ENGINEERMAN

Is that a common problem Anders? I have checked to make sure that the line is free and unobstructed. I noticed that there is a fiting that pushes through what looks like a simple grommet in the booster where the vacuum line from the manifold attaches I don't think that it necessarily has a great seal. Is that an area that commonly contributes to weak boost? Please advise.

Thanks!

Thom

Brake wise the only difference between my S2 and my turbo is the front calipers, and the effective pedal travel on the turbo is about twice that of the S2, so : bigger pistons = more fluid = longer pedal travel.

pete95zhn

You can test this by blocking the vacuum tube to booster ( block both sides of disconnection point ). If pedal feel stays the same, there's your problem.

pete95zhn

Well, this is the only difference with calipers...all bigger ones after M030 calipers use same pistons, ie 36/44mm. Even 6-piston ones have the exact same piston area.
Still the S2 and turbo master cylinder are of same size, or at least they share the same part number...thus there is the effect you mentioned, but it doesn't change from that any more if you change to even bigger calipers, eg. Big Blacks/Reds of even 6-piston ones.

Thom

Adding two more pistons is equivalent to fitting four bigger sized pistons, thus requiring more fluid, meaning a six piston caliper would also require a longer pedal travel as well than even a big red/black caliper. Does that make sense ?
What is actually the piston size used in 6 pot calipers if total piston area is the same as M030/big red/black calipers ?

pete95zhn

6-piston calipers have 6, but smaller in diameter ( 36/32/28mm = 25,54 cm2 ), calipers, so total area is the same. Been there, done that https://rennlist.com/forums/944-turbo-and-turbo-s-forum/268211-brake-project-pictures.html. ( 44/36mm = 25,38 cm2 )

Thom

Thanks for the info.
Are you sure of your calculation on the 6 piston caliper ? I find that 36/32/28 = 24.38 cm², which is a bit less than the 44/36 = 25.38 cm² of the M030 & big red/black calipers ...
Also, what is the weight of the 6 piston caliper and related rotors compared to other calipers/rotors ?

333pg333

Here is a quote from someone else on another forum in regards to this:

"The following is the formula for determining the hydraulic brake system pressure: P=f/a. P is the pressure in pounds per square inch (psi). F is the mechanical force applied to a piston in pounds. A is the surface area of the piston in square inches. So 100lb/2in(squared) yeilds 50psi. This is interesing a larger master cylinder piston for example 100lb/4in(squared) yeilds 25psi. If the master cylinder size is increased the pressure to the caliper cylinders is reduced resulting in a greater input effort by the driver at the pedal. When the Honda CB750 first came on to the market it had one disc brake. Those motorcylclists seeking more braking power quickly added another disc and caliper to the other side of the front end creating an overly sensitive brake. Honda quickly was selling a larger master cylinder to reduce sensitivity and reduce accidents from locking up the front brake. One other thought about brake pedal travel. If any part of the caliper slides/pins/pistions are hanging up (not moving easily-lack of lubercation) will result in a soft or long travel brake pedal."

My reply;

"I'm not sure that I get your theory as if what I understand you to be saying, the larger the m/c the less pressure is created? How about if we change nothing but the m/c and even if we subscribed to what you say, wouldn't I at least have more pressure available to me simply by exerting more force? So I currently have 'X' amount of pressure available to me by exerting all or most of my force with the stock m/c, but if I have a larger m/c then perhaps I have to exert more force to achieve the same psi, but ultimately I should have more psi at my disposal due to the greater piston area of the m/c?
I'm not a physicist or mathematician, but I just can't see how increasing the m/c gives me less. I haven't even given thought to the size of the lines carrying the brake fluid? If we go up in m/c what about increasing the diameter of the lines to carry more fluid and increasing the psi to the calipers. This is just my lame brain thinking out aloud here. "

His return:

"This not my theory. This is how hydraulic systems work. If you have a master cylinder that is the same size as a wheel cylinder, the hydraulic pressure will be only be what your leg can produce. To increase hydraulic pressure at the wheel cylinder, the master cylinder has to be smaller than the wheel cylinder, this is how hydraulics work. The greater the difference between the master cylinder and the wheel cylinder, the less effort will be required by the driver. Brake systems that do not have a power booster will have a smaller master cylinder piston that a system with a with a power booster when the wheel cylinder remain the same size. If you put larger wheel cylinders on the wheel (big reds?), but did not change the master cylinder, it may be possible to remove the power booster. I would not recommend this.

