stock rear S4 tiny pads and little rotors) this is because if you can do a 1.4g stop, only 300lbs will be sitting on either of the two rear wheels, and much less if you are turning. so, the net is that you wont need much braking force in the rear , nor do you want any more than is capable, or you introduce rear end instability.
The proportioning valve also comes into play, only giving a certain rear brake pressure, regardless of the front pedal pressure, so, in fade situations, (brake over heat) you dont end up with the rear end coming around due to lock up with a lot of pedal force up front trying to overcome fade, and cool brakes in the rear locking up due to unusually high pedal force.

again there is NO doubt that most cars have too much rear bias to begin with . I see it all the time at the track in races. well tuned cars, locking up the rears due to this "production " car setting. real race cars can have in car adjustments to optimize the condition. street performance cars that are used in track street applications should go real light on rear bias. go as big as you can up front and dont worry about the rears. in the end , the bigger you go the better your situation will be.

mark kibort

Yep.... i fianlly bit the bullet and did the test , at the track, in a national event. it was obvious it was the wrong thing to do (increase rear bias over the 18bar stock setting). went back to the stock stuff and no other changes and it was back to the best possible configuration. (thats 13" rotors and the most aggressive race pads). Had i been equipped with 14 rotors and stickier tires, that might have not been much of an issue.

karl ruiter

I have S calipers and rotors on my '79. I also updated to the larger booster but left the regulators alone. There is no problem with the rears locking up, which I think would be the issue with too much pressure on the rear. The larger booster makes a substantial difference to the feel, but not the real stopping power.

mark kibort

the larger rotors and calipers help the rear lock up issue. agian, its going to be subtle, only in race type situations or pinner tires. the vacuum booster is for pedal feel and effort.
what you did with the 79 , helps the situation.

I remember with 79 race car (way long time ago) we also didnt have much problem with rear lock up, but we never pushed it as hard on the track as my current car and the slow down speeds to the same problem corners were much slower. (also with 275/305 toyos)

karl ruiter

Yes, my '79 is street use only, so I am not pushing things that hard. Also I have Carerra IIIs, which means a LOT of rubber on the rear.

mark kibort

so, going as big as you want up front, is only a good thing. No concerns at all about bias situation. now, if it was a 911, there might be more of a valid concern about optimizing the rear.. after all, the 911 can effectively use 2x the brake force in the rear on a threshold stop vs a front engine 928