Calculating a Master Cylinder Selection

[Red Morris]

Hey guys and girls. I’m looking for a bit of help here. I have a braking system that has a pattern failure so prevelant that there are pretty much no parts left in the world! It’s the Powermaster III which I believe was on a few early 90s Buicks and Pontiacs so I’m hoping some of you might be familiar with it. It’s a pressure (as opposed to vacuum) system, and even when it’s working it’s got very poor feedback. On top of that, mine is not working. I’m looking at diagnosing it to repair it, but my winter project will be to replace it with a more conventional system. That’s where I need the help.

I’m really struggling to find someone who can help me through the selection of a master cylinder, and maybe booster, that will work with the car and discs I currently have. Most people get defensive when I don’t buy the part they suggest straight away. Instead I have the nerve to ask why this part is right for me! :ohmy:

I can provide pedal ratio, piston areas, car weight, etc., if anyone thinks they can help me, but I won’t bore you any longer if no one can.

Topic

[Bryan Carter]

Is there any reason why you can’t make use of one of the several aftermarket kits sold for doing a vacuum conversion? Like this one as an example: http://www.gbodyparts.com/product_info.php?products_id=239&osCsid=0489b9aa84d91ee525d6074f403dc0f1

[Red Morris]

I didn’t know these existed actually. 🙂

I’ve looked at it and see a few things that would be a problem though. The car is a Lotus Esprit, so there are some unusual challenges.
The brake pedal would be too wide to fit between the clutch and the accelerator, and the brakes live in the front storage area which is quite small and slopes down.
Any booster would need to sit past the clutch master cylinder, not alongside it, and we start getting close to the booster preventing the bonnet from closing.
What’s to say the master cylinder included would supply the psi and fluid movement required for my pistons?

[Red Morris]

Well, I’ve done some calculations, and I’m fairly confident in them, but the outcome doesn’t seem quite right. :unsure:

I have 4 pots on the front with 2 38.1mm pistons and 2 41.3mm pistons each.
On the rear I have single pots at 36mm.
I’ve been told to expect 0.25mm of piston travel.

So…
38.1mm x 4 x 0.25mm = 1140.5cmm
41.3mm x 4 x 0.25mm = 1339.7cmm
2.480 ccm for the front
36.0mm x 4 x 0.25mm = 509.2cmm
0.510 ccm for the rear

That’s a total of 2.990 ccm, so let’s call it 3ccm. And let’s assume we will need some slack for pipe swell and that needing the whole pedal travel is not ideal and double it to 6ccm. Or 0.366cu.

The smallest bore master cylinder I have found (Wilwood TM1 5/8″) will displace 6.6ccm or 0.4cu.

Is this correct? It seems odd to me that beefy brakes like that could be stopped by the smallest available bore.

I really hope it is, because 5/8″ would require little force and allow me to do without a servo, and it should have a nice high psi too. But somehow I think I’ve missed something

[Author]

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