Anyone try using an S10 Master cylinder- Results? (Page 1/2)
Dennis LaGrua SEP 18, 04:56 PM
My brake pedal on my 87GT is spongy even after a thorough bleed ( and some time running) and its probably time to change the master cylinder. I would like to use the S-10 master cylinder to go with my S-10. Looks like it will bolt on and uses the same size brake line fittings. This is a larger bore master cylinder and w the S-10 booster figure that the higher capacity May provide better braking action.
Anyone try this? If so what were the results?

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 09-18-2020).]

FieroWannaBe SEP 18, 06:36 PM
A larger bore will lower the line pressure.
Pline=Fin/Amc
With the same pedal force your line pressure will drop.
As your line pressure drops the force your pads apply to the rotors will drop, since:
Fpad=Pline*Acal
fierogt28 SEP 18, 06:55 PM
Dennis, for fun...take it apart. Check the piston, seals, and bore. Just to give you an idea.

I'd buy a new fiero MC anyways and install it. Make sure your callipers aren't leaking.

Dirty brake systems can ruin seals and bores.

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fierogt28

88 GT, Loaded, 5-speed.
88 GT, 5-speed. Beechwood interior, All original.

fierogt28 SEP 18, 06:56 PM
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fierogt28

88 GT, Loaded, 5-speed.
88 GT, 5-speed. Beechwood interior, All original.

[This message has been edited by fierogt28 (edited 09-18-2020).]

pmbrunelle SEP 18, 09:52 PM
I had a larger bore MC, but with bigger calipers than stock.

I like the feel that the bigger bore MC gives; it gives a stiffer pedal that feels more expensive. Less cheap and flexible.

More pedal force is needed to stop the car, but that brings the braking feel towards the sports-car end of the spectrum, and away from the one-touch lock the tires CUV feel.

After such a modification, make sure that you or whatever other probable driver (spouse?) has the necessary leg strength to threshold brake the car.
Johns 4.9 SEP 19, 12:46 PM
Hey Dennis,
I have tried that master cylinder and booster combo. I have larger calipers on all four corners and it works great. Very nice brake feel.
Cheers
Dennis LaGrua SEP 19, 04:58 PM

quote
Originally posted by Johns 4.9:

Hey Dennis,
I have tried that master cylinder and booster combo. I have larger calipers on all four corners and it works great. Very nice brake feel.
Cheers



My Fiero has standard calipers. While I feel that the far lower cost S-10 master cylinder would work nicely, I just played it safe and ordered a new AC Delco unit. ($130 from Summit Racing Equipment) I believe that the Fiero MC bore is .944" while the 2000 S-10 pickup /BLazer is 1" -nearly identical. That would result in about the same braking pressure. The advantage is that the S-10 Master cylinders are about 1/4 the price of the Fieros.
1Packrat SEP 20, 11:10 AM
Good move! I tried this change you were thinking of doing... It Does NOT work after the change you would need to almost STAND on the brakes to make the car stop. Went back to the original size for the car and all is well. A change to the larger brake booster would be a better choice.
fieroguru SEP 20, 11:55 AM
Using a larger master with stock fiero calipers is a very bad idea... it is a brake downgrade as it will result in lower line pressure = lower clamp force = lower braking force.

Most who have done it with OK to favorable results did it in combination with calipers with a larger piston area. They still have the same lower line pressure (result of the larger master bore) but this lower pressure over a larger than stock caliper piston area allows the caliper clamp force to be the same or higher than stock.

Never mix and match brake components unless you have done the math to know the results you should expect.
pmbrunelle SEP 20, 01:11 PM

quote
Originally posted by fieroguru:
it is a brake downgrade as it will result in lower line pressure = lower clamp force = lower braking force.



...assuming that braking_force/pedal_force is the metric you wish to maximize.

There's a range of workable gains; I wouldn't set the factory gain as a hard lower bound.