Mark Anderson

Thanks for the suggestion Steve,
I was wondering what the effect would be if I reduced the size of the booster.

Lizard928

Why not move the check valve out and just decrease the vacuum the booster gets?

dr bob

Mark--

Larger master cylinder = more effort but less pedal feel.

It may be worthwhile looking at how the pedal is attached and where the rod for the master cylinder attaches. Similar to what the Tilton and other adjustable balance bar systems use, you can adjust the linkage pivot positions to mimic the effects of a larger or smaller MC. Just make sure you get full retraction on the MC when the pedal is up, so the feed/bleed port to the reservoir is clear.

You may also consider a different pad.

mark kibort

awesome job Mark, as usual!
times??

anyway, I havent done much work on modifying the braking system, and still run whatever bias was in the 86 and now 87 and I cant notice a difference. (I probably can use a lot more and I think I might have more with the 87 chassis now.)
I dont think a smaller booster will do it, as it doesnt run out of capacity with short hard brake efforts. a smaller booster would run out of vacuum sooner if you pumped your brakes while killing your engine. Dr Bob, can you explain?

i think that bleeding off pressure could do the trick, but that might need to be tested first, as it might have to be a pretty large leak to really change the "feel". what do you think steve??

mk

James Bailey

Using a smaller booster reduces the area of the diaphram therefore reduces the assist. The vacuum is stored in the "eggcrate" in the fender .

Mark Anderson

Good point but I don't know that I want to modify the pedal. I think it's cast and I would not want to weld on it.

Mark Anderson

1:42 was my best but it was VERY hot out so no records in that heat.

Mark Anderson

Jim if I remeber correctly when I ran without the storage tank it felt boosted at the beginning but lost vacuum as I depressed the pedal.

brianrheffron

For a track only car I don't think you should even be discussing
standard master cylinders, brake servos and vacuum bleed valves.
I don't have a lot of 928 experience yet but for my Triumph Dolomite
period race car I threw out the stock vacuum servo, the tandem master
cylinder and the rear axle anti lock proportioning valve. I made up
a system without a servo but with dual master cylinders and
a brake balance bar operated by a push rod from the standard pedal.
With this set up I could adjust the front/ rear balance to suit venue,
tyres and weather conditions. It's not that difficult to do and is surely
the only worth while set up for a track car. I think that with any other
arrangement you are going to leave easily achievable gains "on the table".
One other point to note concerning really heavy braking.
The front uprights/ stub axles on the Triumph were prone to flexing under
heavy braking causing the discs to push the pads back into the calipers
and this caused very long pedal travel before the brakes "bit".
I put in low pressure retaining valves in the front brake lines to deal with
this pad knock back effect. Do 928s ever suffer from this problem?

mark kibort

Well, thats the answer then. the only question , is about if the early boosters are smaller.

Mark Anderson

Yes they do suffer from flex and bad knock back. I have not head of that type of solution. Who makes the parts?

dr bob

IIRC, it's a steel bar assembly that's welded where the pedal pad hangs on. There's a room on your right there, through the door past Tom, where there are known to be a few spare parts for these cars. Take a look at a pedal and see. I've fabricated adjustable pedals and arms before, and it isn't rocket science by any stretch.

Mark Anderson

HA ha. I thought you were talking about modifying the pedal. Yes the hanger is steel but I'm failing to see how that changes things.

karl ruiter

On the off chance you don't have one, I have a spare good 9" booster that you are welcome to.

Mark Anderson

Thanks Karl but I think I have one.