Brake pedal

[Walt]

Hi all, i have driven my car for close to 8k miles since i first got it on the road, my question is since i have done the donor brakes i cut down the brake arm, i am thinking of getting the Whitby power brake set up to install, so i am thinking i will have to replace the brake arm with a standard sized one.
Am i thinking correct ?
Thanks
Walt

[CraigS]

Yes. Otherwise the brake pedal effort will be too low.

[mike223]

What master cylinder are you using now?

I am using a 21mm bore MC from a 87-88 Tbird with an unmodified 4:1 brake pedal + donor brakes and that setup works great (for me).

That MC is about $36 from autozone brand new.


You ought to try that MC, I can't imagine you'd still need power brakes using a modified pedal with that MC.

You might even end up looking for an unmodified pedal for use with that MC...


The AutoZone part number for that MC is NM1907.

[Walt]

Im using the MC from the donor with out a booster, they have been ok but not great. Donor was an 89 mustang.

[DVANSLEEN]

I have Whitbys power brakes with the modified pedal and love it. Will stop on a dime but with plenty of control. I would try it with your cut down pedal first and then decide.

[Walt]

Mike223, are you using it with or without a proportioning valve? I think it might be worth trying.
Walt

[mike223]

Walt,

I am not using a proportioning valve - but still testing with temporary brake lines at the MC.

I am running 1995 GT single piston calipers in front and 89 TBird in rear (old IRS).

There are many variables in brake bias, but I have autocrossed this setup and it works very well in my car - test yours with care.

I misspoke earlier - the MC I am running is from ~1982 TBird / Granada V6 - The NM1907 part number is correct (and 21mm).


A better option for you probably would be Advance Auto Parts / Carquest P/N MCA39637 - This has the same metric brake line fittings (2- M10x1, M12x1) as your donor MC - where the one I am using (NM1907) only has two standard brake line fittings (9/16"x18, 1/2"x20).

The MCA39637 from Advance / Carquest is simply a different 21mm MC (and would probably be a far simpler / easier change out).


The only master cylinder I see listed on AutoZone's website for a 1989 Mustang is a 1.187" bore (NM2196) - although I suspect most of them (OEM 87-93 power brakes) were probably either 1" or 1-1/16".

I suspect the 21mm MC (MCA39637) was the OEM manual brake MC (but I can't prove that, manual brakes were pretty rare by the late 1980s).


Anyway, I'm running an unmodified OEM brake pedal in a SN95 pedal box - the stock brake pedals in these pedal boxes vary quite a bit, but the pedal I'm using measures about 4:1 mechanical advantage.

The procedure in the FFR manual should yield about a 6:1 ratio depending on execution - so you may find that you need to swap back out to an unmodified brake pedal (also in the original pivot position) due to being too touchy, or maybe short on stroke (probably not a problem, but watch for it).

I can share brake pedal measurements as needed.


If your current MC is 1.187" bore - going to a 21mm (.826" bore) is going to approximately double (200%) your brake line pressures using the same pedal pressure.

If your current MC is 1" bore - going to 21mm bore will increase brake line pressure approximately 50% (150%) using the same pedal pressure.


Good Luck - test with care.

[Walt]

Ok, thanks for the information , i'll do some checking around tomorrow.
Walt