If you have any sticking parts in your brake system i.e. pads that are too tight or stuck in their holders, slide pins or plates that are rusted or stuck or a piston in a cylinder bore that is sticking this will cause long travel at the brake pedal. Provided that you do not have any air in the system."

Still doesn't make sense to me unless I am getting mixed up between the force and the pressure?
I don't see how having a smaller m/c is going to increase pressure or conversely having a larger one will decrease it??

Olli Snellman

Six piston caliper weights about the same than old school big calipers. disk weight is a little bit higher. There are a lot of pictures available of these GT2 calipers & adapters.

<== Look at my avatar, that's what Pete & i have.

facboy

hmm interesting...when i went from my turbo to my turbo S i immediately noticed that the S had less initial bite than the turbo. i always wondered why that is...i've gotten used to it though. i wonder if there is something wrong with the car?

Rich Sandor

Guys I think it has more to do with not bleeding your brakes properly. I know you guys will all insist that you're doing it right, but I'll bet anything there's AIR in them thar brake fluids.

OZ951

Tom my 86 had problems like that and it felt unsafe. A brake dyno revealed one caliper was real bad in particular. I pulled it apart to discover that the pads were almost frozen into the calipers by rust/crud under the spring plate. Ie they could barely move yo squeeze against the disc. They were so tight I had to belt them out with a hammer and drift. I cleaned up the under the spring plate and also trimmed a 1/2mm off the top and bottom of the backing plate of each pad. The pads then fit comfortably and move freely in the calipers. I checked the other calipers and found them all the same. Cleaned them all up and now the brakes feel normal. I am not sure where the pads came from but they seem like they were not an ideal fit for the calipers.

Patrick my 87 runs Big reds with no abs, I used ATE in the US and RBF6000 in Oz, both felt fine. SRF should be a champion fluid and is commonly used on the track. Brake effectiveness was great and easily outbraked the tyres with Pagid Blacks until I got 255s up front.

My 86 runs whatever fluid I have available, plus SS lines, its pedal feel is fine and feels safe/normal as described above but I would like more braking effectiveness from it.

Geneqco

Yeah, I think the compromise is:

Smaller MC = less effort but longer pedal travel (ie softer pedal)

Bigger MC = more effort but shorter pedal travel (ie firmer pedal)

333pg333

Rich, if it were me and my buddies doing it I'd be inclined to believe you, but this is being done by my mechanics who are very experienced and are using a power bleeder, not the old manual 2 person method.

Oh, and our stock ones and the 928 ones are the same. We have a 23mm front and 20mm rear conjoined m/c just for reference.

Plus I had it backwards in my head re a leaking booster. It would make the pedal harder not softer of course. D'oh!

Rich Sandor

If you only use the power bleeder without pushing in the brake pedal, you aren't circulating the fluid the same way.

I find I get the firmest pedal by pressurizing the power bleeder to about 10psi, and then having someone push the pedal 8/10th's down and keep it down - while I open and close the bleeders. Then let off the pedal once the bleeder is closed, and push it in and out a bit, then keep it down again while I do the next bleeder.

(Contrary to popular belief - the factory workshop manual says there is NO SEQUENCE for caliper bleeding.)

Each time the brake pedal is let out after a bleed, it moves fluid around more aggressively than the power bleeder alone does.

Try bleeding the brakes just once the way I described, and see if it makes a difference - it'll be a lot cheaper to try than replacing part$.

Maybe I'm full of ****, but this year, for the first time ever - my brakes at Portland and Mission have been AMAZING under very hard braking - and I do them myself! ATE Blue and Ferodo DS2500's on stock 968 calipers.