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barnym17 05-21-2009 05:01 AM

master cylinder to use?
I have converted my malibu to rear disc using metric calipers and am trying to figure out the best master cyl. to use. These are low drag calipers needing a quick take up cyl. I think, am considering one from an 86 firebird with 4wheel disc brakes.The catch is I'm running manual brakes not power and all these were power.Can I use the manual pedal ratio and get acceptable braking?

aosborn 05-21-2009 01:36 PM

What you are describing will give you a very hard pedal. You will either need to drastically change the pedal ratio or change to a smaller bore master cylinder to get acceptable braking. If going to a smaller bore cylinder, make sure you don't run out of pedal travel. Smaller bore means less volume per pedal application and 4 wheel disks require more volume than disc/drum applications.


barnym17 05-22-2009 04:59 AM

I was thinking of the older vette manual cyl. but am worried about pedal travel as these calipers take extra volume as they pull off the rotor rather far when released.The stock cyl. had a stepped bore originally for the fronts.

cobalt327 05-22-2009 06:41 AM

Consider calling one of the major brake manufacturers/dealers and ask them for a recommendation.

By "reverse engineering" their recommendation as to pedal ratio and (stepped)bore size, you can then choose an OEM equivalent.

barnym17 05-22-2009 09:49 AM

Thanks for the tip.

powerrodsmike 05-22-2009 02:27 PM

This interests me also. :thumbup: I have not done this but have also begun to look into using a stepbore master in a 4 wheel disc manual application.
The step bore section of the master cylinder only works until about 100 psi system pressure is reached, as it is only designed to bring the pads up to the rotors, then the valving in the master lets the smaller bore piston take over to actually build the pressure. The pedal effort to reach 100 psi line pressure will be minimal with any pedal.

Think of the stepbore master as you would a 2 stage compressor, where you can get the necessary volume AND higher output pressure with less HP.

So I would imagine it is not a big deal as far as pedal effort is concerned, as long as you can construct the pedal so you can have a 6 or 7:1 ratio AND still have the pedal configured so that you can have the pushrod actuate the pistons through their entire available stroke.
This is not as hard to do with a swinging pedal as it is an underfloor pedal, as you can always bring the pedal up higher to get your pedal proportions right.
From my experience, if the secondary bore size is 1" or so, a 6:1 ratio will be adequate to apply enough pressure to the metric calipers, you will have a stiff pedal, and a 7:1 might be better if you are going racing and need to build some higher line pressures.

Do you know the bore size and available stroke of the stepbore master you are looking at? A couple of the step bore masters I had looked at had 1 1/4" primary bore, and 7/8 secondary bores but never investigated the stroke length.

None of the aftermarket companies I talked to had anything to tell me about stepbore masters, but I only asked a couple of them, and it's hard to get to a real brake engineer at those companies.

Later, mikey

barnym17 05-23-2009 05:03 AM

The one I am considering has a 24 mm bore almost 1.0 it equals .996 according to a conversion chart.I already have moved the rod to the manual brake position giving more leverage but for now have a stock 79 camaro cylinder on it with 1 1/16 bore.The stock metric cylinder didn't have enough volume to activate the rears with it's 7/8 bore.Could have been a bad cyl.The main problem with the camaro master is there is a residual pressure valve in it somewhere and the back brakes stay applied somewhat after the bleeding procedure.This master is a disc drum master.After looking at the brake setup on the bird it looks like it is using metric front calipers in the front and simular sized in the rear and I figured it wouldn't have a residual valve being designed for 4 wheel disc.I have used this setup for racing but always with a dual cylinder setup with 1 inch bores.

powerrodsmike 05-23-2009 01:00 PM

3 Attachment(s)
The residual pressure valve is easily removed with an easy procedure. I went out and did one for ya in one of my mockup/visual aid master cylinders:) ..took about 5 minutes, including taking pics.
Tap the male flare seat with a 6-32 tap.

Screw a nut onto a 1" x 6-32 screw, stack up enough washers on the screw to keep the nut from falling into the port, then screw the whole stack into the tapped port.

Crank down on the nut and that seat will pop right out.

Remove the RPV valve and spring, (you will see that the RPV prevents the chips from tapping from going in the bore) clean everything with a bit of brakekleen, then reinstall the brass seat. You can seat the brass seat with a tubenut of the appropriate size.

You can probably run that 1" mc for a while, if it develps enough volume, but a 1 1/8" might be necessary. You'll have a low pedal with the 1", but you can see what the pedal effort is like.

Later, mikey

barnym17 05-23-2009 01:14 PM

Thanks Mikey, I will try the 1 1/16 cylinder first, was over at a buddy's we were yacking and 7/8 is the size on the dual master setup.He is using a 2lb residual valve to keep the pads from pulling so far off the rotors eliminating the need for the quick take up master one front and one rear.He uses this setup on all his hobby stocks and e-mods he builds with good success so it stands to reason we should be able to do it with a stock dual master cylinder, same difference still using two pistons one front and one rear.I will pull the residual valve like you show see how it works if the pedal is still low I'll try the 2lb valves . These setups work well on dirt but you can't use as much brake so we'll just have to experiment.If I can get it to lock up I can adjust bias from there.The best part about the bird cyl. is it is only $29.00 vs. $100.00 for a wilwood which is just a dodge truck master anyway.

barnym17 05-23-2009 01:15 PM

By the way Thanks for all your help.

powerrodsmike 05-23-2009 01:45 PM

2 Attachment(s)
I have used 10# inline RPVs on low drag fronts with no problems, it cuts the piston retraction back to about .010" from .020", so you don't have a brake drag problem with that setup. I don't know what RPV rating is in the MC you have, stockers can be as much as 16#, that extra residual line pressure is what might be making the rear brakes drag in your case, with your current setup.

You can test the RPV, the setup is simple.
Adapt a 0-30 psi gauge to the outlet. (in this case I am testing an inline wilwood)
To test an RPV in a mastercylinder, just adapt a gauge anywhere downstream from the outlet, youcan even plumb it into a bleeder on the caliper.

Apply about 20 psi, the let off the pressure...The gauge will tell you what the residual pressure rating is. :)

That is a valve that I took apart and installed a better spring, it now holds about 12 psi. I test them for pressure, as well as time it takes to leak down. It should hold indefinatly, any less than 20 minutes at the rated pressure and I would consider the valve junk.

Yes, as you can see all of my info is gotten from books. :mwink:
Later, mikey

cobalt327 05-23-2009 02:21 PM


Originally Posted by barnym17
$100.00 for a wilwood which is just a dodge truck master anyway.

What year and model? ;)

barnym17 05-24-2009 07:16 AM

If memory serves early 90's 1/2 ton pickup.

barnym17 05-24-2009 07:18 AM

Mikey you got what my dad called "Gotta see how it works itis"Or when he said this to me as a kid was" What did ya tear up now?"

Triaged 05-28-2009 01:38 PM

Why not just put pre-83 non-low-drag calipers in the rear?

Sounds like you were looking at a manual brake S10 master cylinder which should be a good fit with what you have.

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